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TEXTURES OF ROCKS

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					            TEXTURES OF IGNEOUS ROCKS
Texture refers to the way in which individual grains relate to grains immediately surrounding them.
Texture deals with small-scale features seen in hand specimen or under the microscope, such as
the degree of crystallinity, grain size, grain shape, grain orientation, grain boundary relations and
crystal intergrowths. Textures are useful indicators of cooling and crystallization rates and of phase
relations between minerals and magma at the time of crystallization.
Igneous rocks with interlocking crystals have crystalline textures. The crystallinity and dominant
grain size in crystalline igneous rocks are each described y one of a series of three terms, as
shown in the figure below.
                             Names of Special Igneous Rock Textures
               Volcanic                     Plutonic                        Other
             Microlitic                   Poikilitic                       Zoned
             Spherulitic                  Graphic                          Corona
             Vitrophyric                  Ophitic                          Kelphytic rim
             Intersertal                  Subophitic                       Rapakivi
             Intergranular                Diabasic                         Epitaxial
             Felty                        Orthocumulate                    Poikilitic
             Pilotaxitic                  Mesocumulate
             Trachytic                    Adcumulate
             Subophitic                   Symplectic
             Ophitic                      Myrmekitic
             Dictytaxitic                 Seriate
             Glomeroporphyric             Trachytoidal
             Pyroclastic                  Granophyric
             Seriate
             Spinifex
            Source: Raymond, L.A. , 1995, Petrology of Igneous, Sedimentary, Metamorphic Rocks. Wm. C.
            Brown Publishers Company, page 24.


        1.      Textures referring to the crystallinity of an igneous rock
        2.      Textures referring to the granularity of an igneous rock
        3.      Textures referring to the crystal shapes of an igneous rock
        4.      Textures referring to interrelations between the crystals
        5.      Textures referring to orientation and arrangement of the crystals
        6.      Textures referring to intergrowth of the crystals
        7.      Textures referring to overgrowth of the crystals
        8.      Textures referring to radial arrangement of the crystals
        9.      Textures referring to banding of the crystals
        10.     Textures referring to cavity fillings of the crystals
        11.     Other terms referring to the textures of igneous rocks
(Data in Textures of Igneous Rocks page Compiled by Okan Zimitoglu and Seda Cagatay)




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                                                                                                 Textures of Igneous Rocks



1. TEXTURES REFERRING TO THE                                   rock is one devoid of phenocrysts. This term
CRYSTALLINITY                                                  is generally applied only to aphanitic or fine-
                                                               grained rocks. Thus an equigranular granite,
The term Holocrystalline refers to a rock
                                                               for example, devoid of phenocrysts would not
composed of entirely crystalline material.
                                                               be termed aphyric.




Anorthosite sample which is entirely composed of Albitic
                  plagioclase crystal
                                                                 Basalt sample which contains no phenocrysts and is
The term Holohyaline refers to a rock                          composed of plagioclase microphenocrysts and volcanic
composed of entirely glassy material.                           glass. Individual crystals can not be seen by naked eye.
                                                               Pegmatitic refers to a rock that is very
                                                               coarse-grained, dominated by grains > 3 cm
                                                               in length. Although many petrologists reserve
                                                               the term pegmatite for siliceous (granitoid)
                                                               rocks of that grain size, rocks of any
                                                               composition may be pegmatitic.
                                                               Terms Which Are Used According To The
                                                               Absolute Size of The Crystals
                                                               The term coarse-grained implies crystal size
                                                               > 5 mm.
                                                               The term medium-grained implies crystal
   Obsidian sample ( volcanic glass) which is entirely
        composed of siliceous glassy material.
                                                               size between 1-5 mm.
2. TEXTURES REFERRING TO THE                                   The term fine-grained implies crystal size < 1
GRANULARITY                                                    mm.
Terms Which Are Used According to The                          Terms Which Are Used According To The
Visible or Invisible Granular Properties of                    Relative Crystal Sizes
Igneous Rocks                                                  Equigranular refers to an igneous rock
Phaneritic (phanerocrystalline) refers to a                    texture in which the diameters of component
rock in which the grains of the essential                      minerals are comparable, allowing of course
minerals can be discerned without the aid of                   for the inherently tabular and prismatic, rather
microscope                                                     than equant habits of some minerals.




Polished surface of the granitic rock slab. Minerals can be       Biotite granite sample which is composed of nearly
                                                                   equigranular crystals of quartz (white), K-feldspar
           distinguished clearly by naked eye.
                                                               (grayish), plagioclase (grayish with polysynthetic twining)
Aphanitic refers to a rock, which is so fine                              and biotite (reddish brown)minerals.
grained, that individual crystals cannot be
discerned without a microscope. An aphyric


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                                                                                                  Textures of Igneous Rocks



Porphyritic texture connotes one or more
mineral species or a generation of one or
more mineral species that are conspicuously
greater in size than those minerals
constituting the rest of the rock. There are
number of larger grains called phenocrysts,
surrounded by a population of grains of
significantly smaller size, the groundmass.




                                                                Basalt sample with microporphyritic texture. As it is seen
                                                                in the figure, both the phenocrysts and the groundmass
                                                                                are fine grained (aphanitic).




    A porphyric rock which is composed of euhedral-
    subhedral feldspar phenocrysts in a fine grained
                      grounmass.
Seriate texture refers to a situation where
there is a continuous range in grain size of
one or more mineral species from that of                       An aphanitic-porphyritic volcanic rock which is compoded
phenocryst to groundmass size, and in which                       of euhedral sanidine, quartz (not visible) and some
crystals of progressively smaller sizes are                       altered mafic (amphibole) minerals in an aphanitic
                                                                                     groundmass.
increasingly numerous. This texture is
commonly shown by plagioclase in some                          Vitrophyric texture refers to a rock
andesite porphyries.                                           (vitrophyre),  which      is    composed       of
                                                               phenocrysts of intratelluric crystallization in a
                                                               glassy matrix.




A volcanic rock with seriate texture. Feldspar phenocrysts
     especially exhibit a broad interval of grain size.
                                                               A volcanic rock sample containing plagioclase and quartz
Microporphyritic texture refers to a situation                            phenocrysts in a glassy grounmass.
where both the phenocrysts and the
                                                               Microlitic texture is a porphyritic texture in
groundmass are aphanitic.
                                                               which microlites (small fibrous crystals) are
Aphanitic-porphyritic     and     phaneritic-                  enclosed in a glassy groundmass
porphyritic term describes, respectively
some situations where the phenocrysts are
phaneritic but the groundmass is aphanitic
and where both groundmass and phenocrysts
are phaneritic.




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                                                                                                  Textures of Igneous Rocks



3. TEXTURES REFERRING TO THE                                   Dendritic crystals consist of a regular array
CRYSTAL SHAPES                                                 of fibres sharing a common optical orientation
Terms Which Are Used According To The                          and having a branching pattern resembling
Development of Crystal Surfaces                                that of a tree or the veins in a leaf or a feather.
Allotriomorphic refers to a texture in which                   Embayed crystals are those, which have
all the component mineral grains are                           been resorbed by reaction with liquid. While
anhedral.                                                      this may be true of some crystals others have
                                                               embayments, which probably formed during
                                                               growth.




Dunite sample which is composed of entirely olivine (Ol)
minerals and accessory enstatite (En – orthopyroxene)
                      minerals.
Hypidiomorphic refers to a texture in which
the grains of some mineral species are
anhedral, those of others subhedral, and
those of some may even be euhedral. This
texture is typical of granitic rocks in many of                  Rhyolitic volcanic rock sample that contains embayed
which quartz and orthoclase tend to be                         quartz crystal (white colored). Resorbtion of quartz crystal
anhedral and plagioclase and biotite are                             by the melt is clearly seen in the photograph.
subhedral to euhedral.                                         Parallel-growth crystals are aggregates of
                                                               elongate crystals of the same mineral whose
                                                               crystallographic axes are mutually parallel or
                                                               almost so. Although in thin section the
                                                               individual parts of the aggregate may be
                                                               isolated from one another, in the third
                                                               dimension they are probably connected. A
                                                               parallel-growth crystal is therefore a single
                                                               incomplete crystal formed by a particular style
                                                               of skeletal growth.



Norite sample which is composed of euhedral-subhedral
  and anhedral plagioclase minerals (plag - gray-black
     colored minerals with polysynthetic twining) and
      subhedral-anhedral orthopyroxene (opx) and
              clinopyroxene (cpx) minerals.
Idiomorphic refers to a texture in which,
theoretically, all the component mineral grains
are euhedral (a geometrical impossibility in a
holocrystalline rock). The term is commonly
applied to the texture of lamprophyres, which
are      characteristically   densely      and                  Komatiite sample that shows parallel growth of elongate
conspicuously crowded with euhedral mafic                              quenched olivine crystals (blue colored).
phenocrysts.
                                                               Sieve textured crystals are those which
Special Terms Referring To The Special                         contain abundant, small, interconnected, box
Crystal Shapes                                                 shaped glass inclusions, giving the crystals a
Skeletal crystals are those which have                         spongy or porous appearance.
hollows and gaps, possibly regularly                           4.     TEXTURES        REFERRING           TO
developed and usually with particular                          INTERRELATIONS          BETWEEN          THE
crystallographic orientations. In thin section                 CRYSTALS
these spaces appear as embayments and
                                                               Glomeroporphyritic texture results when
holes in the crystal, filled with groundmass                   phenocrysts aggregate in groups; sometimes
crystals or glass.                                             only some of several phenocryst species may
                                                               be so aggregated, suggesting that those


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                                                                                                 Textures of Igneous Rocks



species not involved belong to a later period                  thus only partly enclose individual plagioclase
of     intratelluric   crystallization. Some                   tablets.
glomeroporphyritic clusters represent the
incorporation of texturally more complex
material,     perhaps     consolidated   early
fractionated material.




Olivine basalt sample which is composed of olivine (Ol),
clinopyroxene (Cpx) phenocrysts and plagioclase
microphenocrysts (not seen) in a fine grained                      Gabbro sample that shows clinopyroxene (various
groundmass. Note that the aggregation of clinopyroxene          colored) minerals partly enclosing individual plagioclase
minerals as glomerocrysts.                                               minerals (with polysynthetic twinning).

Poikilitic texture is a general term describing                Intergranular texture is characteristic for
a texture in which one or more mineral                         basaltic flows where the grains of augite and
species may be partly or wholly enclosed by                    accessory opaque minerals small enough to
another mineral species. This texture                          fit between the unoriented array of tabular
indicates crystallization sequence of minerals                 plagioclase microlites. Unlike ophitic texture,
in an igneous rock.                                            adjacent intersticies are not in optical
                                                               continuity and hence are discrete small
                                                               crystals. The feldspars may be in diverse,
                                                               sub-radial or sub-parallel arrangement.




      Biotite minerals (reddish-greenish-dark brown)
 poikilitically enclosed by a large K-feldspar mineral (ligh
                                                               Dolerite sample with intergranular texture. Colored
                            gray).
                                                               minerals are mainly anhedral clinopyroxene and olivine
Ophitic texture is a term generally used for                   minerals, which are enclosed by prismatic plagioclase
gabbroic rocks and refers to a situation where                 minerals.
the dimensions of the augite crystals are                      Intersertal texture is characteristic of tholeiite
substantially larger than those of the                         flows where it may be present in addition to
plagioclase;    several      even    numerous                  intergranular or sub-ophitic textures; although
plagioclase tablets may come to be included                    it is not present in all tholeiite flows but it is
within each grain of augite.                                   diagnostic of tholeiite where it is seen.
                                                               Intersertal texture denotes the presence of
                                                               small, disconnected, patches of a glassy
                                                               mesostasis of acid composition, often
                                                               containing numerous tiny inclusions of opaque
                                                               minerals amongst the rectangular plagioclase
                                                               laths that are not aligned. Glassy mesostasis
                                                               may be altered in older flows generally to
  Gabbro sample that shows extremely large crystals of         varieties of chlorite such as highly colored
   enstatite (En-orthopyroxene) surrounding smaller            chlorophaeite.
                plagioclase (Pl) crystals.
                                                               Diabasic texture is very common in the more
Sub-ophitic texture is a term also used for
                                                               slowly cooled hypabyssal rocks of all basaltic
gabbroic rocks and refers to a situation where
                                                               compositions such as sills and some thick
the augite grains are somewhat smaller, and
                                                               dykes. It is a coarse grained equivalent of the
not much larger than the plagioclase; they will


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                                                                                                Textures of Igneous Rocks



intergranular textures found in volcanic rocks.                unoriented rectangular sections in thin
Diabasic texture is sometimes referred to as                   section; there is no requirement for the
ophitic texture but two terms are not quite                    mesostasis to be glassy or not in the definition
synonymous. Because in diabasic texture                        of orthophyric texture.
both orthopyroxene and opaque mineral as                        Parallel growth texture refers to a single
well as augite may develop a comparable                        elongate skeletal crystal, which in thin section
poikilitic habit with respect to the plagioclase,              appears to consist of a clot of crystals having
whereas ophitic texture specifically connotes                  the same elongation direction and the same
the poikilitic enclosure of plagioclase by augite              optical orientation. In rocks with trachytoid
alone.                                                         texture it is not uncommon for neighboring
                                                               parallel growth crystals to be aligned.




 Olivine basalt sample from Palisades Sill. This rocks is
 composed of relatively coarse grained enstatite (En-
 orthopyroxene), clinopyroxene (Cpx), olivine (Ol) and
                plagioclase (Pl) minerals.
5.      TEXTURES          REFERRING         TO
ORIENTATION AND ARRANGEMENT OF
THE CRYSTALS
Trachytic texture is characteristic for many                    Komatiite sample that shows parallel growth of elongate
trachytes and some phonolites that are                                 quenched olivine crystals (blue colored).
characterized by a high proportion of tabular                  Comb texture refers to elongate, possibly
sanidine microlites in the groundmass that are                 curved, branching crystals sharing the same
very flat in shape and are aligned in sub-                     direction of elongation. The crystals typically
parallel arrangements and thus prone to                        form a band, layer, or fringe with the
strong fluxioning.                                             elongation direction of the crystals inclined at
Trachytoidal texture refers to a sub-parallel                  60-90 to the plane of layering.
arrangement of tabular, bladed or prismatic                    6.      TEXTURES           REFERRING         TO
crystals, which are visible to naked eye.                      INTERGROWTH OF THE CRYSTALS
Hyalopilitic texture refers to a texture in                    In thin section, the junction between two
which plagioclase microlites are set in an                     crystals may appear as a straight line, a
abundant glassy mesostasis (but the                            simple curve or a complex curve; in the third
plagioclase microlites may and generally do                    case the crystals interdigitate or interlock,
show a degree of fluxioning).                                  possibly so intimately that they appear to be
Pilotaxitic texture connotes abundant                          embedded        in    one     another.    These
plagioclase microlites prominently fluxioned in                interpenetrative patterns are all examples of
an overall sub-parallel manner and locally                     intergrowth textures. Usually the crystals
around phenocrysts (but strictly in a                          concerned are anhedral but one or both may
holocrystalline non-glassy matrix).                            be skeletal, dendritic or radiate.
                                                               Consertal texture refers to a situation where
                                                               the boundary between two crystals involves
                                                               interdigitations and hence appears to be
                                                               notched or serrated in thin section.
                                                               Graphic texture is similar to pokilitic texture
                                                               in that a larger grain encloses apparently
                                                               smaller grains, but this texture, which occurs
Pyroxene Andesite sample with orthophyric texture. En -        in pegmatitic granitoid rocks, consists of a
enstatite (orthopyroxene), Cpx - clinopyroxene and Pl –
                plagioclase phenocrysts.                       very large crystal of alkali feldspar enclosing
                                                               smaller crystals of quartz that all have the
Orthophyric texture is quite common in
                                                               same crystallographic orientation. In some
adesites and related rocks, results where the
                                                               cases it can be clearly demonstrated that the
plagioclase microlites have the form of stubby
                                                               apparently separate quartz grains are all
rather than flat tablets so that fluxioning of
                                                               connected and are part of a large, single
these relatively more equidimensional tablets
                                                               dendritic or snowflake quartz crystals.
does not occur and they exhibit squat


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                                                                                                  Textures of Igneous Rocks




 Intergrowth of quartz minerals in K-feldspar crystals to      Myrmekitic texture defined by wormy (rounded)
                  form graphic texture.                        intergrowths of quartz and K-feldspar in plagioclase which
Micrographic texture is compositionally akin                   is adjacent to K-feldspar.
to graphic intergrowth but the term is                         Exsolution lamellae or exsolution blebs
generally used to refer to the small interstitial              refer to a lamellar and bleb-like intergrowths
patches of quartz and alkali feldspar that                     are often attributed to exsolution of the
characterize quartz-diabeses of tholeiitic                     lamellae and blebs of one component from
affinity.                                                      the host crystal (i.e. solid-state reaction).
Granophyric texture delicate intergrowths,
often with an overall radiating habit, of quartz
and alkali feldspar in ternary minimum
proportions. The texture often coarsens
radially outward and passes into a
micrographic texture. This transition may be
apparent on a small scale within the area of
one thin section and may be found also on a
larger scale within intrusive bodies (typically
small epizone intrusions emplaced below
critical vesiculation depth), which may have
granophyric margins and microgranitic
                                                                 Exsolution lamellae of orthopyroxene in clinopyroxene
interiors.                                                                    mineral (blue-purple colored).
                                                               Perthite is very common in igneous rocks and
                                                               consists of quantitatively minor lamellae,
                                                               shreds, patches and rims of an albite
                                                               component within and around host orthoclase
                                                               or microcline. Whatever the orientation in thin
                                                               section, the albite component always has the
                                                               higher birefringence and appears brighter
                                                               under crossed nicols, a useful feature in
                                                               identification, as the exsolution lamellae are
                                                               generally far too small to show any diagnostic
                                                               multiple twinning.
 Granophyric intergrowth of quartz grains in K-feldspar.
Myrmekitic texture connotes a symplectic
intergrowth of quartz and oligoclase occurring
in small cauliflower-shaped embayments into
microcline at oligoclase-microcline boundaries
in katazonal granitic rocks and migmatites, or
more     rarely     at    oligoclase-orthoclase
boundaries in some mesozonal granitic rocks.
Unlike graphic and micrographic textures,
which are produced by crystallization from a
silicate melt, myrmekitic textures reflect
subsolidus replacement effects.
                                                                Exsolution of albite patches in microcline crystal to form
                                                                                     perthitic texture.




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                                                                                                Textures of Igneous Rocks



Antiperthite is not commonly seen in igneous
rocks and it would appear to be strictly a
texture produced during metamorphism. It is
also common in granulites. This texture
consists of numerous tiny blebs and rods of
orthoclase within the host plagioclase.
Mesoperthite refers to the typical perthite of
epizone granitic bodies wherein the albite
component is comparable in abundance to the
orthoclase component may come to form an
anastomosing host around remnants of an
orthoclase component (generally still relatively
                                                                Wormlike intergrowth of very tiny quartz and K-feldspar
more abundant).                                                           minerals in a plagioclase mineral.
Microperthite refers specifically to exsolution                7.      TEXTURES           REFERRING          TO
textures that are visible only on a microscopic                OVERGROWTH OF THE CRYSTALS
scale (some perthitic intergrowths are
                                                               The term overgrowth describes the partial
sufficiently coarse to be conspicuous in hand
                                                               mantling of one mineral either by material of
specimen).
                                                               the same composition or by material of the
                                                               same mineral species but different solid-
                                                               solution composition or by an unrelated
                                                               mineral. This is presumably conforming to a
                                                               sequence of crystallization and without
                                                               conspicuous reaction. The term overgrowth
                                                               carries the additional connotation of
                                                               crystallographic continuity between the two
                                                               participating minerals insofar as their differing
                                                               crystal structures permit this. Where the
                                                               overgrowth forms a more or less continuous
                                                               rim around the enclosed mineral, such as, for
                                                               example, the rim of plagioclase around a
Cryptoperthite refers to even finer-grained                    proportion of orthoclase phenocrysts in some
intergrowths that are only resolvable by X-ray                 textural varieties of rapakivi granite, it may be
methods.       The      apparently      optically              termed a mantle.
homogenous sanidines of older volcanic rocks                   Skeletal or dendritic overgrowths are found
are in fact cryptoperthites (a quickly revealed                in some porphyritic rocks with a glassy or very
by their distinctive values of 2V), their                      fine-grained groundmass may show delicate
submicroscopic pattern of exsolution having                    fibres or plates extending from the corners or
been affected during the passage of time                       edges of the phenocrysts. The overgrowth
rather than during the cooling of the igneous                  and the phenocryst need not to be the same
rock.                                                          mineral.
Symplectic      intergrowth     connotes      an               Corona texture refers to a situation where a
intimate fine-grained intergrowth of two                       crystal o one mineral is surrounded by a rim
minerals in which one mineral has a                            or mantle of one or more crystals of another
vermicular (wormlike) habit; although some                     mineral     (i.e.   olivine    surrounded      by
symplectic intergrowths may well be due to                     orthopyroxene       or    biotite    surrounding
exsolution and others may reflect post-                        hornblende). Such relationships are often
crystallization reaction or metasomatism,                      presumed to result from incomplete reaction
some are of perplexing origin and the non-                     of the inner mineral with melt or fluid to
genetic connotation of the term symplectic is                  produce the equivalent genetic terms
thus useful for descriptive purposes.                          “reaction rim” and “reaction corona” are
                                                               frequently used.




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                                                                                                 Textures of Igneous Rocks



                                                               Epitaxial refers to a situation where one
                                                               mineral grows around another in such a way
                                                               that their crystallographic structures are
                                                               continuous from one to the other.
                                                               Terms Which Are Used For The Crystal
                                                               Zoning
                                                               One or more concentric bands in a single
                                                               crystal are picked out by lines of inclusions or
                                                               by gradual or abrupt changes in solid-solution
                                                               composition of the crystal.


Anhedral amphibole phenocryst (greenish colored interior
part) surrounded by another euhedral amphibole mineral
  (yellowish colored), which is possibly the same type.
Reaction rim strictly results where an early-
formed mineral later reacts with the still
crystallizing magma. A conspicuous example
is the occurrence of prominent “black rims”
composed of small granules of pyroxene and
opaque oxide mineral around hornblende and
biotite phenocrysts in andesite and latite
flows.                                                              Aegirine-augite crystal showing crystal zoning.
                                                                 Note that the concentric euhedral zones in the crystal.
                                                               Normal zoning connotes the gradual
                                                               transition during the growth of a crystal (from
                                                               core to rim) to a relatively low-temperature
                                                               composition in a crystalline solution series. It
                                                               is the anticipated result of fractional
                                                               crystallization where equilibrium has failed to
                                                               keep up with falling liquidus composition.
                                                               Reverse zoning in contrast, connotes the
                                                               transition generally abrupt to a higher
                                                               temperature outer zone in a crystal. Some
                                                               hiatal event such as an accession of fresh
                                                               magma to a magma chamber undergoing
                                                               fractional crystallization or sudden loss of
                                                               volatiles from a sub-volcanic magma chamber
                                                               is responsible for reverse zoning.
                                                               Multiple zoning is used for crystals having
                                                               repeated discontinuous zones. If the zones
                                                               show a rhythmic repetition of width, the
                                                               pattern is known as oscillatory zoning. The
                                                               overall compositional trend of the multiple
                                                               zoning may be normal or reverse or even (in
                                                               which there is no general trend from core to
Basalt sample containig composite pyroxene. Subhedral
                                                               rim). Individual zones may be of uniform or
orthopyroxene phenocryst with rounded margins (inner
 part), is surrounded by clinopyroxene rim (red-yellow         variable composition, such that the zoning
          colored) due to reaction with the melt.              pattern on a composition-distance graph is
Kelyphitic rims refer to a microcrystalline                    square wave, step-like, saw-tooth, curved
overgrowth of fibrous pyroxene or hornblende                   saw-tooth, or some combination of these.
or olivine or garnet. This texture is found in                 Oscillatory zoning generally accompanies
altered coarse-grained igneous rocks, around                   reverse zoning and refers to a succession of
the boundaries of crystals, reflecting partial                 normally zoned shells in a crystal each
degradation of the pyrogenetic mineralogy                      separated by a sharp reverse zone.
under low-grade metamorphic conditions.                        Oscillatory zoning is often evident in the
Rapakivi texture is a special term used to                     plagioclase      phenocrysts        of    andesite,
describe an overgrowth by sodic plagioclase                    originating intratellurically in high level magma
on large, usually round, K-feldspar crystals.


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                                                                                           Textures of Igneous Rocks



chambers where a complex crystallization                       Spherulitic texture is composed of an
history is inferred.                                           aggregate of fibrous crystals of one or more
Convolute zoning is a variety of multiple                      minerals radiating from a nucleus, with glass
zoning in which some of the zones are erratic                  or crystals in between. The acicular crystals
and have non-uniform thickness.                                may be either single, simple fibres or each
                                                               may have branches along its length; any
Sector (or hourglass) zoning is a common
                                                               branches may or may not share the same
feature of pyroxenes in alkali-rich basic and
                                                               optical orientations as their parents. The most
ultrabasic rocks. It has also been seen in
                                                               common occurrence of spherulitic texture is a
plagioclases in a few quickly cooled basalts.
                                                               radiate aggregate of acicular alkali-feldspars
In this type of zoning, a crystal takes the form
                                                               with glass between them, though quartz or
of four triangular segments (sectors) with a
                                                               other minerals may be present, resulting in an
common apex. Opposite sectors are
                                                               intergrowth texture. Axiolites differ from
chemically identical whereas adjacent ones
                                                               spherulites in that radiating fibres extend from
differ in composition (though possibly only
                                                               either end of a linear nucleus (i.e. from a small
slightly) and hence in optical properties. Each
                                                               acicular crystal) rather than a point. They
sector may be homogenous or show
                                                               could be regarded as a variety of overgrowth
continuous or discontinuous or oscillatory,
                                                               texture, as indeed could those spherulites
normal or reverse or ever zoning. In three
                                                               which grow about visible crystals rather than a
dimensions the sectors are pyramid shaped. If
                                                               submicroscopic nuclei.
the sector boundaries are curved, the pattern
can resemble that of an hourglass.                             Variolitic texture is composed of a fan-like
                                                               arrangement of divergent, often branching,
8. TEXTURES REFERRING TO RADIAL
                                                               fibres; usually the fibres are plagioclase and
ARRANGEMENT OF THE CRYSTALS
                                                               glass or granules of pyroxene, olivine or iron
Radiate textures are those in which elongate                   ore occupy the space between. This texture
crystals diverge from a common nucleus.                        differs from spherulitic texture in that no
They are most frequently found in fine-grained                 discrete spherical bodies are identifiable; in
rocks, but not exclusively. A remarkably large                 fact, each fan as seen in thin section is a slice
number of terms exist to describe the various                  through a conical bundle of acicular crystals.
patterns, including, fan, plume, spray, bow-tie,
                                                               9. TEXTURES REFERRING TO BANDING
spherical, sheaf-like, radiate, radial, axiolitic,
                                                               OF THE CRYSTALS
spherulitic and variolitic. All except the last
three are of self evident meaning.                             Textures of this type involve two, or more,
                                                               narrow (up to a few centimeters), sub-parallel
                                                               bands in a rock which are distinguishable by
                                                               differences in texture, and/or color and/or
                                                               mineral proportions. Petrologists also use the
                                                               term layering; while it includes banded
                                                               texture, it is also used for larger scale
                                                               stratification.
                                                               Comb layering refers to elongate, possibly
                                                               curved, branching crystals sharing the same
                                                               direction of elongation. The crystals typically
                                                               form a band, layer, or fringe with the
                                                               elongation direction of the crystals inclined at
                                                               60-90 to the plane of layering.
                                                               Orbicular texture is particularly exotic kinds
                                                               of banding. In this texture, “Orbs” consist of
                                                               concentric shells of rhythmically alternating
                                                               mineral constitution within the shells the
                                                               texture may either be granular or elongate
                                                               crystals may be radially arranged. Orbs may
                                                               reach a few tens of centimeters in diameter.
                                                               Eutaxitic texture is a further variety of
                                                               banded texture and occurs in some tuffs and
Spherulites are radiating masses of fibrous crystals in a      ignimbrites and consists of a regular
glassy matrix. These spherulites are probably composed         alignment of flattened glassy fragments.
of alkali feldspars and some polymorph of SiO2, and in
this cross-polarized shot, appear as round objects with
dark crosses. Note the large phenocryst which forms the
nucleus of one of the spherulites at center-left.



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                                                                                             Textures of Igneous Rocks



10. TEXTURES REFERRING TO CAVITY                               crystals in the host bordering an ocellus are
FILLINGS OF THE CRYSTALS                                       tangentially arranged but sometimes project
These are a collection of textures, which                      into the ocellus. Ocelli are normally less than
feature either holes in the rock, or likely                    5 mm in diameter but may reach 2 cm. Their
former holes which are now partly or                           origin has been ascribed on the one hand to
completely filled with crystals.                               separation of droplets of immiscible liquid
                                                               from magma, and on the other hand to
Vesicular texture refers to a round, ovoid or
                                                               seepage of residual liquid or fluid into
elongates irregular holes (vesicles) formed by
                                                               vesicles.
expansion of gas in magma.
                                                               Miarolitic texture refers to irregularly shaped
                                                               cavities (druses) in plutonic and hypabyssal
                                                               rocks into which euhedral crystals of the rock
                                                               project.
                                                               11. OTHER TERMS REFERRING TO THE
                                                               TEXTURES OF IGNEOUS ROCKS
                                                               Devitrification refers in general to the
                                                               transformation with time of originally glassy
                                                               groundmass or mesostasis material to a fine-
                                                               grained cryptocrystalline or microcrystalline
                                                               product. The patterns of devitrification
                                                               textures      can    be     quite    varied    and
                                                               superimposed, for example, on primary
       Scoria sample containing ovoidal vesicles               features such as perlitic fractures or the
                    (dark regions).                            outlines of shards in an ignimbrite, where
Amygdaloidal texture refers to a former                        varying degrees of primary devitrification may
vesicles which are occupied, or partially                      have occurred varying with height in the
occupied by late stage magmatic and/or post                    cooling unit. Devitrification processes in
magmatic minerals, such as, carbonate,                         ignimbrites occur in two stages: (1) a “primary
zeolites, quartz, chalcedony, analcite, chlorite               devitrification” while the ignimbritic cooling unit
and/or rarely, glass or fine groundmass. The                   is cooling, commonly results in a complete
filled holes are known as “amygdales” or                       fine-grained devitrification of the groundmass
“amygdules”.                                                   of the entire upper portion of the ignimbrite
                                                               cooling unit, often associated with vague
                                                               spherulitic devitrification textures within
                                                               pumice fragments; and (2) a “secondary
                                                               devitrification” with the prolonged passage of
                                                               time will come to affect all glassy material not
                                                               devitrified by the primary devitrification.
                                                               Relict textures refer to such features as
                                                               preservation of shard outlines and perlitic
                                                               cracks in some devitrified ignimbrites and
                                                               glassy rhyolite flows respectively, or the
                                                               presence of small kernels of fresh olivine
                                                               within a crystal that may be nearly completely
                                                               pseudomorphed by a mesh of serpentine.
The oval feature in this photomicrograph is an amygdule:       Felsitic      texture     results    from     slow
a formerly open vesicle which has been filled with a           devitrification over geologically long periods of
secondary mineral(s) precipitated from low-T ground
waters which have penetrated into the rock.                    time of rhyolitic material originally cooled to a
                                                               glass; in felsitic texture aggregates of
Ocellar texture refers to a certain spherical or
                                                               cryptocrystalline      or    very     fine-grained
ellipsoidal leucocratic patches enclosed in
                                                               microcrystalline material extinguish together in
more mafic hosts are known as ocelli (singular
                                                               small patches throughout the rock.
ocellus). Unlike amygdales, the minerals filling
an ocellus can normally all be found in the                    Eutaxitic texture results from varying
host rock; they may include any of; nepheline,                 degrees of flattening and welding of glassy
analcite, zeolite, calcite, leucite, K-feldspar,               shards of groundmass in ignimbrites, which
Na-feldspar,     quartz,     chlorite,   biotite,              are the characteristic volcanic product of
hornblende and pyroxene, or even glass, and                    rhyolite eruptions.
the minerals are commonly distributed in a                     Cumulus texture is best recognized by
zonal arrangement. Often, platy and acicular                   distinctive layering of cumulate rocks in the


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                                                                                                 Textures of Igneous Rocks



outcrop. Where there is only one cumulus                       Monzonitic texture is a specific type of
phase, the cumulate nature of the rocks is                     poikilitic texture in which orthoclase (often
generally evident from the (varied) nature of                  perthitic) poikilitically encloses crystals of
intercumulus growth such as oikocrysts and                     plagioclase (that often display limpid albite
adcumulus growth.                                              rims in this situation) and mafic minerals.
Gabbroic texture results where all the grains                  Aplitic texture refers to a fine-grained rock,
tend to be anhedral with intergrown irregular                  often occurring as narrow aplite veins,
boundaries but may retain an overall                           generally of ideal granite composition, in
subhedral outline (i.e. each cumulus grain has                 which all the felsic minerals are equigranular
acted as a convenient nucleation center for                    and anhedral.
intercumulus growth).




Gabbro sample which is composed of             anhedral
clinopyroxene (colored minerals) and subhedral-anhedral
plagioclase minerals (grayish-white colored).
Granitic texture connotes a granitic rock,
which    is     both    equigranular   and
hypidiomorphic – the common condition.




                                                               Aplite samples with quartz phenocrysts in the fine grained
                                                                  holocrystalline part Of the rock. Fine grained part is
                                                                       composed mainly of quartz and K-feldspar.
                                                               Lamprophyric texture refers to the
                                                               voluminously      porphyritic,  apparently
                                                               panidiomorphic texture that is common in
                                                               lamprophyric rocks.
Biotite granite sample which is nearly equigranular and
composed of Biotite (reddish), quartz (wihe), plagioclase
and K-feldspar (grayish) minerals those are subhedral to
anhedral in habit.




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                                                                                           Textures of Metamorphic Rocks



                          TEXTURES OF METAMORPHIC ROCKS
         Metamorphic rocks are easily recognized in the field on the basis of their distinctive
structures and textures. Textures are a function of grain size, grain shape, intergrain relationships,
grain distribution, and grain orientation. Metamorphic rocks exhibit a variety of textures. These can
range from textures similar to the original protolith at low grades of metamorphism, to textures that
are purely produced during metamorphism and leave the rock with little resemblance to the original
protolith.
         Metamorphic textures may be divided into five major groups-foliated textures, granoblastic
textures, diablastic textures, cataclastic textures, and relict textures. Foliated textures are textures
characterized by an alignment of mineral grains in such a way as to give the rock the appearance
of or the tendency for splitting into layers or flat pieces. Commonly, the minerals in foliated rocks
are predominantly acicular or tabular. Granoblastic textures are those characterized by more or
less equidimensional mineral grains. Diablastic textures are those in which tabular or acicular
minerals are intergrown in a nonfoliated, interlocking, locally radiating manner. Cataclastic textures
are nonfoliated textures characterized by fractured rock materials and mineral grains. Each of
these major textural types may be subdivided into two or more individual types.

                            SOME EXAMPLES OF METAMORPHIC TEXTURES
1. Foliation: Crystallization of platy minerals under a        3. Mylonite: In fault zones the degree of deformation may
directed stress results in a preferred orientation of the      be sufficiently intense that so many recrystallization
plates normal to the direction of maximum stress. In this      nucleation sites are formed that the resulting rock is
picture folition is parallel to the aligned sheets of          extremely fine- grained. The name mylonite is used for
muscovite sandwiched between quartz grains.                    these rocks because their fine grain size is largely the
                                                               result of recystallization in response to high strains.




2. Wavy Foliation: This is a sample of the Ira Phyllite.
Note the wavy foliation and the overall fine-grain size of     4. Weak foliation: Note the fine grain size and the
this rock.                                                     unimpressive foliation in this weakly-metamorphosed
                                                               rock.




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                                                                                           Textures of Metamorphic Rocks



5. Banded texture: Hornfels is a fine-grained contact          8. Crenulation schistosity: The vertical foliation in this
metamorphosed rock. The layers of biotite in this sample       rock intersects and deforms an early schistosity. During
probably respresent original sedimentary bedding.              the development of the crenulation schistosity, quartz is
                                                               removed from the zones in which deformation occurs.
                                                               Consequently, the crenulation schistosity is defined by
                                                               compositional zones enriched in mica.




6. Mesh texture: Serpentine is surrounding and veining
relict clinopyroxene in this low grade meta-mafic rock.

                                                               9. Nematoblastic texture: Fine grained to very coarse
                                                               grained texture characterized by subparallel arangement
                                                               of acicular or elongate prizmatik minerals.




7. Palisades texture: The region of coarser-grained
quartz in the upper center portion of this photomicrograph
was probably originally occopied by coesite, the high-
pressure polymorph of quartz.
                                                               10. Porphyroclastic texture: This large grain is a K-
                                                               feldspar     porphyroclast.    Unlike    porphyroblasts,
                                                               porphyroclasts are not grown in-situ, but rather are
                                                               fragments of pre-existing minerals which were broken up
                                                               during the process of metamorphism.




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                                                                                           Textures of Metamorphic Rocks



11. Porphyroblastic texture: This is an andalusite             14. Poikiloblastic texture: Describes porphyroblasts
porphyroblast with poikiloblastic texture. Also note how       which are riddled with finer grained inclusions of other
the foliation (oriented roughly N-S in this view) is wrapped   minerals. Here, orange tourmaline and gray K-feldspar
around the left and right corners of this grain, suggesting    include numerous, fine- grained quartz and muscovite
synkinematic growth of the andalusite porphyroblast.           mica grains.




12. Porphyroblastic texture: Note the zonal distribution       15. Granoblastic texture: Note how the interlocking
of quartz inclusions in this garnet porphyroblast.             plagioclase grains in this rock meet at ~120 degree triple
                                                               junctions. This feature is characteristic of granoblastic
                                                               texture.




13. Porphyroblastic texture: These stubby crystals are
chloritoid porphyroblasts. You can just barely see the
anomalous green interference color at the edge of some         16. Granoblastic-Polygonal texture: Note how the
of the grains.                                                 interlocking calcite grains in this rock meet at ~120
                                                               degree triple junctions. This feature is characteristic of
                                                               granoblastic texture.




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                                                                                           Textures of Metamorphic Rocks



17. Granobalstic-Polysuture texture: Note the sutured          18. Heterogranobalstic texture: The crystal structure of
grain boundaries between the quartz grains in this rock.       this quartz grain has been deformed (probably by low-
                                                               grade metamorphism) to produce sub-grains.




REFERENCES
http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/metamicro.html
http://www.tulane.edu/~sanelson/geol212/metatexture.htm
Philpotts, A.R., 1989. Petrography of Igneous and Metamorphic Rocks. Prentice-Hall, Inc.,
Englewood Cliffs, New Jersey. First edition, 180 p. Raymond, L. A., 1995. Petrology. Wm. C.
                              rd
Brown Communications, Inc., 3 edition, 742 p.




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                                                                                                           Image Gallery




Fig. 1a. This photomicrograph shows a magmatic
clinopyroxene granite north of Ausable Forks, New York,
which contains coarse mesoperthite in a granulite facies.
Where the granite is slightly-deformed, secondary albite-      Fig. 3. Myrmekite bordering grid-twinned microcline in
twinned plagioclase An20 has replaced the K-feldspar           granodiorite.
(light gray) of the perthite leaving plagioclase lamellae
sticking out like teeth of a comb into the secondary
plagioclase. Note remnant plagioclase lamellae (cream-
yellow) in secondary plagioclase (center, left).




                                                               Fig. 4. Replacement of plagioclase (tan; albite-twinned;
                                                               bottom) by microcline (grid-twinned; dark gray) from
Fig. 1b. This photomicrograph is from the same                 border and interior of plagioclase crystal in granodiorite.
mesoperthite (microcline, black) as in Fig. 1a. Remnant        Note irregular islands of microcline in plagioclase with
plagioclase lamellae (black-gray) project like teeth of a      albite-twinning of the grid-twinning parallel to albite-
comb into both secondary albite- and pericline-twinned         twinning in plagioclase. Note remnant islands of
plagioclase (light gray, left) and secondary plagioclase       plagioclase (top) in optical parallel alignment with larger
(white, upper right).                                          plagioclase crystal at the bottom. Biotite (lower right
                                                               corner; brown).




Fig. 2. Aggregates of myrmekite (left side) formed around
the plagioclase crystals (right side; albite twinned; white    Fig. 5. Unbroken myrmekite adjacent to albite-twinned,
and gray) in diorite, which may have resulted from Ca-         deformed and broken plagioclase crystal (left side) in
metasomatism of sodic plagioclase. (Quartz (cream;             mylonitic gneissic granite.
white). Biotite (brown).




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                                                                                                              Image Gallery




                                                                Fig. 9. Micrographic texture formed by eutectic
Fig. 6. Remnant plagioclase islands (tan) inside                crystallization in microgranite. Quartz (white; cream).
microcline (grid-twinned; gray) in granodiorite. Note that      Microcline (grid-twinned). Biotite (brown).
albite-twinning of the grid-twinning is in parallel optical
alignment with albite-twinning of the plagioclase (light tan;
upper right) and that the plagioclase islands in the
microcline are in parallel optical alignment with the larger
plagioclase crystal (upper right).




Fig. 7. Irregular plagioclase islands (light tan) in a          Fig. 10. String perthite formed by exsolution processes in
microcline crystal (grid-twinned; gray) in granodiorite.        microgranite.
Islands are in parallel optical continuity. Myrmekite grains
(right side). Biotite (brown).




                                                                Fig. 11. Graphic texture of runic quartz (white) in perthitic
                                                                microcline (grid-twinned; dark gray), generated by
                                                                magmatic processes in pegmatite.


Fig. 8. Overgrowth of secondary K-feldspar (dark gray) on
end and sides of albite-twinned plagioclase crystal (light
gray). This is anti-rapakivi texture in granodiorite.




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                                                                                                        Image Gallery




                                                               Fig. 13. Photomicrograph of intergrowth of tourmaline
                                                               (brown) in microcline (grid-twinned) in pegmatite. This
                                                               intergrowth may have resulted from metasomatic
                                                               processes.
Fig. 12. Vein perthite with albite lamellae (light gray) in
microcline (dark gray; grid-twinned), formed by exsolution
processes in pegmatite.




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                                                                                                   Image Gallery




                Definitions of Some of the More Common
                           Petrographic Terms

Acid - applied to igneous rocks with an                        Effusive - applied to material (Lava) poured
abundance of silica (usually 66% or more);                     out through volcanic vents of fissures.
the term felsic is preferred.                                  Euhedral - a term applied to minerals
 Alkaline rocks - a general term applied to                    bounded by their characteristic faces. =
rocks with feldspathoids, or to rocks with                     Idomorphic = Automorphic.
unusually high concentrations of alkalies                      Eutaxitic - a term describing the streaked
(Na2O)                                                         appearance of certain volcanic rocks due to
Allotriomorphic granular - a granular texture                  alternating bands or lenses of different
in which most grains are anhedral. See                         material, color, or texture. May in part be a
aplitic.                                                       fluidal structure, and, in part, depositional.
Amydale or amygdule - a vesicle or gas                         Expansion fractures - fractures formed in
cavity in an igneous rock usually filled with                  surrounding grains when a mineral expands
secondary minerals such as zeolites, calcite,                  upon alteration. Commonly seen around
chlorite, chalcedony, etc.                                     serpentinized olivine.
Amygduloid - a rock (usually basaltic)                         Exsolution - the process of separation. In the
containing amygdules.                                          solid state, the separation of two components
Amygduloidal - a term applied to rocks                         which are capable of existing in solid solution
containing amygdules.                                          at higher temperature (e.g., perthite).
Anhedral - a term applied to minerals not                      Fabric - the shape and arrangement of
bounded by their characteristic faces. =                       crystals and amorphous parts in a rocks.
Allotriomorphic = Xenomorphic.                                 Texture is a function of fabric.
Antiperthite - a perthitic intergrowth in which                Felsic - a rock with a felsitic texture.
plagioclase encloses potash feldspar. See                      Felsitic - an aphanitic texture but applied only
perthite.                                                      to acid rocks.
Aphanitic - a fine-grained texture in which the                Flow structure - a banded structure
individual      constituents      cannot       be              commonly in which platy and elongated
distinguished by the unaided eye. Includes                     minerals show parallel orientation. It is due to
both crystalline and glassy rocks.                             flowage during the middle or later stages of
Aplite - an igneous rock (commonly occurs as                   consolidation.
veins or dike rocks) with an aplitic texture.                  Fluidal structure - similar to flow structure
Aplitic - a fine-grained, sugary texture in                    but commonly applied to aphanitic rocks.
igneous rocks inwhich the constituents are                     Glomeroporphyritic - a porphyritic texture in
anhedral grains. See allotriomorphic granular.                 which the phenocrysts occur in aggregates.
Autolith - an inclusion in an igneous rock to                  Granitic - a granular texture characteristic of
which it is genetically related (i.e., a cognate               a granite. = Hypidiomorphic granular.
inclusion, or endogenous inclusion). Cf:                       Granophyric - a microscopic graphic texture.
xenolith.                                                      = Micrographic =Micropegmatitic
Banded structure - due to alternating layers                   Granular - a texture characterized by grains
of different mineralogical composition or                      of approximately equal size.
texture.
                                                               Granularity - the absolute size of crystals in a
Basic - applied to igneous rocks with a low                    texture. Texture is a function of granularity.
silica content (usually less than 52%); the
                                                               Graphic texture - An intergrowth of two
term          mafic         is         preferred.
                                                               minerals (commonly quartz and potash
Crystallites - tiny embryonic crystals in
                                                               feldspar) giving a pattern resembling
glassy rocks which do not polarize light
                                                               cunieform or semitic characters. The
Deuteric - essentially synonymous with late                    individual grains of each mineral, though
magmatic, but not hydrothermal.                                apparently isolated from each other, have
Devitrification - the process of changing from                 parallel optical orientation over small areas.
the unstable glassy state to the crystalline                   Groundmass - commonly applied to the
state.                                                         matrix of a rock in which phenocrysts are
Diabasic - see ophitic.                                        imbedded.


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Holocrystalline - composed wholly of                           Miarolitic cavities - small openings in
crystalline                             material.              plutonic rocks ( commonly granites) filled or
Holohyaline - composed wholly of glass.                        lined with crystals ( quartz, feldspar,
Hourglass       structure     -    a   structure,              muscovite, fluorite, etc.). These represent the
resembling an hourglass in shape, observed                     last phases of crystallization.
in certain cross-sections of minerals.                         Microcrystalline - a texture in which
Hyalopilitic - a texture where numerous                        individual grains can be seen only under a
microlites (commonly feldspar) are enclosed                    microscope.
in a glassy groundmass.                                        Microgranular - microscopically granular.
Hydrothermal - applied to heated waters                        Micrographic - microscopically graphic. =
from crystallizing magmas and to the effects                   Micropegmatitic, = Granophyric.
produced by it, and to the resultant deposits                  Microlites - Microscopic tabular or prismatic
formed.                                                        crystals. Distinguished from crystallites by
Hypabyssal - applied to those igneous rocks                    their capacity to polarize light.
and bodies more or less intermediate between                   Micropegmatitic - see micrographic. =
the volcanic and plutonic types. It includes                   Micrographic, = Granophyric.
sills and dikes.
                                                               Myrmekite - an intergrowth of vermicular
Hypidiomorphic granular - a granular                           (worm-like) quartz and plagioclase (generally
texture where the mineral constituents show                    oligoclase) usually replacing potash feldspar.
boundaries in part only. (= granitic texture.)                 Possibly a deuteric (late magmatic) effect, or
Hypocrystalline - composed in part of glass                    related to deformational recrystallization in
and in part of crystalline material.                           some                                        cases.
Idiomorphic granular - a granular texture in                   Myrmekitic - the texture found in myrmekite.
which the mineral constituents are mostly                      Ophitic - a texture in which large crystals of
euhedral.                                                      augite enclose lath-shaped crystals of
Inclusion - a foreign body enclosed in a                       plagioclase. As the amount of augite
crystal or rock. See xenolith.                                 decreases and the relative amount of
                                                               plagioclase increases the former may fill only
Intergranular texture - a variety of intersertal
                                                               the interspaces between the latter. Commonly
texture in which the interspaces are filled with
                                                               used synonymously with diabasictexture.
granular material.
                                                               Palagonite - a yellow or orange, isotropic
Intersertal - a texture in which glass or                      mineraloid formed by hydration and other
crystalline material fills the interspaces                     alteration    (devitrification,   oxidation)    of
between larger crystals (commonly feldspar                     sideromelane,         and       constituting     a
laths).                                                        characteristic part of palogonite tuffs. Also
Leuco - when prefixed to rock names                            found as amygdule fillings in some basaltic
indicates      a     leucocratic      character.               lavas and as an alteration of the glassy skins
Leucocratic - a term applied to igneous rocks                  of pillow basalts.
unusually low in mafic minerals relative to the                Pegmatite - originally defined to indicate a
normal rock-type.                                              coarse, graphic texture but now used to
Lithophysae - concentric shells with hollow                    describe an unusually coarse-grained igneous
spaces between developed in shrunken                           rock with a pegmatite habit.
spherulites. The cavities may be lined with                    Perlitic - a structure in glass showing onion-
minute crystals of feldspar, quartz, or                        like cracks produced by contraction upon
tridymite.                                                     cooling.
Mafic - a term for ferromagnesium minerals                     Perthite - an intergrowth of plagioclase
actually present in a rock. Also applied to                    (albite) and potash feldspar. In perthite potash
rocks rich in mafic minerals.                                  feldspar encloses plagioclase but in
Megascopic - applied to observations made                      antiperthite plagioclase encloses potash
by the unaided eye.                                            feldspar. In microcline-perthite, microcline is
Mela-, Melano- - when prefixed to rock                         the host. Microperthite is a microscopic
names indicates a melacratic or melanocratic                   perthite.
character.                                                     Perthitic - describing the intergrowth of
Melacratic, Melanocratic - a term applied to                   potash feldspar and plagioclase. The texture
igneous rocks unusually rich in mafic minerals                 may be due to replacement of host by
related to the normal rock-type.                               included mineral, simultaneous crystallization
                                                               of the two minerals (less probable), or the
Miarolitic - possessing miarolitic cavities.
                                                               separation or unmixing from solid solution as


Trần Đại Thắng-Bộ môn Khoáng thạch –Khoa Địa chất – ĐHKHTN Tp.HCM (sưu tầm v à tổng hợp)                      21
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a result of instability induced bycooling.                     Spherulite - a spheroidal mass of acicular
Phaneric - Phaneritic, = Phanerocrystalline.                   crystals (orthoclase and quartz), radially
Phaneritic - a texture in which nearly all of                  arranged.       Spherulites     are     frequently
the mineral constituents can be distinguished                  encountered in glassy rocks and have
by the unaided eye.                                            essentially the same composition as the glass
                                                               from which they may form.
Phanerocrystalline = Phaneric, = Phaneritic.
Phenocryst - a large crystal in a porphyritic                  Spherulitic - a structure in glassy rocks due
rock.                                                          to the presence of spherulites.
Pilotaxitic - a texture in which lath-shaped                   Strain shadows - the wavy extinction seen in
microlites (commonly feldspar) make up a                       crystals due to strain. Very common in quartz.
felty aggregate, glass being absent or in very                 It is not to be confused with the partial
minor quantities. Characteristic of some                       extinction observed in zoned crystals.
volcanicrocks.                                                 Subhedral - term applied to minerals
Pleochroic halos - colored zones in                            bounded only in part of their characteristic
pleochroic minerals (biotite,etc.) surrounding                 faces. = Hypidiomorphic, = Hypautomorphic.
inclusions of radioactive minerals (zircon, etc.)              Subporphyritic - between porphyritic and
which are characterized by an intensification                  granular.
of the pleochroism or darkening of the host.
                                                               Texture - indicates the mutual relations of
Plutonic - applied to those igneous rocks and                  crystalline and amorphous crystallinity,
bodies which form at significant depths in the                 granularity and fabric. It should not be
earth.                                                         confused with the term structure.
Poikilitic - a texture in which small crystals of              Trachytic - a texture in which tabular crystals
one mineral are enclosed by a much larger                      (feldspar) have subparallel orientation as a
crystal unit. Ophitic and diabasic textures are                result of flowage in a partly crystallized melt.
varieties of poikilitic texture in which
                                                               Ultramafic - a term applied to igneous rocks
plagioclase is enclosed by pyroxene.
                                                               with little or no feldspar but comprised almost
Porphyritic - a texture in igneous rocks in                    entirely of mafic minerals. Chemically they
which       conspicuously       large    crystals              may be considered as those rocks with less
(phenocrysts) are imbedded in a finer-grained                  than 45% silica.
or glassy groundmass.
                                                               Variolite - similar to a spherulite but
Pumiceous - a frothy structure in glassy                       characteristic of mafic rocks.
rocks due to extreme vesiculation in the fluid
                                                               Variolitic - a structure in mafic rocks due to
stage by expanding and escaping gases.
                                                               the presence ofvariolites.
Reaction rim - a peripheral zone of minerals
                                                               Vesicle - an air-bubble in an aphanitic rock
formed around another mineral by reaction of
                                                               formed by expanding and escaping gases in
the latter with the magma or adjacent minerals
                                                               the cooling lava or magma.
(partial resorption).
                                                               Vesicular - a structure in aphanitic rocks due
Resorption - the process whereby earlier
                                                               to an abundance ofvesicles.
formed minerals become unstable in the liquid
and are partly or completely dissolved, re-                    Vitrophyre - volcanic glass that contains
fused, or changed (resorbed).                                  phenocrysts or microphenocrysts.
Scoriaceous - a structure found in mafic lava                  Xenocryst - crystals in igneous rocks that are
with a cindery appearance due to an                            foreign to the body of rock in which they
abundance of large vesicles.                                   occur.
Segregation - a concentration into a mass or                   Xenolith - a fragment or inclusion of rock
streak of certain of the early products of                     foreign to the igneous rock enclosing it. Cf.
crystallization in a magma.                                    autolith. These may be partly reacted upon by
                                                               the magma so as to form new minerals.
Seriate porphyritic - a porphyritic texture in
which there is nearly a complete gradation in                  Zoning- a generally concentric arrangement
grain size from the largest phenocrysts to the                 of slightly different composition material in
average sized grain in the groundmass.                         what appears otherwise to be a homogeneous
                                                               crystal.     Very commonly exhibited             in
Sideromelane - basaltic glass; characteristic
                                                               plagioclase.
of palagonite tuff.




Trần Đại Thắng-Bộ môn Khoáng thạch –Khoa Địa chất – ĐHKHTN Tp.HCM (sưu tầm v à tổng hợp)                      22

				
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