Phyllosilicates.2004.Print

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EPSC210 Introductory Mineralogy

       The phyllosilicates
Recommended reading:

Nesse, Chapter 13, pp. 235-260.

or

Klein & Hurlbut, Chapter 11, pp. 418-
426.
After quartz, phyllosilicates are probably
the most versatile material mined from
the Earth’s crust.

They are used in hundreds of everyday
products because of:
- their cleavage
- a relative chemical inertness
-their ability to exchange ions with fluids
in their surroundings

But not all of them possess all these
attributes…
It’s a relatively small
step from a double chain
to a layer silicate.
Transmission electron microscopy has
revealed that there are minerals with
structures intermediate between
amphiboles and phyllosilicates… alternating
double chains, triple chains and even
quadruple chains.

These minerals cannot be identified in
hand specimen. They are known collectively
as the biopyriboles.
Transmission electron microscope picture
(produced by electron diffraction) showing 3- to
8-chain wide biopyriboles in a clinopyroxene
from a meteorite.
Phyllosilicates can be described as
layered structures. Each layer consist of
two types of sheets (yellow, blue)
I) tetrahedral sheet, where SiO4 units share
their three basal oxygen to form infinite
sheets
                       As a result, the T:O
                       ratio…
                       (where T= Al 3+, Si+4
                       are ions in tetrahedral
                       coordination)
                       …is 2:5 or 4:10
II) octahedral sheet, where MeO6 octahedra
share edges to form infinite sheets.
Depending on its composition, the
octahedral sheet may also be called:

- a “brucite” sheet, identical to the basic
structural unit of the mineral Mg(OH)2.

- a “gibbsite” sheet, identical to the basic
structural unit of one of the polymorphs of
Al(OH)3... Remember, this was a common
component of the rock “bauxite”.
An important subdivision exists within
phyllosilicates based on what fills this type
of sheet.

The valence of the metallic cations filling
the octahedral sheet is either:
3+ (Al3+) … in dioctahedral phyllosilicates,
2+ (Mg2+ or Fe2+) … in trioctahedral
phyllosilicates.

This is the opposite of the valence states!
How were those terms chosen?
Dioctahedral and trioctahedral refer to the
number of cations, in octahedral coordination,
needed to satisfy the valence need of an oxygen
ion that links the tetrahedral and octahedral
sheets.

Mesodesmic bond: half of valence need of O2-is
met by Si+4 (because the e.v. bond strength of the
Si-O bond is +4valence/C.N.4 = 1 e.v. unit).

e.v. bond strength of Mg-O is +2/C.N. 6 = 1/3,
therefore 3 Mg-O bonds needed… trioctahedral

e.v. bond strength of Al-O is +3/C.N. 6 = 1/2,
therefore 2 Al-O bonds needed… dioctahedral
Isodesmic: all bonds of same e.v. strength...
E.g. Na-Cl

Anisodesmic: structure includes bonds of
different e.v. strengths, but none satisfies
exactly half the valence of the anion
(oxygen has valence of 2). No possible
linking of anionic groups below into chains,
sheets, networks…

CaCO3, C+4, c.n.=3, C-O bond: 4/3=1.33
CaSO4, S+6, c.n.=4, S-O bond: 6/4=1.5
CaPO4(OH,F,Cl), P+5, c.n.=4, P-O bond:5/4=1.25
In a trioctahedral
sheet, all sites are
occupied because
each oxygen ion
shared with
tetrahedra needs
3 nearest Mg2+
neighbours.
In a dioctahedral
sheet, two-thirds
of all octahedra
are filled. Each
oxygen needs 2
Al3+ neighbours.
The small spheres,
drawn at some corners
of the octahedra,
represent the OH-
groups.
 They do not bond to
Si+4 ions. H+ provides
one valence unit to its
O2-, octahedral cations
provide the rest.
 OH- groups line up
with the center of
rings found in the
tetrahedral sheets.
 The tetrahedral and
 the octahedral sheets
 are joined into a single
 layer by sharing the
 apical oxygens (tips)
 of tetrahedra) and
 corners of MeO6
 octahedra.

But there a size misfit between the two types of
sheets… This would destabilizs the structure if it
wasn’t adjusting. This problem is dealt with in
different ways in various types of phyllosilicates.
Different sheet combinations make up these
types of layers: t-o, t-o-t, t-o-t +o
 t-o layers


                                  t-o-t layers
                                  with inter-
                                  layer cations



                                 t-o-t layers + o
                                 i.e. interlayer
                                 octahedral
 t-o-t layers                    sheet
Main groups within the phyllosilicates are:

1) serpentine group

2) clay minerals

3) true micas

4) brittle micas

5) chlorite group
Serpentine and clay are not only the
name of groups of minerals.

These terms also refer to rocks made
up mostly of minerals that belong to
the serpentine or clay group.
Serpentine group



three minerals of composition Mg3Si2O5(OH)4

- antigorite, lizardite

- chrysotile (asbestiform variety)

- the neutral T-O layers are held by weak Van
der Waals forces and H..O hydrogen bonds.
In antigorite, the T-O layers are curved but
they reverse orientation regularly. The
result is a “corrugation” (waviness). This also
prevents the layers from slipping easily over
each other (as they do in talc or in kaolinite).




           Mg3Si2O5(OH)4
In chrysotile, the T-O ayers curve and roll
up like a carpet. The fibers are not needle-
like crystals, but rolled up layers!




            Mg3Si2O5(OH)4
The bad name of “asbestos” comes from amphiboles!
Some amphiboles (e.g., glaucophane) grow with a
fibrous habit. In the partial series of glaucophane
to riebeckite, crocidolite has been used as “blue
asbestos”. Over long periods of exposure, its
needle-like crystals are less soluble and more
damaging to lung tissues than chrysotile.




The familiar “tigereye” or “hawkeye” gemstone is created by
the pseudomorphic replacement of crocidolite, the
asbestiform variety of riebeckite, by quartz.
In kaolinite, Al2Si2O5(OH)4 the crystals
accommodate the misfit by not growing large.

White spheres (right)
represent weak
hydrogen bonds
between the sheets.
                     Kaolinite
                     crystals are
                     often less than
                     1 micrometer
                     in diameter...
                     so cleavage not
                     apparent.



Tetrahedra are
rotated to fit the
size of octahedral
sheet.
The term “clay” is also used in earth
sciences to refer to particles of a
size smaller than 5 micrometers.

The glacial clays found as soft
sediment on much of the bedrock in
Quebec is actually a rock “flour”
consisting mostly of crushed quartz
sand (SiO2). It is not necessarily
made up of clay minerals.
In industry, the term “clay” refers to a
fine-grained, earthy material that
becomes plastic when mixed with a small
amount of water.

Clay is the main material used in the
making of pottery. Once fired (“cooked”),
the material turns rock-hard and
waterproof. OH groups were driven off
the clay mineral structures, and they
recrystallized into a new set of mineral
grains with interlocking boundaries.
Clays form by weathering of silicate
minerals in contact with acidic water, at
low temperature (at Earth’s surface).


KAlSi3O8 + 2H+ + H2O
= Al2Si2O5(OH)4 + 2K+ +4SiO2
This equation, given by Nesse,
describes the weathering of
orthoclase/microcline to kaolinite.
Montmorillonite group:

Montmorillonite is dioctahedral. It is
the dominant clay material in altered
volcanic ash.

All members of the group can absorb
water molecules between the sheets.
When they do so, their volume expands
considerably.
montmorillonite   Smectite clays (T-O-T)
                  among the most useful
                  phyllosilicates, largely
                  because of their cation
                  exchange capacity.

                  This property is the result
                  of an increased net negative
                  charge of their layers. This
                  occurs by the substitution
                  of Mg2+ for some Al3+
                  normally present in the
                  octahedral sheets of a t-o-t
                  dioctahedral phyllosilicate.
Smectites swell considerably when the
interlayer ions are replaced by water
molecules. Used as drilling mud, dam plugs.

They tend to exchange weakly bonded
interlayer ions (such as Na+) for other ions
in their surroundings. Used to mop up heavy
metals, even to release medication in pills.
During sedimentary burial, smectite is heated
and, if a source of K is present, its structure
converts to that of illite.

Between 0.8 and 1 K+ cations per formula unit
are incorporated between layers. Since K-O
bonds them together more strongly… no more
swelling when moistened.

Illite is a general term
for mica-like clay minerals
 with a T-O-T layer.
This smectite-to-illite reaction, driven by
the temperature increase at depth,
releases substantial amounts of water and
a decreases in volume of clay-rich rock.

These changes contributes to underground
pressure gradients that are responsible for
the movement of oil and natural gas
through porous rocks.
The source of K for illite is generally K-
feldspar. Its weathering to form illite can
be described by a hydrolysis reaction:

3KAlSi3O8 + 14H2O =
KAl2 (AlSi3)O10(OH)2 + 6Si(OH)4 + 2K+ +
OH-

This Si(OH)4 is silicic acid, the main form
of dissolved silica in natural waters. It is a
common product of weathering of silicates.
Clays are generally studied by X-ray
diffraction.
- Crystal size is too small to determine
their optical properties under the
petrographic microscope (whose resolving
power is limited to about 5 micrometers).
- It is possible to recognize swelling from
non-swelling clays by the changes in d-
spacing they adopt when air dried, and
when ethylene glycol (an organic
compound) replaces interlayer water.
Kaolinite,
Al2Si2O5(OH)4
t-o layer
dioctahedral,



Talc,
Mg3Si4O10(OH)2
t-o-t layer
trioctahedral,
as soft as kaolinite
                         Kaolinite forms by
                         weathering (e.g.
                         feldspars), at the
                         Earth’s surface.
                         Crystals remain very
                         small. Cleavage cannot
                         be seen.

Talc forms at a low
grade of
metamorphism.
Crystals grow larger.
If aligned, the rock
has a foliated fabric.
The mica group (T-O-T):

Hardly any solid solution
between trioctahedral
and dioctahedral
members.
    Most common trioctahedral micas:
     phlogopite KMg3(AlSi3)O10(OH)2
     biotite K(Mg,Fe)3(AlSi3)O10(OH)2

    Most common dioctahedral mica:
     muscovite KAl2(AlSi3)O10(OH)2
 Solid solutions within the mica group
Is this mica dioctahedral or trioctahedral?
  lepidolite K(Li, Al)2-3(AlSi3)O10(O, OH, F)2

Dioctahedral muscovite KAl2(AlSi3)O10(OH)2
  3Li+ = 1 Al3+ (or 1.5Li+ = 0.5 Al3+)

 You can have no more than 3 moles of ions in
the octahedral sheet per formula unit.

Trioctahedral lepidolite (all octahedra filled):
  K (Li1.5Al1.5)(AlSi3)O10(OH)2
The layers of the true micas can be
separated in very thin foliae. Muscovite
and Fe-free phlogopite are used widely as
insulating material in electrical devices.
 feldspar weathering seen under the microscope
Sericite is a name given to fine-grained
muscovite, and it is another common
alteration product of feldspar.
The clay mineral, “illite”, can also be
described as a fine-grained version of
muscovite, but modified by substitutions
such as:
Mg2+ (or Ca2+ ) + Al3+ = K+ + Si4+

An example of a possible illite composition
K0.6(H3O)0.4 Al1.3Mg0.3Fe2+0.1 Si3.5 O10(OH)2 ·(H2O)


     Compare it to the true mica
     muscovite KAl2(AlSi3)O10(OH)2
                            The pseudo-
                            hexagonal habit
                            of biotite...




Phyllosilicates do not have exact hexagonal
symmetry. This is partly because of the size
misfit between tetra- and octahedral sheets.
In addition, the c axis is usually not
perpendicular to the (001) plane.
Many phyllosilicates show
polytypism, i.e. their
layers are stacked with an
offset (i.e. not directly
aligned).

 Different stacking
geometries are possible, and
these different versions of
the same phyllosilicate are
called polytypes.

Kaolinite has two polytypes:
nacrite and dickite.
Polytypism is not polymorphism. It is a
structural variant found only in minerals
with definite sheet structures.

Unlike polymorphs, the symmetry and
environment of the ions is unchanged
within the sheets forming each layer.

The crystallographic system and/or
Bravais type of unit cell changes from one
polytype to another because of the
stacking pattern of the layers.
This is why most micas are monoclinic rather
than hexagonal. Their c axis is inclined relative
to the sheets.




These 3 polytypes of lepidolite show
different degrees of offset among
stacked layers. First two are
monoclinic, the 3rd orthorhombic
“Brittle” micas are scarcer than true micas.
They are found in silica-poor rocks, with
corundum (Al2O3), as alteration minerals. They
are harder, less flexible (cleaved sheets
break easily when they are bent) than true
micas.
Let’s start with a flexible muscovite
KAl2(AlSi3)O10(OH)2

A substitution Al3+ for Si4+ in tetrahedra
requires coupling for charge balance:
       ivAl3+ + Ca2+ = ivSi4+ + K+



The result (margarite) CaAl2(Al2Si2)O10(OH)2
             The chlorite group



T-O-T
layer

extra
octahedral
sheet

T-O-T
layer
                    Chlorite is a low-
                    grade metamorphic
                    mineral. One can
                    find chlorite
                    pseudomorphs of
                    many other
                    ferromagnesian
                    silicates.

Can you guess the most likely composition
of the garnet that was replaced by chlorite
in this pseudomorph?

				
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posted:12/30/2011
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