Last mineral identification test this
Friday, November 26.
Sign up for one of the two shifts …
14 people per shift.
EPSC210 Introductory Mineralogy
Nesse, Chapter 13, pp. 235-260.
Klein & Hurlbut, Chapter 11, pp. 418-
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
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
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
As a result, the T:O
(where T= Al 3+, Si+4
are ions in tetrahedral
…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
The valence of the metallic cations filling
the octahedral sheet is either:
3+ (Al3+) … in dioctahedral phyllosilicates,
2+ (Mg2+ or Fe2+) … in trioctahedral
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
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...
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,
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
each oxygen ion
3 nearest Mg2+
In a dioctahedral
of all octahedra
are filled. Each
oxygen needs 2
The small spheres,
drawn at some corners
of the octahedra,
represent the OH-
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
The tetrahedral and
the octahedral sheets
are joined into a single
layer by sharing the
apical oxygens (tips)
of tetrahedra) and
corners of MeO6
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-t layers + o
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.
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).
In chrysotile, the T-O ayers curve and roll
up like a carpet. The fibers are not needle-
like crystals, but rolled up layers!
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)
between the sheets.
often less than
so cleavage not
rotated to fit the
size of octahedral
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 is dioctahedral. It is
the dominant clay material in altered
All members of the group can absorb
water molecules between the sheets.
When they do so, their volume expands
montmorillonite Smectite clays (T-O-T)
among the most useful
because of their cation
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
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+ +
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
- 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.
as soft as kaolinite
Kaolinite forms by
feldspars), at the
Crystals remain very
small. Cleavage cannot
Talc forms at a low
Crystals grow larger.
If aligned, the rock
has a foliated fabric.
The mica group (T-O-T):
Hardly any solid solution
Most common trioctahedral micas:
Most common dioctahedral mica:
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):
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
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
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
geometries are possible, and
these different versions of
the same phyllosilicate are
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
Let’s start with a flexible muscovite
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
Chlorite is a low-
mineral. One can
Can you guess the most likely composition
of the garnet that was replaced by chlorite
in this pseudomorph?