Last mineral identification test this Friday, November 26.

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Last mineral identification test this Friday,
November 26. Sign up for one of the two shifts
14 people per shift.
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EPSC210 Introductory Mineralogy The
phyllosilicates
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Recommended reading Nesse, Chapter 13, pp.
235-260. or Klein Hurlbut, Chapter 11, pp.
418-426.
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• After quartz, phyllosilicates are probably the
  most versatile material mined from the Earths
  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

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Its a relatively small step from a double chain
to a layer silicate.
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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.
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Transmission electron microscope picture
(produced by electron diffraction) showing 3- to
8-chain wide biopyriboles in a clinopyroxene from
a meteorite.
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Phyllosilicates can be described as layered
structures. Each layer consist of two types of
sheets (yellow, blue)
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I) tetrahedral sheet, where SiO4 units share
their three basal oxygen to form infinite sheets
As a result, the TO ratio (where T Al 3, Si4
are ions in tetrahedral coordination) is 25 or
410
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II) octahedral sheet, where MeO6 octahedra share
edges to form infinite sheets.
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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.
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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?
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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 Si4 (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
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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, C4, c.n.3, C-O bond
4/31.33 CaSO4, S6, c.n.4, S-O bond
6/41.5 CaPO4(OH,F,Cl), P5, c.n.4, P-O
bond5/41.25
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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.
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The small spheres, drawn at some corners of the
octahedra, represent the OH- groups. They do
not bond to Si4 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.
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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 wasnt adjusting. This problem is dealt with
in different ways in various types of
phyllosilicates.
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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 sheet
t-o-t layers
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Main groups within the phyllosilicates are 1)
serpentine group 2) clay minerals 3) true
micas 4) brittle micas 5) chlorite group
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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.
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• 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.

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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
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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
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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.
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In kaolinite, Al2Si2O5(OH)4 the crystals
accommodate the misfit by not growing large.
White spheres (right) represent weak hydrogen
bonds between the sheets.
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Kaolinite crystals are often less than 1
micrometer in diameter... so cleavage not
apparent.
Tetrahedra are rotated to fit the size of
octahedral sheet.
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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.
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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.
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Clays form by weathering of silicate minerals in
contact with acidic water, at low temperature (at
Earths surface). KAlSi3O8 2H H2O
Al2Si2O5(OH)4 2K 4SiO2 This equation, given
by Nesse, describes the weathering of
orthoclase/microcline to kaolinite.
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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.
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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.
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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.
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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.
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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.
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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.
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• 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.

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Kaolinite, Al2Si2O5(OH)4 t-o layer dioctahedral,
Talc, Mg3Si4O10(OH)2 t-o-t layer trioctahedral, a
s soft as kaolinite
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Kaolinite forms by weathering (e.g. feldspars),
at the Earths 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.
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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
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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
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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.
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feldspar weathering seen under the microscope
Sericite is a name given to fine-grained
muscovite, and it is another common alteration
product of feldspar.
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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 Al1.3Mg0.3Fe20.1 Si3.5
O10(OH)2 (H2O)
Compare it to the true mica muscovite
KAl2(AlSi3)O10(OH)2
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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.
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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.
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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.
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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
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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.
Lets 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
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The chlorite group
T-O-T layer extra octahedral sheet T-O-T layer
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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?