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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=1.33CaSO4, S+6, c.n.=4, S-O bond: 6/4=1.5CaPO4(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

t-o-t layerswith inter-layer cations

t-o-t layers + oi.e. interlayer octahedral 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.

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.

montmorillonite

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 termfor 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.

Talc,Mg3Si4O10(OH)2 t-o-t layertrioctahedral,as soft as kaolinite

Kaolinite,Al2Si2O5(OH)4 t-o layerdioctahedral,

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 compositionK0.6(H3O)0.4 Al1.3Mg0.3Fe2+

0.1 Si3.5 O10(OH)2 ·(H2O)

Compare it to the true micamuscovite KAl2(AlSi3)O10(OH)2

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.

The pseudo-hexagonal habit of biotite...

Many phyllosilicates show polytypism, i.e. theirlayers 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.

These 3 polytypes of lepidolite show different degrees of offset among stacked layers. First two are monoclinic, the 3rd orthorhombic

This is why most micas are monoclinic rather than hexagonal. Their c axis is inclined relative to the sheets.

“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-Tlayer

extraoctahedral sheet

T-O-Tlayer

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?