Earth Materials Lecture 8: Phyllosilicates

Question 1

Define Phyllosilicates

Answer 1

Minerals that contain silicon (Si) and oxygen (O) in some multiple of Si2O5.

Phyllosilicates are made up of sheets of atoms arranged in two patterns: tetrahedra and octahedra.

Question 2

Describe tetrahedra and octahedra in Phyllosilicates

Answer 2

Tetrahedra

A central silicon atom surrounded by four oxygen atoms.

Three of the oxygen atoms in each tetrahedron are shared with other tetrahedra, forming a hexagonal array

Octahedra
cations surrounded by large, anions of oxygen, hydroxyl, and sometimes fluorine.

Both Tetrahedra and Octahedra have triangular faces

Question 3

Describe Phyllosilicate structure

Answer 3

Sheets bonded by weak van der Waal’s forces producing the perfect basal (001) cleavage and
“ease of gliding”.

layers can be linked by cations to satisfy charge imbalances.

Question 4

What is Van der Vaals forces

Answer 4

Van der Vaals forces are forces of attraction and repulsion between molecules that have charges that vary depending on their polar region.

This is the same attraction which is in hydrogen bonding.

Question 5

What are the cation substitions for the Phyllosilicates

Answer 5

Cation substitutions:
➢ Tetrahedral layers: Al3+ or Fe3+ for Si4+

➢ Octahedral layers: Principal are Al3+ (dioct); Mg2+ ,Fe2+ (trioct).
− Also Fe3+, Ti, Ni, Zn, Cr, Mn.
− Mid O2- replaced by OH-, Cl-, F-

Question 6

List the (Main/most common) micas

Answer 6

Muscovite
Biotite
Phlogopite
Paragonite
Lepidolite

Question 7

Define Muscovite

Answer 7

K²Al⁴(Si⁶Al²O²⁰)(OH,F)⁴

A structure in which two-thirds of the octahedral sites are occupied by trivalent cations, such as Al and Fe.

RI’s 1.552 – 1.616, varies with Fe and Mn content.

Hexagonal tabular habit but with ~ditrigonal symmetry.

Can be intergrown with biotite

Question 8

Describe the optical properties of Muscovite

Answer 8

Colour also variable depending on impurities.

Colourless to pale cream in PPL

Occasionally weakly pleochroic

Moderate relief

Look for cleavage trace

Mod to high birefringence:
high 2nd to low 3rd orders.
Increases with Fe content

Straight extinction

Question 9

Define Biotite

Answer 9

K,Mg, Fe,Al,Ti.

Trioctahedral with Mg, Fe or Al in O-sheets, and interlayer K.

Each oxygen or hydroxyl ion is surrounded by three divalent cations

RI’s 1.53 – 1.7, variable with chemistry

Hexagonal tabular/platy habit.

Question 10

Describe the optical properties of Biotite

Answer 10

Browns, yellowish, reddish, greenish in PPL.

Strongly pleochroic.

Moderate relief.

OAP (010).
High birefringence 3rd orders +
and often masked by bodycolour.

Extinction is straight

Pleochroic haloes in some
- zircons.

Question 11

List the clay minerals which are Phyllosilicates

Answer 11

Illite Group
K,Al4(Si8-xAlx)O204

Kaolinite Group
Al4Si4O10(OH)8 (dickite, nacrite, halloysite)

Chlorite Group
(Mg,Fe2+)10Al2
Al2Si6O2016
Clinochlore (Mg) – Chamosite (Fe) solid solution.

Smectite Group (e.g. Montmorillonite, saponite etc)
− (Ca0.5,Na)0.7(Al,Mg,Fe)4-6
(Si,Al)8O204
.nH2O

Vermiculite Group (Mg,Ca)(Mg,Fe3+,Al)6
[(Si,Al)8O20]4.nH2O

Serpentine
Mg3Si2O54 (lizardite, antigorite, chrysotile)

Talc
Mg6Si8O204

Question 12

Describe the formation and occurrence of clay minerals

Answer 12

Form in a range of surface and subsurface environments and
conditions of temperature, pressure and pH.

Weathering, sedimentation, diagenesis, low-grade
metamorphism and hydrothermal systems incl. seafloor.
− Magmatic intrusions, ore veins, fumaroles etc.

In hydrothermal systems, from ~100 to 350 or more Celsius and
2-7 pH, but generally 5.5-7.

Hydrothermal systems often zoned by Temperature and pH

Breakdown of primary silicates and glasses.

In weathering, leaching of elements into solution and
precipitation in situ or transported away.

Slow but can be accelerated by climate, increased
concentrations of elements etc.

Smectites, illite, kaolinite from felsic rocks.

Chlorite, serpentine, and talc from mafic and ultramafic rocks.