1. William D. Nesse Copyright © 2012, by Oxford University Press, Inc.
CHAPTER 13
Sheet Silicates
Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

2. Sheet Silicates are made of two types of sheets: O or octahedral sheets and T or tetrahedral sheets. These two sheets are bonded to form layers which are stacked and bonded to form the mineral structure.
Octahedral sheets are made of two planes of OH-ions and the octahedral voids are occupied by bivalent Fe2+ or Mg2+ ions, or by trivalent Al3+ or Fe 3+ ions. If the cations are bivalent, all three octahedral sites are filled forming trioctahedral sheets – Mg3(OH)6 same as Brucite. If the cations are trivalent, only two of the three sites are filled forming dioctahedral sheets – same as in Gibbsite – Al2(OH)6

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Figure 13.2 Schematic plan view of trioctahedral and dioctahedral sheets.
Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

4. How many oxygens are shared? (Z:O ratio)
Figure 11.2 Silicate structures.
How many oxygens are shared? (Z:O ratio)
a. 0 – isolated (1:4) (how is the charge balanced?)
b. 1 – disilicates (2:7)
c. 2 – ring silicates (1:3)
d. 2 – single chain (1:3)
e. 2 and 3 – double chain (4:11)
f sheet silicates (2:5)
g. 4 – framework (1:2)

5. For a dioctahedral TO layer: 1T sheet + 1O sheet =TO layer + hydroxyl
Tetrahedral sheets consists of tetrahedrally coordinated cations which can be Si4+, Al3+ or less commonly Fe3+. The tetrahedra are arranged in a ring like mesh such that each tetrahedra shares three oxygens and the one, called the apical oxygen is not shared – giving the sheet a formula of Si2O52- or when Al3+ or less commonly Fe3+ substitutes for Si4+, the –ve charge of the sheet will increase
The T and O are joined when the apical oxygen replaces on OH- ion in the O layer. This forms a TO or 1:1 layer– composed of three layers of anion
For a dioctahedral TO layer:
1T sheet + 1O sheet =TO layer + hydroxyl
Si2O52-+ Al2(OH)6 = Al2Si2O5(OH)4 + 2(OH)-
Equivalent trioctahedral layer is Mg3Si2O5(OH)4
A TOT or 2:1 layer is formed by sandwiching a O layer between two T layers. It consists of four layers of anions.
2T sheet + 1’O’ sheet = TOT layer + hydroxyl
2Si2O52-+ Al2(OH)6 = Al2Si4O10(OH)2 + 4(OH)- Equivalent trioctahedral layer is Mg3Si4O10(OH)2
Figure 13.1 Sheet silicate sheets and layers.
Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

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1:1 Layer silicates
Consists of repeating TO layers
Dioctahedral : kaolinite Al2Si2O5(OH)4
Trioctahedral : Serpennite Mg3Si2O5(OH)4
Layers are electrically neutral, bonding between layers are by weak vanderWaal’s or hydrogen bond  soft minerals
2:1 Layer silicates
Consists of repeating TOT layers
Although TOT layer is electrically neutral, substitution (e,g., Al3+ for Si4+) gives the layers a net negative charge.
In Talc (Trioctahedral) and Pyrophyllie (dioctahedral), Si4+ occupies all tetrahedral sites hence TOT layers are charge neutral, bonding between layers are by weak vanderWaal’s or hydrogen bond  soft minerals, greasy feel
TOT + c Structure
Dioctahedral
Al2Si4O10(OH)2  Al2(AlSi3O10)(OH)2 + K+ = KAl2(AlSi3OH10)(OH)2 (Muscovite)
Tricotahedral: Mg3Si4O10(OH)2  Mg3(AlSi3O10)(OH)2 + K+ = KMg3(AlSi3O10)(OH)2 (Phlogopite)
Both ionic and vanderWaal’s bond between layers  hardness 2-3
Figure 13.3 Schematic layer silicate unit structure.
Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

7. William D. Nesse Copyright © 2012, by Oxford University Press, Inc.
Brittle Micas:
Dioctahedral: Margarite  half the tetrahedra Si4+ is replaced Al3+ and to balance the 2 –ve charges divalent cations like Ca2+ occupy the interlayer sites: CaAl2(Al2Si2O10)(OH)2
No equivalent tricotahedtal brittle micas is found – the most common trioctahedral brittle mica is clintonite  1/3rd of octahedral sites contain Al3+ (instead of Fe2+ or Mg2+) and is balanced by additional Al3+ for Si4+ substitution: CaMg2Al(Al3SiO10)(OH)2
TOT + O Structure aka 2:1 + 1 layer example: Chlorite group
TOT (e.g., Talc) sheet + O (e.g., Brucite) sheet = chlorite structure
M3Si4O10(OH)2.Mg3(OH)6
In reality, Al3+ replaces about 1/3rd of Si4+ in the tetrahedral sites causing negatively charged tetrahedral sheets which is balanced by Al3+ substituting for Fe2+ or Mg2+ in octahedtral sheets which makes O sheets positively charged. The stronger ionic bonds gives chlorite hardness of 2-3 – same as the micas.
Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

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Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

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Polytypism:
Top tetrahedral layers must be offset compared to the bottom sheet in order to fit in the sites in the central OH- sheet.
The offset can take place along any of the three pseudo-hexagonal axes
Different polytypes depending on the periodicity of offsets i.e., they go zig-zig-zig /zag-zag-zag or zig-zag/zig-zag and so on.
Figure 13.4 Polytypism in the micas.
Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

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Figure 13.5 Serpentine structures.
Introduction to Mineralogy,
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Figure 13.6 Serpentine.
Introduction to Mineralogy,
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William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

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Figure 13.7 Muscovite. (a) Muscovite “book” from a granitic pegmatite. (b) Photomicrograph of muscovite (M) with dark biotite (B) and clear quartz in a mica schist.
Introduction to Mineralogy,
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William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

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Biotite is a solid solution between four components
Figure 13.8 Compositional range of common biotite (shaded). Adapted from Deer and others (1992).
Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

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Figure 13.9 Biotite. (a) Foliated biotite grains in mica schist. Note the dark pleochroic halos around radioactive inclusions. (b) Pale laths of phlogopite-rich biotite (B) with calcite (C) in marble.
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William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

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Figure Approximate range of nγ (shaded) as a function of mole fraction octahedral Fe in biotite.
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Figure Lepidolite composition. The common range is shown with the shaded area.
Introduction to Mineralogy,
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Figure Glauconite pellets (G) with rounded quartz (Q) grains and elongate detrital muscovite (M). Calcite forms the cement between grains, and the opaque mineral is hematite.
Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

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Figure Chlorite.
Introduction to Mineralogy,
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William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

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Figure Approximate variation of chlorite optical properties as a function of composition. After Albee (1962).
Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.

20. William D. Nesse Copyright © 2012, by Oxford University Press, Inc.
The term clay has been used for sediments or rocks, size of clastic particles and minerals. We will discuss only clay as a mineral
Very difficult to recognize in fine grained sediments
1:1 clay mineral: Kaolinite (dioctahedral) and serpentinite (trioctahedral)
2:1 clay mineral: Smectite
lower negative charge in TOT layers requires less interlayer cations leaving many interlayer sites vacant where water can move in and out making smectite an expanding clay.
Illite (higher negative charge in TOT layer requiring more interlayer cations)
Vermiculite – forms by alteration of Biotite – also expanding clay
Mixed Layer Clays: Structure made of different combination of 1:1 and 2:1 clay layers.
Figure Clay mineral structure.
Introduction to Mineralogy,
Second edition
William D. Nesse Copyright © 2012, by Oxford University Press, Inc.