Sheet Silicates

• Built on a plane of bridging O, leaving an unsatisfied apical charge for each tetrahedra.

• Result is a 'sheet' of tetrahedra which is the basis for group name and physical properties.

• Structure usually composed of sandwiches of tetrahedral and octahedral layers.
• Composition of sheets: Si₂O₅ (count T and O in net)

What are the choices for charge compensation for the apical O ions?

At the same height as, and centered within the ring of 6 apical oxygens, is located an OH^- ion, to create the lower surface of an octahedral sheet. (See Fig. 13.79 in the text - note the triangular facets.)

How many OH^- are needed to finish an octahedral site layer ?

Thus: charge balance is accomplished

• (i) by octahedral cations and
• (ii) by interlayer cations.

What are the likely octahedral cations? The usual suspects:

• Mg, Fe²⁺, Fe³⁺, Al, Mn, Ti, Li

Substitution of Al for Si ? YES

Basic formula: [octahedral cation]⁶⁺ Si₂O₅ (OH)₄

How is 6+ achieved with above list?

Hydrous minerals, low T formation, abundant elements (e.g., Mg, Al)

2+ and 3+ cations -> 6+ ??

• Trioctahedral
• Dioctahedral

SERPENTINE (lizardite, chrysotile, antigorite) Mg₃Si₂O₅(OH)₄

* Structural Modulation:

The general issue of how the tetrahedral sheet and other parts of the structure can change dimensions to allow various compositional variants to occur.

• (2 tetrahedral sheets and one octahedral sheet)

Note the layer stagger in the figures in the text (13.90, 13.91, 13.92)! - What does this mean for the symmetry ?

As above, both dioctahedral and trioctahedral versions!

Dioctahedral = PYROPHYLLITE Al₂Si₄O₁₀(OH)₂

Trioctahedral = TALC Mg₃Si₄O₁₀(OH)₂

K [...]⁶⁺ (Si₃Al)O₁₀ (OH)₂

Dioctahedral = MUSCOVITE

Trioctahedral = BIOTITE

Also: Lepidolite: K (Li,Al) etc.

WHAT IF: INSERT A COMPLETE OCTAHEDRAL SHEET INTO INTERLAYER ?

Remember the simple brucite and gibbsite structures.

Dioctahedral and trioctahedral (and combinations of these) = chlorite

Interesting things that happen in layer silicates:

POLYTYPISM: Polytypes are structures that differ essentially only because essentially identical layers are stacked in different ways. 1M, 2M, 3T

INTERLAYERING - complex minerals

NOTE

Structures of

• Kaolinite: (497K) Al₂Si₂O₅(OH)₄
• Talc: (382K) Mg₃Si₄O₁₀(OH)₂
• Muscovite: (444K) KAl₂(AlSi₃)O₁₀(OH)₂
• Biotite: (618K) K(Mg,Fe)₃(AlSi₃)O₁₀(OH,F)₂ (Mg,Si,F endmember example)
• Chlorite: (499K) (Mg,Al)₆(Si,Al)₄O₁₀(OH)₈

• Chloritoid (234K) (actually an orthosilicate): Mg₂Al₄O₂(SiO₄)₂(OH)₄