Plane of bridging O, unsatisfied apical charge
Result is a 'sheet' of tetrahedra: basis for group name and physical properties
Composition of sheets: Si2 O5 (count T and O in net)
Charge compensation for O- ?
Two apical O arranged, with OH (HOW MANY per Si2O5?), to create upper surface of octahedral sheet
How many to finish octahedral sites ? Thus: charge balance
What are the likely octahedral cations ?
Mg, Fe2+, Fe3+, Al, Mn, Ti, Li
Substitution of Al for Si ? YES
SIMPLE CASE OF ONE TETRAHEDRAL AND ONE OCTAHEDRAL LAYER: 1:1 LAYER SILICATES
Basic formula: [octahedral]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+ ??
SERPENTINTE (lizardite, chrysotile, antigorite
* 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.
WHAT IF, INSTEAD OF (OH) alone, APICAL OXYGENS (and OH) COMPLETE OCTAHEDRA?
2:1 LAYER SILICATES (2 tetrahedral sheets and one octahedral sheet)
[....]6+ Si4O10 (OH)2
Note the layer stagger!! - symmetry ?
Dioctahedral and trioctahedral versions!
Dioctahedral = PYROPHYLLITE
Trioctahedral = TALC
WHAT IF CHARGE ON APICAL O IS ALSO COMPENSATED BY INTERLAYER CATIONS?
2:1 LAYER SILICATES = MICAs!
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 ?
Dioctahedral and trioctahedral (and combinations of these) = chlorite!
POLYTYPISM: Polytypes are structures that differ essentially only because essentially identical layers are stacked in different ways.
INTERLAYERING - complex minerals
David Palmer (Technical information)