Page 45 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS
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1 1 1 lcltulised tetrahedral layer of the sheet silicates Figure 2.3
(a) Sheet silicates
(b) sheet silicates,
the three
polytypes.

Key
0 o'-
e Cal+, Na+
0 Mg2+, Fe 2 +
0 Si4+. Al h \ I \
Ill• ''i''~'C'- of the tetrahedra all point in the same direction
1111 til" ca-.e upwards). Such a tetrahedral sheet may be depicted
111 'II''' -.cction as:
\'-----~ _/ or I t \
Ill , ...,, () layers are joined together by octahedral laye rs; either
( I I )J I) layers. called Gibbsite layers and depicted by the letter G,
1 1 1 ~ ~ ~ .• 1-1:-0 H) layers. called Brucite layers and depicted by
double chains viewed at right I ill "'Ill' I 13 .
Figure 2.2
angles to the c axis: the chains
Double chain double chain parallel to the c are linked together by various I h) 2 layer unit (I tetrahedral layer and I octahedral
silicates. axis as occurs in the amphiboles atoms in the positions shown \'i== ==t/ layer; called a I : I type)
I G I }
---- 7
11cxt I : I unit /
Phyllosilicates
111 I lypc represented by kaolinite - serpentine is similar with a B layer replacing the G layer
When three oxygens are shared between tetrahedra, phyllosilicates or
sheet silicates result. The composition of such a silicate sheet is
[ Si.0 10 ] ~- . Phyllosilicates exhibit 'stacking', in which a sheet of brucite
} 3 '''" ,,;, (2 '""'h'd"l '"' I oo"h'd"l; o'll'd '2 ; llyp<)
2
2
composition containing Mg +, Fe • and (OH)- ions, or a sheet of gibbsite
composition containing AP • and (OH)- ions, is stacked on to an alkali atoms here - K , Na, etc.
[ Si.O,o] silicate sheet or sandwiched between two [ Si 4 0 10 ] silicate ---- 7
next 2 : I unit '
sheets (Fig. 2.3a). Variations in this stacking process give rise to several 111 type with muscovite , illite and montmorillonite having G octahedral layers, and biotite
( )
related mineral types called polytypes. Three main polytypes exist, each Jl l.tyc t ~: the three layer units are joined together by mo novalent alkah tons. Montmo nllomte
of which is defined by the repeat distance of a complete multilayered IIIIIY no t possess any ato ms in this plane and may have an overall negative charge. Water
lllllkntlc~ may enter the structure alo ng these inter-unit planes
unit measured along the crystallographic axis. The 7 A, two layer struc-
ture includes the mineral kaolin ; the 10 A, three layer structure includes
B
the clay minerals montmorillonite and illite, and also the micas; and the
14 A, four layer structure includes chlorite. Figure 2.3b gives simplified
4 layer unit (2 tetrahedral and 2 octahedral; called a 2 : 2 type)
detzils of the main polytypes. These multilayer structures are held I BorG I
together by weakly bonded cations (K+, Na•) in the micas and other
10 A and 14 A polytypes. In some other sheet silicates, only Vander I \
Waals bonding occurs between these multilayer structures. The sheet
silicates cleave easily along this weakly bonded layer, and all of them tll'\1 2 : 2 unit (3) 14 A type as represented by chlorite
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