Page 42 - A Practical Introduction to Optical Mineralogy

CRYSTAL CHEMISTRY
2 Silicate minerals single chain silicates because the ( SiO. r-tetrahedra are linked together
to form chains of composition [ SiO,] ~- stacked together parallel to thee
axis, and bonded together by cations such as Mg' +, Fe'+, Ca'• and Na•
(Fig. 2.1). Chain silicate minerals always have a prismatic habit and
exhibit two prismatic cleavages meeting at approximately right angles
2.1 Crystal chemistry of silicate minerals on the basal plane, these cleavages representing planes of weakness
between chain units. The pyroxenes are single chain inosilicates. Varia-
All silicate minerals contain silicate oxyanions [ SiO.]"-. These units tions in the structure of the single chain from the normal pyroxene
take the form of a tetrahedron, with four oxygen ions at the apices and a structure produces a group of similar, though structurally different,
silicon ion at the centre. The classification of silicate minerals depends minerals (called the pyroxenoids, of which wollastonite is a member).
on the degree of polymerisation of these tetrahedral units. In silicate Double chain silicates also exist in which double chains of composi-
minerals, a system of classification commonly used by mineralogists tion [ Si 4 0 11 ] ~- are stacked together, again parallel to the c crystal-
depends upon how many oxygens in each tetrahedron are shared with lographic axis, and bonded together by cations such as Mg'•, Fe'+, Ca'+,
other similar tetrahedra. Na• and K• with (OH)- anions also entering the structure (Fig. 2.2).
Double chain minerals are also prismatic and possess two prismatic
Nesosilicates cleavages meeting at approximately 126° on the basal plane, these
Some silicate minerals contain independent [ Si0 4 ]"- tetrahedra. These cleavages again representing planes of weakness between the double
minerals are known as nesosilicates, orthosilicates, or island silicates. chain units. The amphiboles are double chain inosilicates.
The presence of [ SiO.] units in a chemical formula of a mineral often
indicates that it is a nesosilicate, e.g. olivine (Mg,Fe),Si0 or garnet
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(Fe,Mg etc.),Al,Si,O ,, which can be rewritten as (Fe,Mg etc.),
AI,[ SiO.L. Nesosilicate minerals include the olivine group, the garnet
group, the AI,SiO, polymorphs (andalusite, kyanite, sillimanite),
zircon, sphene, staurolite, chloritoid, topaz and humite group minerals.
Cyclosilicates
Cyclosilicates or ring silicates may result from tetrahedra sharing two
oxygens, linked together to form a ring, whose general composition is
[ Si.xO,.r] >x-, where x is any positive integer. The rings are linked
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together by cations such as Ba'+, Ti +, Mg'•, Fe'•, AP• and Be'+, and Key
oxycomplexes such as [ BO,J'- may be included in the structure. A 0 oo-
typical ring composition is [ Si 6 0 ,.] , _ and cyclosilicates include tour-
maline, cordierite and beryl, although cordierite and beryl may be • Ca1+. Na+
included with the tektosilicates in some classifications. 0 Mg 2 +, Fc2+
0 Si<+. AI'+
Sorosilicates
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Sorosilicates contain [ Si,0 7 ] .::- groups of two tetrahedra sharing a com-
mon oxygen. The [Si,0 7 ] • - groups may be linked together by Ca'+,
AP +, Mg'+, Fe>+ and some rare earth ions (Ce'•, La'• etc.), and also
contain ( OH)- ions in the epidote group of minerals. Besides the epidote
group, sorosilicates include the melilites, vesuvianite (or idocrase) and
pumpellyite.

lnosilicates
single chains viewed at right
When two or two and a half oxygens are shared by adjacent tetrahedra, angles to the c axis: the chains
single chain parallel to the c are linked together by various
inosilicates or chain silicates result. Minerals in this group are called
axis as occurs in the pyroxenes atoms in the positions shown
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