Page 46 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS AhSiOs POL YMORPHS

exhibit this perfect cleavage parallel to the basal plane. Minerals belong- Then the main properties of each mineral are given in the following
ing to this group include micas, clay minerals, chlorite, serpentine, talc order: colour, pleochroism, habit, cleavage, relief, alteration, birefrin-
and prehnite. gence, interference figure, extinction angle, twinning and others
(zoning etc.). Of course, only those properties which a particular min-
Tektosilicates eral possesses are actually given, and the important properties are
When all four oxygens are shared with other tetrahedra, tektosilicates or marked with an asterisk.
framework silicates form. Such a framework structure, if composed Some mineral descriptions may include a short paragraph on their
entirely of silicon and oxygen, will have the composition SiO, as in distinguishing features and how the mineral can be recognised from
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quartz. However, in many tektosilicates the silicon ion (Si +) is replaced other minerals with similar optical properties.
by aluminium (AP +). Since the charges do not balance, a coupled The description ends with a short paragraph on the mineral occur-
substitution occurs. For example, in the alkali feldspars, one aluminium rences, associated minerals and the rocks in which it is found.
ion plus one sodium ion enter the framework structure and replace one
silicon ion and, in addition, fill a vacant site. This can be written
Al 2 Si0 5 polymorphs Nesosilicates
AP + + Na+:;:::::: Si + + 0 (vacant site) Andalusite Al,SiO, orthorhombic
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0.983: 1:0.704
In plagioclase feldspars a slightly different coupled substitution is C = CL
required since the calcium ion is divalent: I
2AP+ + Ca'+ :;:::::: 2Si + + 0 (vacant site)
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This type of coupled substitution is common in the feldspar minerals,
and more complex substitutions occur in other tektosilicate minerals or
mineral groups. Tektosilicates include feldspars, quartz, the felds-
pathoid group, scapolite and the zeolite group.
The classification of each mineral or mineral group is given in the
descriptions in Section 2.2.

- - -- - -- -- b = (3
2.2 Mineral descriptions

The thin-section information on the silicate minerals is laid out in the
same way for each mineral as follows:

Group Crystal chemistry
Mineral name Composition (note: Fe means Fe'+) Crystal system
Drawing of mineral (if needed)
n. = 1.629-1.649 } . .
n = 1.633-1.6 RI variation in all polymo~phs IS due to ferne
Rl data 53
n: = 1. _1. iron and manganese entenng structure
Birefringence (<'>): Maximum birefringence is given for each mineral. 638 660
Any variation quoted depends upon mineral com- a = o.oo9-0.011
position. 2V. = 78°-86° -ve (a prism section is length fast)
Uniaxial or biaxial data with sign +ve (positive) or -ve (negative). OAP is parallel to (010)
1
Specific gravity or density Hardness D = 3.13-3.16 H = 6 12-7%
34 35