Page 144 - A Practical Introduction to Optical Mineralogy

CARBONATES
3 The non-silicates Figure 3.1 The
structure of
calcite CaCO,.

3.1 Introduction

Minerals which are not silicates have been grouped together in this
chapter for the description of their properties. However, unlike the
silicates, the crystal structures and chemical variation of members of the
group are not easily related to mineralogical properties and mode of
occurrence. Even subdivision of the group into transparent and opaque The triangular nature of the (C0,) - radical dominates the structure
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minerals is impractical, as closely related minerals and even compo- of the carbonates and results in trigonal (rhombohedral) or
sitional varieties of the same mineral may vary in opacity. For example, orthorhombic (pseudo-hexagonal) symmetry. The critical factor con-
sphalerite is transparent when it is pure zinc sulphide but becomes more trolling the type of symmetry is the radius of the dominant metallic
opaque with increasing iron substitution of zinc. cation; for elements such as Mn, Fe, Mg with radius less than about
The non-silicates can usually be regarded as accessory minerals in 1.0 A the carbonates are trigonal, but for elements such as Ba, Sr, Pb
most rocks, yet they are major components in some rock types, e.g. with larger radii the carbonates are orthorhombic. Calcium lies close in
halides in evaporites, sulphides in massive sulphide deposits and carbo- radius value to the critical size, and this explains the existence of CaCO,
nates in limestones. as two minerals, calcite (trigonal) and aragonite (orthorhomic).
Minerals of the following non-silicate groups appear in this chapter: Although aragonite is considered to be a high pressure polymorph of
carbonates (CO~-), sulphides (S - ), oxides (0 - ), halides (Cl-, F-), CaCO,, it can grow at low pressures provided that the solution chemistry
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hydroxides (OH-), sulphates (SO~-), phosphates (PO!-), tungstates is correct. However, it is metastable and usually inverts to calcite during
(WO~- ) and native elements. Within each group the minerals are recrystallisation processes such as diagenesis.
described in alphabetical order. The relationship of optical and physical
properties to chemical composition and structure is outlined only for the
Figure 3.2 CaC0 3 calcite
first four groups. Carbonates in the
In this chapter, where appropriate, thin-section information is as CaCO,-MgCO,-FeCO,
described in Section 1.3 and presented for the silicates in Chapter 2. The system.
polished-section information, using reflected light, is as described in
Section 1.6.

3.2 Carbonates CaMg(CO,)z , --------~
dolomite ~
ankerite
The carbonates, of which the most well known example is calcite
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CaCO,, contain a discrete (C0,) - radical that may be considered as a
single anion in the structure but is in fact a trigonal planar complex. This
complex, with carbon in the centre of an equilateral triangle formed by
three oxygens, is shown in the carbonate structure in Figure 3.1. There
are relatively few common carbonates of rock-forming significance, and
most can be considered as secondary or replacive minerals forming on
alteration of metal-bearing precursor minerals, e.g. cerussite PbCO, MgC0 3 FeC01
after galena PbS. Some secondary carbonates contain structural water, magnesite sideriie
e.g. malachite Cu 2 CO,(OH) 2 after chalcopyrite CuFeS 2 •
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