Page 245 - A Practical Introduction to Optical Mineralogy

Appendix E Gangue minerals

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::l The gangue minerals referred to here are the minerals that commonly
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0 accompany ore minerals in hydrothermal deposits. Although they are
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transparent and are best studied using transmitted-light microscopy, it is
useful to be able to recognise the common gangue minerals in reflected
light (see Fig. 1.7). The minerals listed all have low reflectance values
-~ but the eye can determine small differences in brightness even at these
~ low values. The carbonates are exceptional in having large bire-
....
" E fringences and this results in distinct bireflectance; the resulting strong
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<') anisotropy is often masked by internal reflections. It is relatively easy to
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ll) recognize a mineral as being a carbonate but it is difficult, as it is in thin
~ section, to determine the type of carbonate. As well as using optical
-~ properties, physical and textural properties can be used in recognising
'" the gangue minerals:
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Quartz Lack of cleavage but irregular fractures ; crystal shape,
especially pyramidal terminations; lack of alteration.
Barite Several sets of cleavage traces; bladed or tabular crystals;
radiating aggregates.
Fluorite Octahedral cleavage giving up to three cleavage traces
and triangular cleavage pits; cubic crystals.
K-feldspar Several cleavage traces; alteration.
Carbonates Rhombohedral cleavage resulting in up to three cleavage
traces; multiple twinning; rhomb shaped crystals.

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