Page 242 - A Practical Introduction to Optical Mineralogy

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(/) 0.. Appendix D Mineral
identification chart

This simple chart shows the optical properties of the common ore
minerals listed in order of relative polishing hardness, and it can be
used as an aid to mineral identification. Reflectance values (R %) are
given with minerals plotted in their correct position, but some minerals
are plotted in brackets in a second position because of their variable
appearance.

Procedure
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·o.. "' ro (1) Determine whether the unknown mineral is isotropic, weakly
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0 0 an isotropic or distinctly anisotropic. Weak anisotropy is seen using
.~ E slightly uncrossed polars, whereas distinct anisotropy is easily vis-
ible with exactly crosssed polars.
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"'" (2) Note whether the mineral is colourless, slightly coloured or col-
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::c: 00 oured in PPL; white to grey minerals are considered to be colour-
> r- less for this purpose. Take care to consider the colour in relation to
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several adjoining minerals.
(3) Estimate the brightness (reflectance) of the mineral as a percen-
tage. This is usually rather difficult unless some minerals in the
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:::> section have already been identified. An uncertainty of ±5 is
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8 typical for estimates of R % unless a good reference mineral is also
in the section. If the mineral is distinctly bireflecting then estimate
the minimum and maximum reflectance values.
( 4) Whether the mineral is hard or soft can usually be determined by
t its polishing behaviour, e.g. pits persist in hard minerals, soft
minerals scratch easily. Also, hard minerals tend to stand proud of
the surface whereas soft minerals are scoured out. The Kalb light
line can be used to compare polishing hardness with other minerals
in the section.
~ If the properties of the unknown mineral appear to correspond to one of
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0 the minerals on the chart then it is best to check all the mineral's
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characteristics with the information given for the selected mineral in the
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N~ description of the ore minerals before concluding the identification.
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