Page 243 - A Practical Introduction to Optical Mineralogy
P. 243

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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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