Page 220 - A Practical Introduction to Optical Mineralogy

COLOUR OF MINERALS IN PPL

Cubic 5.1.4 Identification of minerals using reflectance measurements
Isometric
Accurate spectral reflectance curves are now available for all the com-
mon ore minerals. An identification scheme described by Bowie and
Simpson (1980) is based on reflectance values at the four standard
cube 100 dodecahedron 110 wavelengths 4 70, 546, 589 and 650 nm. Having measured several grains
of the unknown mineral at the four standard wavelengths, the results are
Uniaxial compared with the reference values on linear charts. If certain
identification cannot be achieved immediately, then microhardness
Hexagonal m measurements or qualitative properties may be used to supplement the
w
prism IOiO • . . . ' quantitative measurements. An advantage of their method is that
although sophisticated research microscopes are required for accurate
basal pinacoid 0001 rhombohedron lOll determination of spectral reflectance curves, relatively simple apparatus
can be used to provide satisfactory reflectance values at the four stan-
Tetragonal dard wavelengths.
A 6 5.2 Colour of minerals in PPL

v.PY"mid Ill .
basal pinacoid 001 dipyramid 101 Recognition of the colour of minerals in polished section is useful in
prism 110 pnsm 100 their identification, but unfortunately most minerals are only slightly
coloured and the actual colours seen are easily changed. Colour change
Low symmetry may be real, for example it may result from slight tarnishing; or it maybe
Triclinic illusory, for example it may be caused by a varying background colour
8J OQ:J to a better understanding of colour and its use in mineral identification.
I~
due to differing associated phases.
The application of quantitative colour theory to ore minerals has led

pinacoids Ill, IiI . I i l. IIi basal pinac()id 001 basal pinacoid 001 The colour perceived by the observer depends on the nature of the light
source, the spectral reflectance curve of the mineral and how the
pinacoids 110, 110 pinacoids 010, 100
observer interprets the spectral distribution of the light reaching his eye
Monoclinic in terms of the mineral's surroundings. There is also the possibility of
slight imperfection in the observer's colour vision. Obviously anyone
'
. who is severely colour blind is going to have great difficulties in using
00 reflected light microscopy.
The quantitative colour system used is that of the Commission Inter-
basal pinacoid 001 pinacoid 101 nationale d'Eclairage 1931 (Judd 1952). If the standard data and their
front pinacoid I 00 basal pinacoid 00 I pinacoid I Oi
side pinacoid 010 prism 110 side pinacoid 010 theoretical treatment are accepted, the only measurement required to
obtain quantitative colour values of a polished section of a mineral is its
· spectral reflective curve. This curve represents the modification made by
.
·
·.
.
"
"
Oclhochombi< ~. the polished surface of the mineral to the white colour of the light
.·
.
·
·
..
··
..
'
.·.·
·
Ill
.··
0
source. A surface with 100% reflectance at all visible wavelengths would
•,
obviously appear bright white (the colour of the source lamp). All
basal pinacoid 001 minerals have reflectances much less than 100%, and since R % varies
basal pinacoid 001
dipyramid Ill prism 110 front pinacoid I 00 with wavelength this leads, but not in a simple way, to colour. Using the
side pinacoid 010
CIE (1931) colour diagram (described in detail in Section 5.2.1), miner-
Figure 5.4 Crystal symmetry. als can be plotted and their colours compared quantitatively as well as
209