Color measurement of some natural and synthetic minerals -I.

Satoru Nakashima, Isoji Miyagi, Eiji Nakata, Hitoshi Sasaki,
Shinji Nittono, Tsuyoshi Hirano, Takayuki Sato and Hisato Hayashi
Rep. Res. Inst. Natural Resources, Mining College, Akita Univ.
No. 57, pp 57-76 (1992)

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Abstract:

A simple colorimetry of rocks has been developed by using Minolta
chromameter CR-300 and spectrophotometer CM-1000. Colors of rock
powders and blocks can be rapidly and quantitatively described in a L* 
(white-black)-a*(red-green)-b*(yellow-blue) color space. Colorimetry
of synthesized Ni-hydrotalcite and Ni and Co-smectites indicates
possible chemical forms of Ni and Co in these clay minerals. Color
changes of precipitates in aqueous solutions during the synthesis of
jarosite indicate the possible chemical forms of intermediate more or
less amorphous phases and suggest jarosite formation pathways. Colors
of natural amphiboles from volcanic rocks can be an indicator of their 
Fe2+/Fe3+ ratios coupled with their water contents. Colorimetry on
altered rocks containing chlorite and zeolites indicates that a* value 
can be taken as an indicator of degree of chloritization. Chlors of
fresh, weathered, hydrothermally altered granites together with
primarily pink and red granites were measured. The comparison with
colors of powder mixtures of goethite and hematite in alumina suggests 
that the weathering yields mainly goethite-like yellow-brownish
minerals, while the hydrothermal alteration forms hematite with some
presence of FeOOH. These mixtures can also be used a a
semi-quantitative measure of FeOOH and Fe2O3 contents of these
granitic rocks. The colorimetry and diffuse reflectance spectroscopy
of 1mm steps on weathering and alteration fronts indicate that these
fronts tend to be enriched in Fe2O3 component. The formation processes 
of Fex (O, OH, H2O)y -> FeOOH -> Fe2O3 along the alteration front can
be then characterized by this method. All these results from the first 
systematic colory study of minerals and rocks in a quantitative way,
which should be continued on much more wide variety of synthetic and
natural minerals.