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Example chart from Webmineral
Optical mineralogy Petrology {{petrology-stub
In optical mineralogy
Optical mineralogy is the study of minerals and rocks by measuring their optical properties. Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrographic microscope. Opti ...
an interference colour chart, first developed by Auguste Michel-Lévy
Auguste Michel-Lévy (7 August 184427 September 1911) was a French geologist. He was born in Paris.
Biography
He became inspector-general of mines, and director of the Geological Survey of France. He was distinguished for his researches on extru ...
, is a tool to identify mineral
In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...
s in thin section
In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron ...
using a petrographic microscope
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology whi ...
. With a known thickness of the thin section, minerals have specific and predictable colours in cross- polarized light, and this chart can help identify minerals. The colours are produced by the difference in speed in the fast and slow rays, also known as birefringence.
When using the chart, it is important to remember these tips:
* Isotropic and opaque (metallic) minerals cannot be identified this way.
*The stage of the microscope should be rotated until maximum colour is found, and therefore, the maximum birefringence.
*Each mineral, depending on the orientation, may not exhibit the maximum birefringence. It is important to sample a number of similar minerals in order to get the best value of birefringence.
*Uniaxial minerals can look isotropic (always extinct) if the mineral is cut perpendicular to the optic axis
An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight.
The optical axis is an imaginary line that defines the path along which light propagat ...
(this situation can be revealed with the conoscopic interference pattern A conoscopic interference pattern or interference figure is a pattern of birefringent colours crossed by dark bands (or ''isogyres''), which can be produced using a geological petrographic microscope for the purposes of mineral identification and in ...
).
References
*Nesse, W. D., 1991, Introduction to Optical Mineralogy, 2nd edition.External links
Example chart from Webmineral
Optical mineralogy Petrology {{petrology-stub