Interference colour chart
Encyclopedia
An interference colour chart, first developed by Auguste Michel-Lévy
, is an optical mineralogy
tool to identify mineral
s in thin section
using a petrographic microscope
. 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:
Auguste Michel-Lévy
Auguste Michel-Lévy was a French geologist. He was born in Paris.He became inspector-general of mines, and director of the Geological Survey of France. He was distinguished for his researches on extrusive rocks, their microscopic structure and origin; and he employed the polarizing microscope...
, is an optical mineralogy
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...
tool to identify mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s in thin section
Thin section
In optical mineralogy and petrography, a thin section is a laboratory preparation of a rock, mineral, soil, pottery, bones, or even metal sample for use with a polarizing petrographic microscope, electron microscope and electron microprobe. A thin sliver of rock is cut from the sample with a...
using a petrographic microscope
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 which focuses on detailed descriptions of rocks...
. 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
Birefringence
Birefringence, or double refraction, is the decomposition of a ray of light into two rays when it passes through certain anisotropic materials, such as crystals of calcite or boron nitride. The effect was first described by the Danish scientist Rasmus Bartholin in 1669, who saw it in calcite...
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When using the chart, it is important to remember these tips:
- Isotropic and opaque (metallic) minerals cannot be identified this way.
- Rotate the stage in order to get the maximum colour, and therefore, the maximum birefringence.
- Each mineral, depending on the orientation, may not exhibit the maximum birefringence. It is important to try and find multiple like 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 axisOptic axis of a crystalThe optic axis of a crystal is the direction in which a ray of transmitted light suffers no birefringence . Due to the internal structure of the crystal , light propagates along the optical axis differently than in other directions...
. (But this situation can be revealed with the Conoscopic interference patternConoscopic interference patternA conoscopic interference pattern or interference figure is a pattern of rings caused by optical interference observed when diverging light rays travel through a non isotropic substance. It is the best way to determine if a mineral is uniaxial or biaxial and also for determining optic sign in...
).