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Concept
Optical mineralogy
Summary
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. Optical mineralogy is used to identify the mineralogical composition of geological materials in order to help reveal their origin and evolution.
Some of the properties and techniques used include:
Refractive index
Birefringence
Michel-Lévy Interference colour chart
Pleochroism
Extinction angle
Conoscopic interference pattern (Interference figure)
Becke line test
Optical relief
Sign of elongation (Length fast vs. length slow)
Wave plate
William Nicol, whose name is associated with the creation of the Nicol prism, is likely the first to prepare thin slices of mineral substances, and his methods were applied by Henry Thronton Maire Witham (1831) to the study of plant petrifactions. This method, of significant importance in petrology, was not at once made use of for the systematic investigation of rocks, and it was not until 1858 that Henry Clifton Sorby pointed out its value. Meanwhile, the optical study of sections of crystals had been advanced by Sir David Brewster and other physicists and mineralogists and it only remained to apply their methods to the minerals visible in rock sections.
A rock-section should be about one-thousandth of an inch (30 micrometres) in thickness, and is relatively easy to make. A thin splinter of the rock, about 1 centimetre may be taken; it should be as fresh as possible and free from obvious cracks. By grinding it on a plate of planed steel or cast iron with a little fine carborundum it is soon rendered flat on one side, and is then transferred to a sheet of plate glass and smoothed with the finest grained emery until all roughness and pits are removed, and the surface is a uniform plane. The rock chip is then washed, and placed on a copper or iron plate which is heated by a spirit or gas lamp.
Official source
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