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
History 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.
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. A microscopic glass slip is also warmed on this plate with a drop of viscous natural Canada balsam on its surface. The more volatile ingredients of the balsam are dispelled by the heat, and when that is accomplished the smooth, dry, warm rock is pressed firmly into contact with the glass plate so that the film of balsam intervening may be as thin as possible and free from air bubbles. The preparation is allowed to cool, and the rock chip is again ground down as before, first with carborundum and, when it becomes transparent, with fine emery until the desired thickness is obtained. It is then cleaned, again heated with an additional small amount of balsam, and covered with a cover glass. The labor of grinding the first surface may be avoided by cutting off a smooth slice with an iron disk armed with crushed diamond powder. A second application of the slitter after the first face is smoothed and cemented to the glass will, in expert hands, leave a section of rock so thin as to be transparent. In this way the preparation of a section may require only twenty minutes.
Microscope
The microscope employed is usually one which is provided with a rotating stage beneath which there is a polarizer, while above the objective or eyepiece an analyzer is mounted; alternatively the stage may be fixed, and the polarizing and analyzing prisms may be capable of simultaneous rotation by means of toothed wheels and a connecting rod. If ordinary light and not polarized light is desired, both prisms may be withdrawn from the axis of the instrument; if the polarizer only is inserted the light transmitted is plane polarized; with both prisms in position the slide is viewed in cross-polarized light, also known as "crossed nicols". A microscopic rock-section in ordinary light, if a suitable magnification (e.g. around 30x) be employed, is seen to consist of grains or crystals varying in color, size, and shape.
Characteristics of minerals
Color Some minerals are colorless and transparent (quartz, calcite, feldspar, muscovite, etc.), while others are yellow or brown (rutile, tourmaline, biotite), green (diopside, hornblende, chlorite), blue (glaucophane). Many minerals may present a variety of colors, in the same or different rocks, or even multiple colours in a single mineral specimen called colour zonation. For example, the mineral tourmaline may have concentric zones of colour ranging from brown, yellow, pink, blue, green, violet, or grey, to colorless. Every mineral has one or more, most common tints. Recently, the field of digital mineral study and identification has been developing based on the optical properties of minerals, including through the use of artificial intelligence technologies .
Habit & Cleavage
The shapes of the crystals determine in a general way the outlines of the sections of them presented on the slides. If the mineral has one or more good cleavages, they will be indicated by sets of similarly oriented planes called cleavage planes. The orientation of cleavage planes is determined by the crystal structure of a mineral and form preferentially through planes along which the weakest bonds lie, thus the orientation of cleavage planes can be used in optical mineralogy to identify minerals.
Refractive Index & Birefringence Information regarding the refractive index of a mineral can be observed by making comparisons with the surrounding materials. This could be other minerals or the medium in which a grain is mounted. The greater the difference in Optical relief the greater the difference in refractive index between the media. The material with a lower refractive index and thus lower relief will appear to sink into the slide or mount, while a material with higher refractive index will have higher relief and appear to pop out. The Becke line test can also be used to compare the refractive index of two media.
Pleochroism
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