Optical mineralogy
Most commonly, rock and mineral samples are prepared as thin sections or grain mounts for study in the laboratory with a petrographic microscope.
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.
The rock chip is then washed, and placed on a copper or iron plate which is heated by a spirit or gas lamp.
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.
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.
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.
), while others are yellow or brown (rutile, tourmaline, biotite), green (diopside, hornblende, chlorite), blue (glaucophane).
For example, the mineral tourmaline may have concentric zones of colour ranging from brown, yellow, pink, blue, green, violet, or grey, to colorless.
[4] Some minerals decompose readily and become turbid and semi-transparent (e.g. feldspar); others remain always perfectly fresh and clear (e.g. quartz), while others yield characteristic secondary products (such as green chlorite after biotite).
Various methods of detailed observation may be applied, such as the measurement of the size of the elements of the rock by the help of micrometers, their relative proportions by means of a glass plate ruled in small squares, the angles between cleavages or faces seen in section by the use of the rotating graduated stage, and the estimation of the refractive index of the mineral by comparison with those of different mounting media.
[2] If the analyzer is inserted in such a position that it is crossed relatively to the polarizer, the field of view will be dark where there are no minerals or where the light passes through isotropic substances such as glass, liquids and cubic crystals.
These are often found to give more precise results than are obtained by observing only the position in which the mineral section is most completely dark between crossed nicols.
If all the sections are of the same thickness, as is nearly true of well-made slides, the minerals with strongest double refraction yield the highest polarization colors.
Sections perpendicular to an optic axis of a biaxial mineral under the same conditions show a dark bar which on rotation becomes curved to a hyperbolic shape.
If the section is perpendicular to a "bisectrix" (see Crystallography) a black cross is seen which on rotation opens out to form two hyperbolas, the apices of which are turned towards one another.
This is an element of great importance in the study of the history and classification of rocks, and is almost completely destroyed by grinding them to powder.
