Transmitted Light
In uniaxial minerals like tourmaline, the sections more or less perpendicular to the main crystallographic axis corresponds to the basal section and will not show any pleochroism. In biaxial minerals, there will be two isotropic sections, each perpendicular to one of the optical axes, also without pleochroism. In one of the images, basal section of biotite (brown), without pleochroism. In the other image, basal section of tourmaline (bluish), also without pleochroism.
Very common, occurs in many types of rock. Brown color, may have gray or yellow tones, may have weak pleochroism. Its color, being so strong, often hardly changes between PPL and XPL. Granular, only sometimes with rhombic basal sections as in the images below. Monoclinic. High relief. Fractured, 1 good cleavage, often absent. Possible zoning and twins. B(+)(2V=17-56º).
Soft pink color without pleochroism. Only the thulite variety, of Mn, shows moderate pink to yellow pleochroism. Orthorhombic. High relief, rhombic or hexagonal basal sections. One perfect cleavage. In CPL, anomalous colors (blue-gray, brown) or of lower 1st order. Parallel extinction, ES(-), B(+) (2V 0-69º)
Colorless, but may show pale colors in green, brown or yellow. High relief, bad cleavage, granular or in short prisms. Tetragonal. In XPL, anomalous colors (blue, brown, purple, etc.). Extinction tends to parallel, ES(-),zonation common. U(-), can be anomalous B(-) with 2V from 17-33º.
Varied and well-defined colors: yellow, yellow-brown, brown, dark blue, pale green, black. Weak pleochroism possible, stronger if colors are more intense. Very high relief, parallel extinction, ES(-), U(-). Common accessory in magmatic and metamorphic rocks. Images in PPL.
Colors from yellowish brown to dark brown. Very weak pleochroism in yellowish, reddish, orange to brown. Orthorhombic. Very high relief, no cleavage. In XPL, strong colors up to 3rd order. Parallel extinction. B(+), can simulate being uniaxial. Accessory mineral (schists/gneisses).
Weakly pleochroic in red-brown to yellow-brown. Tetragonal. Very high relief, granular or acicular crystals. The color is almost the same in PPL and in XPL (like titanite). Parallel extinction, ES(+), twins possible. No zoning, no alteration, dark halos around it possible. U(+). Images in PPL.
Strong green colors, almost the same in PPL and in XPL. They form rounded aggregates of crystals so small that pleochroism is difficult to see. Celadonite (at left below) occurs only in cavities and fractures of volcanic rocks. Glauconite (at right below) occurs only in sediments and marine sedimentary rocks. Images in PPL.
Most commonly gray-green to dark green, may be colorless, pale to pink or blue. No pleochroism. Relief very high. No twins or cleavage. Cubic. Anhedral grains, cubes or octahedrons, may show zonation. In XPL, isotropic. When colorless or pink, it is similar to garnet (which does not have triangular sections). Rare mineral. Images in PPL.
Pale to deep violet-brown (almost black), yellow. Relief very high, bad cleavage. Orthorhombic, pseudo-cubic, pseudo-hexagonal. Cubic or octahedral forms. In XPL never completely extinguishes: dark to light gray colors. It has polysynthetic twins, similar to those of leucite. Very rare mineral.
Well-defined pink color, may suggest pleochroism, but does not. Cubic. Mineral usually in rounded grains. High relief, very fractured and without cleavage. It can have many inclusions. ISOTROPIC; it has no interference colors, extinction, etc.
Strong brown color, may suggest pleochroism, but does not. Cubic. Mineral generally in polygonal grains of high relief, without cleavage. May have inclusions. ISOTROPIC, it has no interference colors, extinction, ES, etc. Exclusive to alkaline volcanic rocks!
Yellow to dark brown color with slight pleochroism. High relief, usually massive, usually in submicroscopic crystals. The yellow or brown color remains the same in CPL because it is very strong. Sulphate of secondary formation. Image in PPL; the color in XPL is practically the same.
Intense yellow or green color. Clay mineral of secondary origin (alteration), often in volcanic rocks. Submicroscopic sizes, conclusive identification must use other techniques. In XPL shows strong colors, masked by the yellow color of the mineral. Images in PPL.
