Transmitted Light
Relief high or very high. In PPL, usually colorless, may be gray, pinkish or brown (melanite), but never with pleochroism. Cubic (tetragonal!). Never shows cleavage, but usually has many fractures. Grains normally rounded, rarely euhedral. Inclusions are common. In XPL, almost always isotropic, but may be anisotropic and sectorized. Rarely altered. It is a common rock-forming mineral.
Relief very high. Rare mineral. In PPL may be colorless, pale pink or pale blue, green-gray (common), without pleochroism, twinning or cleavage. When colorless or pinkish, similar to garnet (which does not have triangular sections). Cubic, anhedral grains, cubes or octahedrons, may be zoned. In XPL, isotropic.
Relief high. Rare mineral. In PPL, colorless (if with Fe, yellowish to brown, without pleochroism). Cubic, anhedral grains, octahedrons, granular or massive. Cleavage {001} perfect and {111} good. In XPL, it is isotropic. Occurs in dolomitic marbles and easily alters to brucite.
Relief moderate. In PPL, colorless, rarely pale violet or pale green. Irregular, partial, zoning in violet colors is common. Cubic. Perfect (111) cleavage: or two cleavages intercept at 70º/110º or three at 60º/120º. Usually anhedral crystals, sometimes cubes. In XPL, isotropic, anomalous birefringence is possible. The image shows a horizontal fracture in granite filled with fluorite.
In allanite, metamictization (destruction of its structure due to radiation from the Rare Earth Elements they contain), progressively makes the relief lower and the mineral becomes isotropic. Usually brownish, with pleochroism if unaltered, may show some relict pleochroism when almost isotropic. It also generates a pattern of radial fractures around it due to swelling and may show an epidote corona (image on the right). Monoclinic.
Relief very high. Rare mineral. Orthorhombic, pseudo-cubic, pseudo-hexagonal, cubic or octahedral forms. Poor cleavage. In PPL, pale to deep violet-brown (almost black), yellow. In XPL, it never completely extinguishes: dark to light gray colors. Shows polysynthetic twins, such as leucite.
