* Apatite Chemical composition: Ca5(PO4)3(F,Cl,OH) * Class: phosphate * Crystal system: hexagonal; 6/m * Crystal habit: commonly as prismatic crystals terminated by first order pyramids often with basal planes. Also can be in massive granular to compact masses. * Twinning: twins are rare * Specific gravity: 3.15 - 3.20 * Index of refraction: 1.63 - 1.64 * Birefringence: weak to moderate (0.001-0.015) * Pleochroism: weak to moderate * Hardness: 5 * Color: usually in shades of yellow, green or brown, more rarely colorless, violet or blue. * Luster: vitreous to sub-resinous, occaisionally a chatoyant effect is visible. * Transparency: transparent to opaque * Cleavage:poor on {0001} * Fracture: uneven, brittle * Streak: white to yellow-gray
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