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Hydroxyapatite
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Hydroxyapatite is a naturally occurring mineral form of calcium apatite with the formula Ca₅(PO₄)₃, often written Ca₁₀(PO₄)₆(OH)₂ to denote that the crystal unit cell comprises two entities. It is the hydroxyl endmember of the complex apatite... Wikipedia
Birefringence: δ = 0.007
Color: Colorless, white, gray, yellow, yellowish green
Crystal class: Dipyramidal (6/m); H-M symbol (6/m)
Crystal habit: As tabular crystals and as stalagmites, nodules, in crystalline to massive crusts
Crystal system: Hexagonal
Formula (repeating unit): Ca5(PO4)3OH
Fracture: Conchoidal