hydroxyapatite hydroxyapatite hydroxyapatite, also called hydroxylapatite (ha), is a naturally occurring mineral form of calcium apatite with the formula ca5(po4)3(oh), but it is usually written ca10(po4)6(oh)2 to denote that the crystal unit cell comprises two entities. hydroxyapatite is the hydroxyl endmember of the complex apatite group. the oh− ion can be replaced by fluoride, chloride or carbonate, producing fluorapatite or chlorapatite. it crystallizes in the hexagonal crystal system. pure hydroxyapatite powder is white. naturally occurring apatites can, however, also have brown, yellow, or green colorations, comparable to the discolorations of dental fluorosis. hydroxyapatitehydroxyapatite crystals on matrixgeneralcategoryphosphate mineral apatite groupformula(repeating unit)ca5(po4)3ohima symbolhapstrunz classification8.bn.05crystal systemhexagonalcrystal classdipyramidal (6/m) h-m symbol (6/m)space groupp63/munit cella = 9.41 å, c = 6.88 å; z = 2identificationformula mass502.31 g/molcolorcolorless, white, gray, yellow, yellowish greencrystal habitas tabular crystals and as stalagmites, nodules, in crystalline to massive crustscleavagepoor on and fractureconchoidaltenacitybrittlemohs scale hardness5lustervitreous to subresinous, earthystreakwhitediaphaneitytransparent to translucentspecific gravity3.14–3.21 (measured), 3.16 (calculated)optical propertiesuniaxial (-)refractive indexnω = 1.651 nε = 1.644birefringenceδ = 0.007