Whitlockite
>
Whitlockite is a mineral, an unusual form of calcium phosphate. Its formula is Ca₉(PO₄)₆PO₃OH. It is a relatively rare mineral but is found in granitic pegmatites, phosphate rock deposits, guano caves and in chondrite meteorites. Wikipedia
Crystal habit: rhombohedral crystals, often tabular, massive, microcrystalline crusts and as "cave pearls"
Crystal system: Trigonal
Density: 3.13
Formula (repeating unit): Ca9(Mg,Fe++)(PO4)6(PO3OH)
Fracture: Brittle
Luster: Vitreous to resinous