Brushite

Brushite
240px Possible Brushite crystals (not confirmed) found in bat guano in Jamaica
General
Category	Phosphate mineral
Formula (repeating unit)	CaHPO4·2H2O
Strunz classification	08.CJ.50
Crystal symmetry	Monoclinic prismatic H-M symbol: (2/m) Space group: I2/a
Unit cell	a = 6.265 Å, b = 15.19 Å, c = 5.814 Å; β = 116.47°; Z=4
Identification
Color	Colorless to pale or ivory-yellow
Crystal habit	Prismatic to tabular acicular crystals; typically powdery or earthy
Crystal system	Monoclinic
Cleavage	Perfect on {010} and {001}
Tenacity	Brittle
Mohs scale hardness	2.5
Luster	Vitreous, pearly on cleavages
Diaphaneity	Transparent to translucent
Specific gravity	2.328
Optical properties	Biaxial (+)
Refractive index	nα = 1.539 - 1.540 nβ = 1.544 - 1.546 nγ = 1.551 - 1.552
Birefringence	δ = 0.012
2V angle	Measured: 59 to 87°
Solubility	Readily in HCl
Other characteristics	Piezoelectric
References

Brushite is a phosphate mineral with the chemical formula CaHPO₄·2H₂O. It forms colorless to pale yellow monoclinic prismatic crystals and as powdery or earthy masses. It is the phosphate analogue of the arsenate pharmacolite and the sulfate gypsum.

Discovery and occurrence

Brushite was first described in 1865 for an occurrence on Aves Island, Nueva Esparta, Venezuela, and named for the American mineralogist George Jarvis Brush (1831–1912). It is believed to be a precursor of apatite and is found in guano-rich caves, formed by the interaction of guano with calcite and clay at a low pH. It occurs in phosphorite deposits and forms encrustations on old bones. It may result from runoff of fields which have received heavy fertilizer applications. Associated minerals include tanarakite, ardealite, hydroxylapatite, variscite and gypsum.

Brushite is the original precipitating material in calcium phosphate kidney stones.