Brushite

Not to be confused with Brucite or Magnesium hydroxide.

Brushite is a phosphate mineral with the chemical formula CaHPO₄·2H₂O. Crystals of the pure compound belong to the monoclinic space group C2/c and are colorless.^[2][5] It is the phosphate analogue of the arsenate pharmacolite.

Brushite
Brushite (white) on montmorillonite
General
Category	Phosphate mineral
Formula	CaHPO4 · 2 H2O
IMA symbol	Bsh[1]
Strunz classification	8.CJ.50
Crystal system	Monoclinic
Space group	Ia (no. 9)
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
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	[2][3][4][5][6]

Discovery and occurrence

Brushite was first described in 1865 as an occurrence on Aves Island, Nueva Esparta, Venezuela, and named for the American mineralogist George Jarvis Brush (1831–1912).^[4] 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 the runoff of fields which have received heavy fertilizer applications.^[4] Associated minerals include tanarakite, ardealite, hydroxylapatite, variscite and gypsum.^[2]

Brushite is the original precipitating material in calcium phosphate kidney stones.^[7] It is also one of the minerals present in dental calculi.