Whitlockite

Whitlockite
An example of the mineral Whitlockite on display at the Royal Ontario Museum.
General
Category	Phosphate minerals
Formula (repeating unit)	Ca9(Mg,Fe++)(PO4)6(PO3OH)
IMA symbol	Wht[1]
Strunz classification	8.AC.45
Crystal system	Trigonal
Crystal class	Ditrigonal pyramidal (3m) H-M symbol: (3m)
Space group	R3c
Unit cell	a = 10.33, c = 37.103(5) [Å]; Z = 3
Identification
Color	Colorless, gray-white, light pink, light yellow
Crystal habit	rhombohedral crystals, often tabular, massive, microcrystalline crusts and as "cave pearls"
Cleavage	None
Fracture	Brittle
Tenacity	Brittle
Mohs scale hardness	5
Luster	Vitreous to resinous
Streak	White
Diaphaneity	Transparent
Density	3.13
Optical properties	Uniaxial (−)
Refractive index	nω = 1.629 nε = 1.626
Birefringence	δ = 0.003
Other characteristics	Piezoelectric and pyroelectric
References	[2][3][4]

Whitlockite is a mineral, an unusual form of calcium phosphate. Its formula is Ca₉(MgFe)(PO₄)₆PO₃OH.^[2][3][4] It is a relatively rare mineral but is found in granitic pegmatites, phosphate rock deposits, guano caves and in chondrite meteorites.^[4] It was first described in 1941 and named for Herbert Percy Whitlock (1868–1948), American mineralogist and curator at the American Museum of Natural History in New York City.^[3]

With regards to periodontal dentistry, magnesium whitlockite comprises one component of many of the inorganic content of calculus. It is found primarily in subgingival calculus (as opposed to supragingival calculus). It is also found more in posterior as opposed to anterior regions of the oral cavity.