Brockite

Brockite
Microphoto of brockite on thorite (black). Field view: 2 x 2 mm.
General
Category	Phosphate mineral
Formula (repeating unit)	(Ca,Th,Ce)PO4·H2O
IMA symbol	Bck
Strunz classification	8.CJ.45
Crystal system	Hexagonal
Crystal class	Trapezohedral (622) H-M symbol: (622)
Space group	P6222 or P6422
Unit cell	a = 6.98 Å, c = 6.40 Å; Z = 3
Identification
Color	Reddish brown, yellow (red brown due to inclusions of hematite)
Crystal habit	Rarely as Stubby hexagonal prisms rare; common as granular massive aggregates, cryptocrystalline
Cleavage	None observed
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	3 - 4
Luster	Greasy to vitreous
Diaphaneity	Translucent to opaque
Specific gravity	3.9 (measured)
Optical properties	Uniaxial (+)
Refractive index	nω = 1.680 nε = 1.695
Birefringence	δ = 0.015
Other characteristics	Radioactive
References

Brockite is a rare earth phosphate mineral with formula: (Ca,Th,Ce)PO₄·H₂O. It crystallizes in the hexagonal system in the chiral space group 180 or its enantiomorph 181. It is typically granular to massive with only rare occurrence of stubby crystals. It is radioactive due to the thorium content.

Discovery and occurrence

Brockite was first described in 1962 for an occurrence in the Bassick Mine area, Querida, Wet Mountains, Custer County, Colorado, US. It was named for Maurice R. Brock, of the U.S. Geological Survey.

Brockite occurs in granite and granite pegmatite as an accessory mineral. Associated minerals include monazite, bastnasite, xenotime, thorite, zircon, apatite, rutile and hematite.