Perite

Perite is a mineral that has a general chemical formula of PbBiO₂Cl. The name is given for Per Adolf Geijer, a Swedish economic geologist with the Geological Survey of Sweden, who discovered the mineral in 1960 outside of Långban, Sweden. Perite is orthorhombic, space group Cmcm {C2/m 2/c 21/m}. In terms of its optical properties, Perite is anisotropic which means the velocity of light varies depending on direction through the mineral (i.e. it is birefringent). Its calculated relief is 1.45–1.461, which is moderate. It is colorless in plane polarized light, and it is weakly pleochroic. Perite is found in areas near igneous extrusions in places like the Western United States, Southern Australia, and scattered around Europe.

Perite
Perite, from Blue Bell Claim, San Bernardino County, California, United States of America
General
Category	Halide mineral
Formula (repeating unit)	PbBiO2Cl
IMA symbol	Pe
Strunz classification	3.DC.30
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Bmmb
Unit cell	a = 5.62 Å, b = 5.57 Å, c = 12.42 Å; Z = 4
Identification
Formula mass	483.63 g/mol
Color	Yellow
Crystal habit	Platey crystals, pseudotetragonal
Cleavage	Fair on {001}
Mohs scale hardness	3
Luster	Adamantine
Streak	Yellow
Diaphaneity	Translucent
Specific gravity	8.16
Optical properties	Biaxial (+)
Refractive index	ncalculated = 2.29 – 2.3
Pleochroism	Trichroic
References

Crystallography

Perite belongs to the orthorhombic crystal system, and has the properties of 2/m 2/m 2/m crystal class. Therefore, perite has three mirror planes and three twofold rotation axes.