Perite
Perite is a mineral that has a general chemical formula of PbBiO2Cl.[3] 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 Langban, Sweden.[5] Perite is orthorhombic, which means crystallographically, it contains three axes of two of equal length (the bases a and b) and one of a little longer or shorter length. All three bases intersect at a 90° angles. It belongs to the 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 | |
|---|---|
.jpg) Perite, from Blue Bell Claim, San Bernardino County, California, United States of America | |
| General | |
| Category | Halide mineral |
| Formula (repeating unit) | PbBiO2Cl |
| 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 | ncalculated2.29 - 2.3 |
| Pleochroism | Trichroic |
| References | [1][2][3][4][5] |
Crystallography
Perite belongs to the Orthorhombic crystal system, and has the properties of 2/m 2/m 2/m crystal class. Therefore, perite has 3 mirror planes and 3 twofold rotation axes.