Beraunite
| Beraunite | |
|---|---|
Crystalline beraunite on green kidwellite, Coon Creek Mine, Polk County, Arkansas, US. Size: 3.4 × 3.3 × 2.8 cm | |
| General | |
| Category | Phosphate minerals |
| Formula | Fe2+ Fe3+5(OH)5(PO4)4·4H2O |
| IMA symbol | Bru |
| Strunz classification | 8.DC.27 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | C2/c |
| Unit cell | a = 20.953(8) Å, b = 5.171(1) Å, c = 19.266(4) Å; β = 93.34°; Z = 4 |
| Identification | |
| Color | Dull greenish to greenish brown when fresh, may be color banded; reddish brown to hyacinth-red, blood-red on exposure |
| Crystal habit | Tabular crystals common, may be in coarse radially fibrous aggregates, globular or discoidal, and in crusts |
| Twinning | On {100}, may be interpenetrant |
| Cleavage | On {100}, good |
| Mohs scale hardness | 3–4 |
| Luster | Vitreous, pearly on cleavages, resinous on fractures |
| Streak | Olive-drab when fresh; yellow, brownish yellow on exposure |
| Diaphaneity | Translucent |
| Specific gravity | 2.8–3.08 (measured); 2.894 (calculated) |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.775 nβ = 1.786 nγ = 1.815 |
| Birefringence | δ = 0.040 |
| Pleochroism | X = pale flesh-pink, yellow, blue-green; Y = pale flesh-pink, yellow, pale olive-green; Z = carnelian-red, reddish brown, olive-green. |
| 2V angle | Measured: 30° to 60°, Calculated: 66° |
| References | |
Beraunite is an iron phosphate mineral. It was first described by August Breithaupt for an occurrence in Beraun currently in the Czech Republic. Beraunite occurs as a secondary mineral in iron ore deposits, and as an alteration product of primary phosphate minerals in granite pegmatites.
Beraunite crystallizes in the monoclinic crystal system with point group 2/m. Beraunite's formula is Fe2+ Fe3+5(OH)5(PO4)4·4H2O. Aluminium and zinc may substitute in the structure.