Vivianite group
| Vivianite | |
|---|---|
Vivianite tabular crystal, transparent, with a deep green color. Crystal size: 82 mm x 38 mm x 11 mm. From Huanuni mine, Dalence Province, Oruro Department, Bolivia
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| General | |
| Category |
Phosphate mineral Vivianite group |
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Formula (repeating unit) |
Fe2+ Fe2+ 2(PO 4) 2·8H 2O |
| Strunz classification | 8.CE.40 (10 ed) 7/C.13-40 (8 ed) |
| Dana classification | 40.3.6.1 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (same H-M symbol) |
| Space group | B2/m |
| Unit cell | a = 10.086 Å, b = 13.441 Å c = 4.703 Å; β = 104.27°; Z = 2 |
| Identification | |
| Formula mass | 501.61 g/mol |
| Color | Colorless, very pale green, becoming dark blue, dark greenish blue, indigo-blue, then black with oxidation |
| Crystal habit | Flattened, elongated prismatic crystals, may be rounded or corroded; as stellate groups, incrustations, concretionary, earthy or powdery |
| Twinning | Translation gliding |
| Cleavage | Perfect on {010} |
| Fracture | Fibrous |
| Tenacity | Flexible, sectile |
| Mohs scale hardness | 1.5-2 |
| Luster | Vitreous, pearly on the cleavage, dull when earthy |
| Streak | White, altering to dark blue, brown |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.68 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.579 - 1.616, nβ = 1.602 - 1.656, nγ = 1.629 - 1.675 |
| Birefringence | δ = 0.050 - 0.059 |
| Pleochroism | Visible; X = blue, deep blue, Indigo-blue; Y = pale yellowish green, pale bluish green, yellow-green; Z = pale yellowish green, olive-yellow |
| 2V angle | Measured: 63° to 83.5°, Calculated: 78° to 88° |
| Dispersion | r < v, weak |
| Ultraviolet fluorescence | Not fluorescent |
| Melting point | 1,114 °C (2,037 °F) |
| Solubility | Easily soluble in acids |
| Alters to | Metavivianite |
| References | |
Vivianite (Fe2+
Fe2+
2(PO
4)
2·8H
2O) is a hydrated iron phosphate mineral found in a number of geological environments. Small amounts of manganese Mn2+, magnesium Mg and calcium Ca may substitute for iron Fe2+ in the structure. Pure vivianite is colorless, but the mineral oxidizes very easily, changing the color, and it is usually found as deep blue to deep bluish green prismatic to flattened crystals.
Vivianite crystals are often found inside fossil shells, such as those of bivalves and gastropods, or attached to fossil bone.
It was named by Abraham Gottlob Werner in 1817, the year of his death, after John Henry Vivian (1785–1855), a Welsh-Cornish politician, mine owner and mineralogist living in Truro, Cornwall, England. John Vivian discovered the mineral at Wheal Kind, in St Agnes, Cornwall.
Vivianite group minerals have the general formula A3(X04)2·8H20, where A is a divalent transition metal cation and X is either phosphorus or arsenic, and they are monoclinic.
Group members are:
...
Wikipedia