Vauxite
| Vauxite | |
|---|---|
Vauxite from the Siglio XX Mine, Llallagua, Bolivia. Specimen size 2 cm
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| General | |
| Category | Phosphate minerals |
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Formula (repeating unit) |
Fe2+Al2(PO4)2(OH)2.6H2O |
| Strunz classification | 8.DC.35 |
| Dana classification | 42.11.14.1 |
| Crystal system | Triclinic |
| Crystal class | Pinacoidal (1) (same H-M symbol) |
| Space group | P1 |
| Identification | |
| Formula mass | 441.86 g/mol |
| Color | Blue, becoming greenish on exposure |
| Crystal habit | Minute tabular crystals, radial aggregates and nodules |
| Twinning | On {010}, twin and composition plane. |
| Cleavage | Fractured |
| Mohs scale hardness | 3.5 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 2.39 to 2.40 |
| Optical properties | Biaxial (+) |
| Refractive index | Nx=1.551, Ny=1.555, Nz=1.562 |
| Birefringence | r>v |
| Pleochroism | (strong): X, Z colorless, Y blue |
| Other characteristics | Fluorescent. Not radioactive |
| References | |
Vauxite is a phosphate mineral with the chemical formula Fe2+Al2(PO4)2(OH)2·6(H2O). It belongs to the laueite – paravauxite group, paravauxite subgroup, although Mindat puts it as a member of the vantasselite Al4(PO4)3(OH)3·9H2O group. There is no similarity in structure between vauxite and paravauxite Fe2+Al2(PO4)2(OH)2·8H2O or metavauxite Fe3+Al2(PO4)2(OH)2·8H2O, even though they are closely similar chemically, and all minerals occur together as secondary minerals. Vauxite was named in 1922 for George Vaux Junior (1863–1927), an American attorney and mineral collector.
The space group is P1, which means that the only symmetry element for the crystal is a center of symmetry. The crystal is built up of identical units, called unit cells, which are stacked together so that, in the absence of imperfections, they completely fill the space occupied by the crystal. The unit cell is a rhomboid (each face is a parallelogram, and opposite pairs of faces are equal) with side lengths a, b and c. The angles between the sides are denoted by the Greek letters α, β and γ, where α is the angle between sides b and c, β between c and a, and γ between a and b. For vauxite, the reported values of these parameters differ slightly from reference to reference, as different researchers have studied different samples, but all agree that a = 9.1 Å, b = 11.6 Å, c = 6 Å, α = 98.3°, β = 92° and γ = 108°, to the accuracy stated. Detailed reported values of the lattice parameters are:
Within each unit cell there are two units of the formula Fe2+Al2(PO4)2(OH)2·6H2O.
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