Vauxite

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.

Unit cell

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: