Crystal structure and crystal chemistry of melanovanadite, a natural vanadium bronze

The crystal structure of melanovanadite from Minas Ragra, Peru, has been determined in space group P1. The triclinic unit cell (nonstandard) has a = 6.360(2), b = 18.090(9), c = 6.276(2) Å, α = 110.18(4)°, β = 101.62(3)°, γ = 82.86(4)°, V= 662.5 ų. A subcell with b′ = b/2 was found by crystal-structure analysis to contain CaV₄O_l0⋅5H₂O. The subcell has a layer structure in which the vanadate sheet consists of comer-shared tetrahedral VO₄ and double square-pyramidal V₂O₈ groups, similar to that previously found in synthetic CsV₂O₅. Refinement of the full structure (R = 0.056) showed that the Ca atom, which half-occupies a general position in the subcell, is 90% ordered at one of these sites in the whole unit cell. Bond length-bond strength estimates indicate that the tetrahedra contain V⁵⁺, and the square pyramids, V⁴⁺. The measured density is 2.55 g/cm³; the calculated density is 2.53 g/cm³. Heating experiments show that four H₂O are lost at 75°C, two more H₂O are lost at 95°C, and all water is lost between 200 and 250°C, at which point the crystals become amorphous to X-rays.