Metavivianite, Fe2+Fe3+2(PO4)2(OH)2·6H2O: new data and formula revision

Abstract The composition, structure, X-ray powder diffraction pattern, optical properties, density, infrared, Raman and Mössbauer spectra, and thermal properties of a homogeneous sample of metavivianite from the Boa Vista pegmatite, near Galiléia, Minas Gerais, Brazil are reported for the first time. Metavivianite is biaxial (+) with α = 1.600(3), β = 1.640(3), γ = 1.685(3) and 2V meas = 85(5)°. The measured and calculated densities are D meas = 2.56(2) and D calc = 2.579 g cm –3 . The chemical composition, based on electronmicroprobe analyses, Mössbauer spectroscopy (to determine the Fe 2+ :Fe 3+ ratio) and gas chromatography (to determine H 2 O) is MgO 0.70, MnO 0.92, FeO 17.98, Fe 2 O 3 26.60, P 2 O 5 28.62, H 2 O 26.5; total 101.32 wt.%. The empirical formula is (Fe 3+ 1.64 Fe 2+ 1.23 Mg 0.085 Mn 0.06 ) Σ3.015 (PO 4 ) 1.98 (OH) 1.72 ·6.36H 2 O. Metavivianite is triclinic, P , a = 7.989(1), b = 9.321(2), c = 4.629(1) Å, α = 97.34(1), β = 95.96(1), γ = 108.59(2)°, V = 320.18(11) Å 3 and Z = 1. The crystal structure was solved using a single-crystal techniques to an agreement index R = 6.0%. The dominant cations in the independent sites are Fe 2+ and Fe 3+ , with multiplicities of 1 and 2, respectively. The simplified crystal-chemical formula for metavivianite is F e2+ (Fe 3+ , Fe 2+ ) 2 (PO 4 ) 2 (OH,H 2 O) 2 ·6H 2 O; the endmember formula is Fe 2+ Fe 3+ 2 (PO 4 ) 2 (OH) 2 ·6H 2 O, which is dimorphous with ferrostrunzite.