Metavivianite, Fe2+Fe23+(PO4)2(OH)2•6H2O: new data and formula revision

The composition, structure, X-ray powder diffraction pattern, optical properties, density, infrared, Raman and Mossbauer spectra, and thermal properties of a homogeneous sample of metavivianite from the Boa Vista pegmatite, near Galileia, Minas Gerais, Brazil are reported for the first time. Metavivianite is biaxial (+) with alpha = 1.600(3), beta = 1.640(3), gamma = 1.685(3) and 2V(meas) = 85(5)degrees. The measured and calculated densities are D-meas = 2.56(2) and D-calc = 2.579 g cm(-3). The chemical composition, based on electron-microprobe analyses, Mossbauer spectroscopy (to determine the Fe2+:Fe3+ ratio) and gas chromatography (to determine H2O) is MgO 0.70, MnO 0.92, FeO 17.98, Fe2O3 26.60, P2O5 28.62, H2O 26.5; total 101.32 wt%. The empirical formula is (Fe1.643+Fe1.232+Mg0.085Mn0.06)(Sigma 3.015)(PO4)(1.98)(OH)(1.72)center dot 6.36H(2)O. Metavivianite is triclinic, P (1) over bar, a = 7.989(1), b = 9.321(2), c = 4.629(1) angstrom, alpha = 97.34(1), beta = 95.96(1), gamma = 108.59(2)degrees, V = 320.18(11) angstrom(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 Fe2+ and Fe3+, with multiplicities of 1 and 2, respectively. The simplified crystal-chemical formula for metavivianite is Fe2+(Fe3+,Fe2+)(2)(PO4)(2)(OH,H2O)(2)center dot 6H(2)O; the endmember formula is Fe2+Fe(PO4)(2)(OH)(2)center dot 6H(2)O, which is dimorphous with ferrostrunzite.