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

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 2Vmeas= 85(5)°. The measured and calculated densities are Dmeas= 2.56(2) and Dcalc= 2.579 g cm–3. The chemical composition, based on electronmicroprobe analyses, Mössbauer spectroscopy (to determine the Fe2+:Fe3+ratio) and gas chromatography (to determine H2O) is MgO 0.70, MnO 0.92, FeO 17.98, Fe2O326.60, P2O528.62, H2O 26.5; total 101.32 wt.%. The empirical formula is (Fe3+1.64Fe2+1.23Mg0.085Mn0.06)Σ3.015(PO4)1.98(OH)1.72·6.36H2O. Metavivianite is triclinic, P1, 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) Å3and 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)2·6H2O; the endmember formula is Fe2+Fe3+2(PO4)2(OH)2·6H2O, which is dimorphous with ferrostrunzite. [ABSTRACT FROM AUTHOR]