Metavivianite, Fe 2+ Fe 3+ 2 (PO 4 ) 2 (OH) 2 ·6H 2 O: new data and formula revision N. V. CHUKANOV 1, *, R. SCHOLZ 2 , S. M. AKSENOV 3 , R. K. RASTSVETAEVA 3 , I. V. PEKOV 4 , D. I. BELAKOVSKIY 5 , K. KRAMBROCK 6 , R. M. PANIAGO 6 , A. RIGHI 6 , R. F. MARTINS 6 , F. M. BELOTTI 7 AND V. BERMANEC 8 1 Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka, Moscow region 142432, Russia 2 Federal University of Ouro Preto (UFOP), Mining School, Geology Department, Ouro Preto, Minas Gerais, Brazil 3 Institute of Crystallography, Russian Academy of Sciences, Leninsky Prospekt 59, Moscow 117333, Russia 4 Faculty of Geology, Moscow State University, Vorobievy Gory, Moscow 119991 Russia 5 Fersman Mineralogical Museum of the Russian Academy of Sciences, Leninsky Prospekt 18-2, Moscow 119071, Russia 6 Federal University of Minas Gerais (UFMG), ICEx, Physics Department, Belo Horizonte, Minas Gerais, Brazil 7 Federal University of Itajuba ´ (UNIFEI), Campus Itabira, Itabira, Minas Gerais, Brazil 8 Geological Department, Faculty of Sciences and Mathematics, University of Zagreb, Zagreb, Croatia [Received 4 March 2012; Accepted 22 April 2012; Associate Editor: Stuart Mills] ABSTRACT The composition, structure, X-ray powder diffraction pattern, optical properties, density, infrared, Raman and Mo ¨ssbauer spectra, and thermal properties of a homogeneous sample of metavivianite from the Boa Vista pegmatite, near Galile ´ia, Minas Gerais, Brazil are reported for the first time. Metavivianite is biaxial (+) with a = 1.600(3), b = 1.640(3), g = 1.685(3) and 2V meas = 85(5)º. The measured and calculated densities are D meas = 2.56(2) and D calc = 2.579 g cm À3 . The chemical composition, based on electron- microprobe analyses, Mo ¨ssbauer 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 ) S3.015 (PO 4 ) 1.98 (OH) 1.72 ·6.36H 2 O. Metavivianite is triclinic, P1 ¯ , a = 7.989(1), b = 9.321(2), c = 4.629(1) A ˚ , a = 97.34(1), b = 95.96(1), g = 108.59(2)º, V = 320.18(11) A ˚ 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 Fe 2+ (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. KEYWORDS: metavivianite, formula revision, crystal chemistry, Minas Gerais, Brazil. Introduction METAVIVIANITE has a complex history. It was discovered at the Big Chief mine, near Glendale, South Dakota, USA and was originally thought to be dimorphous with vivianite, Fe 3 (PO 4 ) 2 ·8H 2 O, and isostructural with symplesite, Fe 3 (AsO 4 ) 2 ·8H 2 O (Ritz et al., 1974). The original description was based on a heterogeneous sample and is incomplete. The density was not measured due to the presence of impurities and the hardness was not determined. Although most of the crystals in the sample were described as either opaque or translucent, the optical properties of rare pale grains were reported, resulting in incorrect values for the refractive indices. The original infrared (IR) data is now known to include contributions * E-mail: chukanov@icp.ac.ru DOI: 10.1180/minmag.2012.076.3.20 Mineralogical Magazine, June 2012, Vol. 76(3), pp. 725–741 # 2012 The Mineralogical Society