Zoned phosphosiderite-metavariscite crystals from Eduardo Mine, Conselheiro Pena, Minas Gerais, Brazil

Abstract

Zoned phosphosiderite-metavariscite crystals are recorded for the first time, in a phosphate-rich granitic pegmatite, at the Eduardo mine, Conselheiro Pena, Minas Gerais, Brazil. Metavariscite is described for the first time in Brazil. Phosphosiderite and metavariscite occur as small friable, globular agglomerates, up to 2 mm, with purple to lilac color, in miarolitic cavities up to 1 cm in the pegmatite. Phosphosiderite, ideally Fe3+(PO4).2H2O, and metavariscite, ideally Al(PO4).2H2O, are isostructural. Empirical formulae from EDS analysis (H2O by difference) are, respectively, (Fe3+0.65Al0.24)Ʃ0.89[PO3.67(OH)0.33].0.96H2O and (Al0.61Fe3+ 0.23)Ʃ0.84[PO3.52(OH)0.48].0.97H2O. The differences between ideal and empirical formulae are attributed to the instability of the crystals under the electron beams. Only the peaks for phosphosiderite were observed in the X-ray powder diffraction pattern due to the small proportion of metavariscite in the mixture. Unit cell parameters calculated for phosphosiderite are: a 5.368(7), b 9.778(10), c 8.710(9) Å, β 90.9(1)º, V 457.1(9) ų.