Abstract

A set of rock crystals with pleochroic inclusions were analyzed by optical microscopy, scanning electron microscopy, Raman spectroscopy and energy-dispersive spectrometry (EDS). The inclusions occur in fracture planes, have a discoid form with radial and concentric structure, diameters ranging from a few micrometers to several millimeters and thicknesses smaller than 50 μm. They have a pronounced pleochroism ranging from yellow to green when fresh and a brown color when oxidized. The Raman spectrum of the inclusion is characterized by a set of bands associated to the ν1 and ν3 PO4 tetrahedra stretching region (at 1186, 1137, 1061, 981 and 937 cm−1), overlapping bands associated to the ν2 and ν4 bending phosphate tetrahedra (at 638, 616, 576 and 463 cm−1), a suite of overlapping vibrations of XO6 octahedra (382, 333, 299 and 241 cm−1), a broad band between 3000 and 3500 cm−1 associated to the OH stretching vibration of molecular water and a sharp band at 3575 cm−1 corresponding to the stretching vibrations of OH groups. The spectrum and compositional data indicating iron, manganese and phosphorous as the main elements point to rockbridgeite, with the empirical formula (Fe2+, Mn2+)Fe3+4(PO4)3(OH)5 and an Fe2+:Mn2+ ratio close to 1.

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