Rockbridgeite inclusion in rock crystal from Galileia region, Minas Gerais, Brazil.

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 mm. 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 n1 and n3 PO4 tetrahedra stretching region (at 1186, 1137, 1061, 981 and 937 cm 1), overlapping bands associated to the n2 and n4 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 (Fe2þ, Mn2þ)Fe3þ 4(PO4)3(OH)5 and an Fe2þ:Mn2þ ratio close to 1.