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

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 ν 1 and ν 3 PO 4 tetrahedra stretching region (at 1186, 1137, 1061, 981 and 937 cm −1 ), overlapping bands associated to the ν 2 and ν 4 bending phosphate tetrahedra (at 638, 616, 576 and 463 cm −1 ), a suite of overlapping vibrations of X O 6 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 (Fe 2+ , Mn 2+ )Fe 3+ 4 (PO 4 ) 3 (OH) 5 and an Fe 2+ :Mn 2+ ratio close to 1.