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 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.