Raman spectroscopy of various phosphate minerals and occurrence of tuite in the Elga IIE iron meteorite

The Raman spectra of phosphate minerals: apatite, merrillite, panethite, brianite, czochralskiite, maricite, new phosphate with composition of Na₂Fe(Mg,Ca)(PO₄)₂ and high-pressure phase tuite from the Elga IIE iron meteorite are characterized. Panethite, brianite, czochralskiite and new Na-Fe phosphate were described for the first time. The basic features of all studied phosphates include strong ν₁ symmetric stretching mode of PO₄ ³⁻ ions at 950–990 cm⁻¹, antisymmetric ν₃ vibrations in the 1000–1175 cm⁻¹ region and weak bands in the 400–500 cm⁻¹ region, which correspond to ν₂ bending mode and in the 550–640 cm⁻¹ region, related to ν₄ bending mode. Weak bands at less than 300 cm⁻¹ represent lattice modes. Most of the studied phosphates have three non-equivalent positions of phosphorus atoms in the crystal structures and can be derived from glaserite, K₃Na(SO₄)₂, arrangement. Two or three of non-equivalent [PO₄] tetrahedra typically correspond to two or three strong bands in the region of ν₁ vibrations. High-pressure mineral tuite has similar composition and was formed after merrillite. P-O vibrations in equivalent [PO₄] tetrahedra produce major Raman band of tuite at 972 cm⁻¹. The Elga parent-body experienced a shock pressure of at least 12–15 GPa according to minimum pressures of tuite formation in the static high-pressure experiments. This is the first finding of tuite in iron meteorites and the only second finding of high-pressure mineral in them.