Abstract
A series of Ti-substituted pyrope crystals was synthesized in the system MgO-(Na2O)-Al2O3-TiO2-SiO2-H2O at PH20 = Ptot between 25 and 30 kbars and 975 and 1000° C, using graphite heated piston-cylinder devices. The crystals, ranging up to 500 μm in diameter, were studied by X-ray, electron-microprobe and FTIR-microscope spectrometric techniques. The pyrope crystals were colourless when hem/mt or mt/wu buffers were used during the synthesis, and pale blue with the wu/iron buffer and in unbuffered runs. Sodium was not found in the synthetic crystals, titanium was always near 0.06 Ti atoms pfu, independent on the Ti-excess used in the starting material. A substitution Al2+[6]+Si4+[4]+4O2-= Ti4+[6]+□[4]+[(OH)3O^5-, providing charge balance for octahedral Ti4+-substitution is found to be compatible with all properties (number, widths, position, integrated intensity) of the stretching vibrations of defect hydroxyls, which have energies 3684, 3568, 3525 cm-1.
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This work was made possible through a research fellowship, generously provided by the Alexander von Humboldt-Stiftung, Bonn, Bad Godesberg, to V.M.K.
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Khomenko, V.M., Langer, K., Beran, A. et al. Titanium substitution and OH-bearing defects in hydrothermally grown pyrope crystals. Phys Chem Minerals 20, 483–488 (1994). https://doi.org/10.1007/BF00203218
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DOI: https://doi.org/10.1007/BF00203218