Titanium substitution and OH-bearing defects in hydrothermally grown pyrope crystals

✍ Scribed by V. M. Khomenko; K. Langer; A. Beran; M. Koch-Müller; T. Fehr

• Book ID: 104662123
• Publisher: Springer-Verlag
• Year: 1994
• Tongue: English
• Weight: 640 KB
• Volume: 20
• Category: Article
• ISSN: 0342-1791
• DOI: 10.1007/bf00203218

✦ Synopsis

A series of Ti-substituted pyrope crystals was synthesized in the system MgO-(Na20) -A1203 -TiO2 -SIO2-H20 at Pa20=Ptot between 25 and 30 kbars and 975 and 1000 ~ C, using graphite heated piston-cylinder devices. The crystals, ranging up to 500 gm in diameter, were studied by X-ray, electron-microprobe and FTIRmicroscope spectrometric techniques. The pyrope crystals were colourless when hem/rot 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 A13 + [61 ..]_ Si 4 + t41 + 40 2-= Ti 4 +[61 + [~[4l_t_ [(OH)sO] 5-, providing charge balance for octahedral Ti 4 +-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.