Titanium substitution and OH-bearing defects in hydrothermally grown pyrope crystals
Tóm tắt
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.
Tài liệu tham khảo
Ackermann L, Cemič L, Langer K (1983) Hydrogarnet substitution in pyrope: a possible location for “water” in the mantle. Earth Planet Sci Let 62:208–214
Aines RD, Rossman GR (1984a) Water content of mantle garnets. Geology 12:720–723
Aines RD, Rossman GR (1984b) The hydrous component in garnets: pyralspite. Am Mineral 69:1116–1126
Begley AL, Sclar CB (1984) Experimental evidence for the existence of hydropyrope. EOS 65:1091
Bell DR, Rossman GR (1992) The distribution of hydroxyl in garnets from the subcontinental mantle of southern Africa. Contrib Mineral Petrol 111:161–178
Burns RG (1972) Mixed valencies and site occupancies of iron in silicate minerals from Mössbauer spectroscopy. Can Jour Spectrosc 17:51–59
Cemič L, Geiger CA, Hoyer H, Koch-Müller M, Langer K (1990) Piston-cylinder techniques: pressure and temperature calibration of a pyrophyllite-based assembly by means of DTA measurements, a salt-based assembly, and a cold sealing sample encapsulation method. N Jahrb Mineral Monatsh 2:49–64
Geiger CA, Langer K, Bell DR, Rossman GR (1989) Realbau of pyrope single crystals grown hydrothermally (abs.). International Conference Crystal structure, microstructure and properties of minerals and ceramic materials, Bochum University, FRG:55–56
Geiger CA, Langer K, Bell DR, Rossman GR, Winkler B (1991) The hydroxide component in synthetic pyrope. Am Mineral 76:49–59
Hammer VMF, Beran A (1991) Variations in the OH concentration of rutiles from different geological environments. Mineral and Petrol 45:1–9
Kühberger A, Fehr T, Huckenholz HG, Amthauer G (1989) Crystal chemistry of a natural schorlomite and Ti-andradites synthesized at different oxygen fugacities. Phys Chem Minerals 16:734–740
Novak GA, Gibbs GV (1971) The crystal chemistry of the silicate garnets. Am Mineral 56:791–825
O'Neill B, Bass JD, Rossman GR, Geiger CA, Langer K (1991) Elastic properties of pyrope. Phys Chem Minerals 17:617–621
Pouchou JL, Pichoir F (1984) A new method for quantitative X-ray microanalysis. I. Application to the analysis of homogeneous samples. Rech Aerosp 3:13–38
Reid AM, Brown RW, Dawson JB, Whitfield GG (1976) Garnet and pyroxene compositions in some diamondiferous eclogites. Contrib Mineral Petrol 58:203–220
Rossman GR, Aines RD (1991) The hydrous components in garnets: grossular-hydrogrossular. Am Mineral 76:1153–1164
Schwartz KB, Nolet DA, Burns RG (1980) Mössbauer spectroscopy and crystal chemistry of natural Fe-Ti garnets. Am Mineral 65:142–153
Shannon RD (1976) Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides. Acta Cryst A32:751–767
Sobolev NV (1974) Deep-seated inclusions and the problem of upper mantle composition. Novosibirsk Nauka (in russian) p 264