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The hydroxide component in synthetic pyrope

Geiger, Charles A. and Langer, Klaus and Bell, David R. and Rossman, George R. and Winkler, Björn (1991) The hydroxide component in synthetic pyrope. American Mineralogist, 76 (1-2). pp. 49-59. ISSN 0003-004X.

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A series of pyrope single crystals up to 2 mm in size was synthesized over a range of hydrothermal pressures of 20.0 to 50.0 kbar and temperatures of 800 to 1200 °C using different starting materials (oxides, glass, gel) and fluid fluxes (H_2O, NaOH, HCl). The crystals were characterized by optical, SEM, microprobe, and X-ray techniques. Single crystal Fourier-transform infrared (FTIR) spectroscopy was used to measure the incorporated structural OH^-. Spectra measured in the region of 4000-3000 cm^(-1) wavenumbers were different for all samples grown from oxides or glass vs. those grown from the gel at temperatures less than 1000 °C. In spectra obtained at room temperature the former are characterized by a single OH^- stretching vibration at 3629 cm^(-1), full widths at half-height (FWHH) = 60 cm^(-1), which is present regardless of the synthesis conditions (P, T or fluid flux). At 78 K, the single band splits into two narrow bands of FWHH of 11 cm^(-1) each. The unit-cell dimension of pyrope increases up to 0.004 Å with the incorporation of OH^-. The best interpretation of these data is that OH^- defects are introduced into the pyrope structure as a hydrogarnet component where (O_4H_4)^(4-) = SiO^(4-)_4, i.e., by the substitution Si^(4+) + 4O^2 - = ^[4]□ + 40H^-. The amount of OH^- substitution into pyrope ranges from 0.02 to 0.07 wt% expressed as H_2O. The infrared (IR) spectra of pyropes grown from a gel starting material, at temperatures less than 1000 °C, display four band spectra, which indicate that OH^- substitution is not governed solely by the hydrogarnet substitution. Natural pyrope-rich garnets generally have lower OH^- concentrations and more complicated IR spectra than the synthetic pyrope crystals grown from oxides. This is assumed to be caused by crystal chemistry differences and probably different mechanisms of OH^- incorporation.

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Rossman, George R.0000-0002-4571-6884
Additional Information:© 1991 Mineralogical Society of America. Manuscript Received October 9, 1989. Manuscript Accepted November 7, 1990. We would like to thank H. Reufffor skillful preparation of the sample mounts. D. Ackermand at Christian Albrechts University, Kiel and F. Galbert at Zelmi of the Technische Universitlit Berlin helped in the microprobing. H. Newesely generously assisted in making the SEM photos at the Free University in Berlin. Some of the high pressure syntheses were done at the Ruhr University through the courtesy of W. Schreyer and H.J. Massonne. We also thank P.J. Wyllie for access to the high pressure facility at Caltech. C.R. Ross II made some of the initial X-ray refinements. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft, Bonn-Bad Godesberg under grant La 324/25, and the National Science Foundation (USA) grant EAR-8618200. Gerlind Siebeker carefully typed the final version of the manuscript.
Group:UNSPECIFIED, Division of Geological and Planetary Sciences
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Deutsche ForschungsgemeinschaftUNSPECIFIED
Bonn-Bad GodesbergLa 324/25
Issue or Number:1-2
Record Number:CaltechAUTHORS:20130514-102848064
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Official Citation: Charles A. Geiger, Klaus Langer, David R. Bell, George R. Rossman, and Bjoern Winkler The hydroxide component in synthetic pyrope American Mineralogist, February 1991, v. 76, p. 49-59
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:38485
Deposited By: Ruth Sustaita
Deposited On:14 May 2013 18:28
Last Modified:03 Oct 2019 04:57

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