The hydroxide component in synthetic pyrope


Charles A. GEIGER, Klaus LANGER

Institut für Mineralogie und Kristallographie, Technische Universität Berlin, Ernst-Reuter-Platz 1, D-1000 Berlin 12, Germany


David R. BELL, George R. ROSSMAN

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A.


Bj˘rn WINKLER
Technische Universität Berlin, Ernst-Reuter-Platz 1, D- 1000 Berlin 12, Germany


ABSTRACT

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 (H2O, 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 reglon of 4000-3000 cm-1 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 ┼ 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 (O4H4)4- = SiO44-, i.e., by the substitution Si4- + 4O2- = {4}[] + 4OH-. The amount of OH- substitution into pyrope ranges from 0.02 to 0.07 wt% expressed as H2O. 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.


American Mineralogist, Volume 76, pages 49-59, 1991