The hydroxide component in synthetic pyrope

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 (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.