The hydroxide component in synthetic pyrope

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 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 aI 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 (O₄H₄)⁴⁻ = SiO⁴⁻₄, 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.