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.

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