Single crystals of pure enstatite (Mg2Si2O6) were synthesized under water-saturated conditions at 6 GPa and 1250 °C and variable silica activity. Run products were investigated using a novel technology: a FTIR spectrometer equipped with a focal plane array detector enabling IR-imaging with a spatial pixel resolution of 2.7 μm. IR spectra within the OH-absorption region show strong pleochroic behavior: absorption bands at 3687 and 3592 cm−1 show strongest absorptions for E||nβ, whereas absorption bands at 3362 and 3067 cm−1 show strongest absorptions for E||nγ. Furthermore, absorption bands are sensitive to the silica activity—the ratio of the integral absorbance of the absorption bands at 3687 and 3592 cm−1 to the absorption bands at 3362 and 3067 cm−1 increases with increasing Mg/Si-ratio of the bulk charge. The most probable interpretation is an assignment of the high-energy absorption bands to tetrahedral (T-site) defects caused by a lower availability of Si and the low-energy absorption bands to octahedral (M-site) defects caused by a lower availability of Mg. All crystals show an internal zonation pattern with an increasing T-site to M-site defect ratio from core to rim, which is interpreted to be caused by changing silica-activity and T during the experiments. The defect ratio and the zonation pattern are applied as a monitor of crystal growth conditions.

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