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In previous experiments with synthetic pure MgSiO₃ enstatite, we showed that the incorporation of water is not necessarily coupled to chemical defects. However, water solubility increases in the presence of aluminium. In a new series of experiments, we measured water solubility in natural, pale-brown, gem-quality enstatite crystals from Tanzania. These crystals contained 1.5 wt% Al₂O₃ and 2 wt% FeO, among other minor impurities. The water content of the starting material was 214 ppm by weight, calculated from polarized IR-spectra. The crystals were annealed in the presence of a free hydrous fluid phase at 1100°C and 0.5 to 100 kbar. For the experiments above 10 kbars (piston-cylinder or multianvil press) we used a Ni-NiO-buffer to fix the oxygen fugacity. The runs in the low pressure range (0.5 - 2.0 kbar, TZM-bombs) were unbuffered with respect to oxygen fugacity. To determine water contents after the annealing experiments, polarized FTIR spectra were measured on oriented crystals. The water distribution inside the crystals was found to be homogeneous for run durations of several days.

Polarized infrared spectra of annealed natural enstatite are shown in Fig. 3.5-7. The spectra show additional bands if compared to pure MgSiO₃. However, there appears to be no major difference in the shape of the spectra between buffered and unbuffered experiments. The absolute water solubility in the natural, aluminous enstatite appears to be somewhat higher than for pure MgSiO₃ (Fig. 3.5-8). According to our previous work, this difference can be attributed to the presence of aluminium. The iron content of the crystals and other trace impurities do not appear to significantly affect water solubility.
 

Fig. 3.5-7: Polarized FTIR spectra of natural, aluminous enstatite water-saturated at 15 kbar, 1100°C. Run duration was 4 days, total water content is 232 ppm.

Fig. 3.5-8: Water solubility in natural, aluminous enstatite and in pure synthetic enstatite at 1100°C.