SUPERHYDROUS PHASE B: A STRUCTURAL AND SPECTROSCOPIC STUDY

Experimental studies have shown that dense hydrous magnesium silicates (DHMS) could be important hosts for H₂O in the earths mantle and their dehydration may be responsible for deep focus earthquakes. Superhydrous phase B is one of the most water-rich of the DHMS. Although it has been known for more than 10 years, its crystal structure, especially in respect to hydrogen, is still under debate. Therefore, we synthezised superhydrous phase B of different compositions and at different P and T: Sample 1, Mg_8.48Ni_1.81Si_3.01O₁₄(OH,D)₄, (22 GPa and 1400 °C); sample 2, Mg_9.68Si_3.10O₁₄(OH)₄, (22 GPa and 1200 °C) and sample 3, Mg_9.81Si_3.02O₁₄(OH)₄, (23 GPa, temperature range 1900 to 1600 °C). Polarized infrared spectroscopy, in-situ HP IR spectroscopy, Raman-spectroscopy and single crystal X-ray diffraction indicate that superhydrous phase B crystallize in at least three different polymorphs, which can be distinguished by the number of their OH bands in their IR spectra. Sample 1 exhibits one OH band at 3373 cm^-1 which shifts linearely with pressure to 3377 cm^-1 (16.4 GPa). This sample could be refined in space group Pnnm (final wR2=14%) with just one hydrogen position. Sample 2 exhibits 2 intense OH bands, at 3407 and 3347 cm^-1, and one very weak band at 3288 cm^-1. The band at 3407 shifts with pressure to 3423 (15.3 GPa), whereas the position of the second band is independent on P. The data indicate that it crystallizes in a subgroup of Pnnm, most probably Pnn2 with two different hydrogen positions. Microprobe analyses and X-ray diffraction indicate vacancies on the Mg sites and we preliminarely assign the weak OH band to vibrations of OH dipoles associated with Mg vacancies. The spectrum of Sample 3 is similar to that of sample 2 but with the weak band at 3288 cm^-1 being much stronger than in sample 2. Electron microprobe analyses also indicate a considerable amount of vacancies on the Mg sites of sample 3 which could be due to the higher temperature of formation. Kudoh et al. (1993) report a third polymorph of superhydrous phase B which crystallizes in space group P2₁mn. Detailed phase equilibria studies are needed to fully understand the phase relations of the three polymorphs. On a first glance, the data indicate that partioning of other components (here NiO) into superhydrous phase B favors crystallization in space group Pnnm