Thomas, S.-M., S. D. Jacobsen, C. M. Holl, C. R. Bina, Z. Liu, Y. Ye, J. R. Smyth, and D. J. Frost, Structure and compressibility of iron- and aluminum-bearing phase D, Eos, Transactions of the American Geophysical Union, 89, Fall Meeting Supplement, T13C-1972, 2008.
We are studying the structure and spectroscopic properties of Al- and Fe-bearing phase D, a dense hydrous magnesium silicate and potential carrier of subducted water into the deep mantle. Gem-quality single-crystals of Phase D measuring up to 0.5 mm in size were synthesized at 25 GPa and 1400 °C in a multianvil apparatus with approximate composition, MgFe0.15Al0.3Si1.5H2.5O6. Mössbauer spectroscopy indicates that all the iron is ferric, Fe3+. Unpolarized single-crystal Raman spectroscopy at ambient conditions reveals in the low-frequency range lattice modes at 202, 377, 491, 588, 682, 800, 1114 and 1277 cm-1. In the high-frequency range broad bands at 2204, 2487, 2863 and 3482 cm-1 can be observed. High-pressure infrared spectroscopy shows little variation in the position of the main OH band at 2863 cm-1 up to the maximum pressure of 43.5 GPa, with dν/dP = -1.3 cm-1/GPa, which is not indicative of hydrogen bond symmetrization at high pressure. Thermal expansivity of the trigonal unit cell was measured between 140 and 300 K, giving linear coefficients of thermal expansion for the a-axis, c-axis, and volume of 4.8(6) x 10-6, 7.6(7) x 10-6, and 17(2) x 10-6 K-1, respectively. Structure refinements were carried out by single-crystal X-ray diffraction at 273 and 100 K. A hydrogen position located between the SiO6 octahedral layers was refined at (0.54, 0, 0.09) with an O-H distance of 0.85 Å, similar to pure-Mg phase D . Upon cooling to 100 K, the d(O. . .O) hydrogen bond distance decreases minimally from 2.646(1) Å to 2.641(1) Å, and the O-H distance is reduced to 0.81 Å at 100 K, further evidence that hydrogen bond symmetrization does not occur in phase D on contraction of the structure. In addition, we have identified a second hydrogen position from difference Fourier synthesis, located along the octahedral edge of the silicon site, suggesting a substitution of (Al,Fe)3+ + H+ = Si4+ on that site. Finally, the isothermal equation of state was studied using single-crystal synchrotron X-ray diffraction to 26 GPa in a diamond-anvil cell loaded in a helium pressure medium. We obtain a bulk modulus of about 164 GPa when the pressure derivative dK/dP = 4.4 is free to refine (where this deviation of dK/dP from 4.0 is statistically significant at the 98% confidence level). Here, we will discuss potential H-sites, substitution mechanisms as well as implications of the stability and properties of Al- and Fe3+-bearing phase D within cold subduction zones in the deep mantle.
References:  Yang et al. (1997), Am. Min., 82, 651-654.Copyright © 2008 American Geophysical Union