Crystal structure of the high-pressure phase Mg-4(MgAl)Al-4[Si6O21/(OH)(7)]: an analogue of sursassite

The crystal structure of the synthetic high-pressure phase Mg-4(MgAl)Al-4[S i6O21/(OH)(7)], presently referred to as MgMgAl-pumpellyite in the literatu re, has been determined by the Rietveld method, and found to be isostructur al with sursassite rather than pumpellyite. This phase is therefore a sligh tly modified Mg-analogue of sursassite Mn42+Al2Al4[Si6O22/(OH)(6)], with Mg for Mn and one Al replaced by Mg+H. For material synthesized at 5 GPa and 600 degrees C, the following monoclinic cell-dimensions (Z = 1, space group = P2(1)/m) were derived: a = 8.5424(8) Angstrom, b = 5.7117(3) Angstrom, c = 9.6484(6) Angstrom,beta = 108.298(4)degrees, V = 447.0(1) Angstrom(3). B esides domain boundaries the material was well crystallized and HRTEM inves tigations did not suggest any stacking disorder along [001] or sursassite-p umpellyite intergrowth.