Crystal structure of the high-pressure phase Mg (sub 4) (MgAl) Al (sub 4) [Si (sub 6) O (sub 21) /(OH) (sub 7) ]; an analogue of sursassite

The crystal structure of the synthetic high-pressure phase Mg (sub 4) (MgAl)Al (sub 4) [Si (sub 6) O (sub 21) /(OH) (sub 7) ], presently referred to as MgMgAl-pumpellyite in the literature, has been determined by the Rietveld method, and found to be isostructural with sursassite rather than pumpellyite. This phase is therefore a slightly modified Mg-analogue of sursassite Mn (super 2+) (sub 4) Al (sub 2) Al (sub 4) [Si (sub 6) O (sub 22) /(OH) (sub 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 (sub 1) /m) were derived: a = 8.5424(8) Aa, b = 5.7117(3) Aa, c = 9.6484(6) Aa, beta = 108.298(4) degrees , V = 447.0(1)Aa (super 3) . Besides domain boundaries the material was well crystallized and HRTEM investigations did not suggest any stacking disorder along [001] or sursassite-pumpellyite intergrowth.