The crystal structure of the synthetic high-pressure phase Mg4(MgAl)Al4[Si6O21/(OH)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 Mn2+4Al2Al4[Si6O22/(OH)6], with Mg for Mn and one Al replaced by Mg+H. For material synthesized at 5 GPa and 600°C, the following monoclinic cell-dimensions (Z = 1, space group = P21/m) were derived: a = 8.5424(8) Å, b = 5.7117(3) Å, c = 9.6484(6) Å, β = 108.298(4)°, V = 447.0(1) Å3. Besides domain boundaries the material was well crystallized and HRTEM investigations did not suggest any stacking disorder along [001] or sursassite-pumpellyite intergrowth.

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