The crystal structure of amesite-2H1 from the Postmasburg Mn ores of South Africa has been refined to R = 4.7% using Siemens P4 rotating-anode MoKα X-ray data and SHELXL-93 software. Partial ordering of tetrahedral Si,Al and especially of octahedral Mg,Al has reduced the ideal P63cm symmetry to triclinic P1 (refined as C1). Tetrahedral rotation of 15° moves the basal oxygens toward the octahedral cations in the same layer but away from the OH H-donors of the next layers. The ordering pattern of AlIV and AlVI is unique in that the locus of Al-rich sites consists of 2 intersecting sets of zigzag lines parallel to X2 and X3, plus 2 Al-rich octahedra in layer 2 instead of the usual 1 site.
