REFINEMENT OF AN AMESITE-2H(1) POLYTYPE FROM POSTMASBURG, SOUTH-AFRICA

The crystal structure of amesite -2H(1) from the Postmasburg Mn ores o f South Africa has been refined to R = 4.7% using Siemens P4 rotating- anode MoK alpha X-ray data and SHELXL-93 software. Partial ordering of tetrahedral Si,Al and especially of octahedral Mg,Al has reduced the ideal P6(3)cm symmetry to triclinic PI (refined as CI). Tetrahedral ro tation of 15 degrees moves the basal oxygens toward the octahedral cat ions in the same layer but away from the OH H-donors of the next layer s. The ordering pattern of Al-IV and Al-VI is unique in that the locus of Al-rich sites consists of 2 intersecting sets of zigzag lines para llel to X-2 and X-3, plus 2 Al-rich octahedra in layer 2 instead of th e usual 1 site.