Refinement of an Amesite-2H1 Polytype from Postmasburg, South Africa

Hong Zheng and Sturges W. Bailey
Department of Geology and Geophysics, 1215 West Dayton Street, University of Wisconsin-Madison, Madison, Wisconsin 53706
Present address: 8109 West 96th Street, Bloomington, Minnesota 55438
Deceased November 30, 1994.

Abstract: 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.

Key Words: Amesite-2H1 • Distortions • Ordering Patterns • Postmasburg • Refinement • South Africa • Twinning

Clays and Clay Minerals; June 1997 v. 45; no. 3; p. 301-310; DOI: 10.1346/CCMN.1997.0450301
© 1997, The Clay Minerals Society
Clay Minerals Society (www.clays.org)