The Crystal Structure of Bannisterite

Peter J. Heaney1, Jeffrey E. Post and Howard T. Evans Jr.
Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560
U.S. Geological Survey, Reston, Virginia 22092
1 Present address: Department of Geology and Geophysical Sciences, Princeton University, Princeton, New Jersey 08544.

Abstract: The crystal structure of bannisterite, a modulated, mica-like mineral species, of general composition Ca0.5(K,Na)0.5(Mn,Fe,Mg,Zn)10(Si,Al)16O38(OH)8 · nH2O, has been solved and refined for specimens from Franklin Furnace, New Jersey (FF), and Broken Hill, Australia (BH). The crystals are mono-clinic in space group A2/a, with (for FF) a = 22.265(1) Å, b = 16.368(1) Å, c = 24.668(2) Å, β = 94.285(5)°; and (for BH) a = 22.286(1) Å, b = 16.386(1) Å, c = 24.575(2) Å, β = 94.355(7)°; Z = 8. Refinement with anisotropic thermal factors reached Rw = 0.034 (FF) and 0.039 (BH). Like stilpnomelane and ganophyllite, bannisterite has a modified 2:1 trioctahedral layer structure in which some of the tetrahedra are inverted towards the interlayer region and linked to inverted tetrahedra in the opposite layer. The octahedral sheet is strongly corrugated along b. The tetrahedral sheet consists of 5-, 6-, and 7-fold rings, and bond distance calculations indicate that Al is concentrated into two of the four inverted tetrahedra. The interlayer Ca, K, and H2O species are highly disordered, as indicated by anomalously large temperature factors and partial occupancies. Localized differences in the Al/Si arrangements in the inverted tetrahedra induce disorder among the interlayer cations.

Key Words: Bannisterite • Interlayer cations • Layer structure • Modulated structure • X-ray structure refinement

Clays and Clay Minerals; April 1992 v. 40; no. 2; p. 129-144; DOI: 10.1346/CCMN.1992.0400201
© 1992, The Clay Minerals Society
Clay Minerals Society (www.clays.org)