The Crystal Chemistry and Paragenesis of Honessite and Hydrohonessite: The Sulphate Analogues of Reevesite

D. L. Bish* and A. Livingstone
Department of Geological Sciences, Harvard University, 20 Oxford Street, Cambridge, MA 02138, USA
Department of Geology, The Royal Scottish Museum, Edinburgh, EH1 1JF, Scotland
*Present address: Los Alamos Scientific Laboratory, Los Alamos, New Mexico, USA.

Abstract: Honessite and hydrohonessite from Unst, Shetland, and Linden, Wisconsin, are sulphate-containing members of the pyroaurite group and are analogous to reevesite. The structural formula can be written (Ni5.55Mg0.10Fe2.353+)(OH)16(SO4)1.18.xH2O for the Unst material, with sulphate groups replacing the interlayer carbonate groups in reevesite. Microprobe analyses of the Unst material show amounts of SO3 in excess of that required to balance all trivalent cations and this may be due to the presence of an amorphous nickel sulphate material. We find no evidence for the presence of trivalent nickel in honessite.

The Unst material occurs in both 8.8 (honessite) and 11.1 Å (hydrohonessite) forms, depending on composition, humidity, and temperature, similar to carrboydite, motukoreaite, and SO4-exchanged takovite. The Wisconsin material occurs only with an 8.8 A basal spacing. The increase from the 7.7 Å basal spacing of reevesite is due to the difference in molecular geometry of carbonate and sulphate groups. The presence of the sulphate molecule is unambiguously shown by the infra-red spectra, and the observed bands are consistent with the sulphate groups lying in the interlayer with their trigonal axes parallel to c. The Unst material is found intimately mixed in variable proportions with reevesite, distributed in patches on chromite, and the mixed nature is revealed by X-ray powder diffraction, by variations in total SO3 content, and by the presence of absorptions due to both SO42− and CO32− in the infra-red spectra.

Mineralogical Magazine; September 1981 v. 44; no. 335; p. 339-343; DOI: 10.1180/minmag.1981.044.335.15
© 1981, The Mineralogical Society
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