American Mineralogist, Volume 96, pages 179–187, 2011 0003-004X/11/0001–179$05.00/DOI: 10.2138/am.2011.3531 179 The crystal structure of stichtite, re-examination of barbertonite, and the nature of polytypism in MgCr hydrotalcites Stuart J. MillS, 1, * PaMela S. Whitfield, 2 Siobhan a. WilSon, 1, † Joanne n. WoodhouSe, 1, ‡ GreGory M. diPPle, 1 Mati raudSePP, 1 and Carl a. franCiS 3 1 Mineral Deposit Research Unit, The University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada 2 Institute for Chemical Process and Environmental Technology, National Research Council of Canada, Montréal Road, Ottawa, Ontario K1A 0R6, Canada 3 Harvard Mineralogical Museum, 24 Oxford Street, Cambridge, Massachusetts 02138, U.S.A. abStraCt Stichtite, ideally Mg 6 Cr 2 CO 3 (OH) 16 ∙4H 2 O, from Stichtite Hill, Tasmania, Australia, and barber- tonite, also ideally Mg 6 Cr 2 CO 3 (OH) 16 ∙4H 2 O, from the Kaapsehoop asbestos mine, South Africa, have been studied by powder X-ray diffraction and their structures have been refined using the Rietveld method. Stichtite from Stichtite Hill crystallizes in the rhombohedral space group R3 m, with unit- cell parameters a = 3.09575(3) and c = 23.5069(6) Å, V = 195.099(6) Å 3 , with Z = 3/8. Barbertonite from the Kaapsehoop asbestos mine crystallizes in the hexagonal space group P6 3 /mmc. The co-type specimens of barbertonite were found to be intergrown mixtures consisting of barbertonite and stichtite. Unit-cell parameters of barbertonite from the co-type specimens were a = 3.09689(6), c = 15.6193(8) Å, and V = 129.731(8) Å 3 and a = 3.09646(6), c = 15.627(1) Å V = 129.76(1) Å 3 , and Z = ¼. Rietveld refinements of both stichtite and barbertonite show that they are polytypes rather than polymorphs and do not represent distinct mineral species. Several possible nomenclature systems are discussed for the naming of hydrotalcite minerals and groups. Raman band assignments are also presented for stichtite from Stichtite Hill. Stichtite and hydrotalcite minerals make up a large proportion of the ore at the Mount Keith nickel mine in Western Australia. Bulk powder diffraction shows the ore contains 6.1 wt% stichtite and 5.6 wt% iowaite. Hydrotalcite group minerals provide an important potential reservoir of CO 2 . At Mount Keith, the amount of CO 2 mined as stichtite could exceed 45 000 metric tons per year, while exchange of Cl for CO 3 could fix in excess of 40 000 metric tons CO 2 per year if end-member iowaite is reacted to form pyaroaurite. Keywords: Stichtite, barbertonite, polytype, hydrotalcite, carbon sequestration, Stichtite Hill, Kaapsehoop asbestos mine, X-ray diffraction introduCtion Stichtite, ideally Mg 6 Cr 2 CO 3 (OH) 16 ∙4H 2 O, is the Cr-analog of hydrotalcite, which occurs in Cr-bearing serpentinites, ophiolites, and greenstone belts in several localities worldwide. Originally identified as “kämmererite” [Cr-rich clinochlore – Nordenskiöld (1840); Lapham (1958)] in 1891, stitchtite was recognized as a non-silicate in 1910 (Petterd 1910) and was formally described in 1914 from Stichtite Hill (41°53′9″S, 145°26′16″E), Dundas, Tasmania, Australia (Twelvetrees 1914). It was named stichtite after Robert Sticht, general manager of the Mt. Lyell mines in Tasmania, and a world-renowned metallurgist. The most compre- hensive work on the crystal chemistry and provenance of stichtite was done by Ashwal and Cairncross (1997), who summarize all previous data on stichtite. Bottrill and Graham (2006) provide further information on the Tasmanian occurrences of this mineral and Mondal and Baidya (1996) provide information on stichtite from the Nuasahi chromite deposits, eastern India. Barbertonite, also ideally Mg 6 Cr 2 CO 3 (OH) 16 ∙4H 2 O, was described by Frondel (1941) as the hexagonal polymorph of stitchtite from Kaapsehoop asbestos mine, Kaapsehoop, Barber- ton district, Mpumalanga, South Africa. Barbertonite was initially identified by X-ray diffraction films which showed a similarity to the diffraction patterns of sjögrenite [Mg 6 Fe 2 CO 3 (OH) 16 ∙4H 2 O] and manasseite [Mg 6 Al 2 CO 3 (OH) 16 ∙4H 2 O]. Ashwal and Cairn- cross (1997) report an unsuccessful attempt to identify and isolate barbertonite from the type material (housed in the collections of the Harvard Mineralogical Museum). The difficulty involved in isolating and identifying this mineral have led several sources to report barbertonite’s status as a mineral to be “questionable” (e.g., Clark 1993). The related mineral, sjögrenite, crystallizes in hexagonal space group P6 3 /mmc (No. 194), with a = 3.103 and c = 15.52 Å, which corresponds to the 2H 1 polytype. Members of the sjögrenite group, which include manasseite and barbertonite, * E-mail: smills@eos.ubc.ca † Present address: Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, Indiana 47405- 1405, U.S.A. ‡ Present address: Golder Associates Ltd., 1-25 Scurfield Bou- levard, Winnipeg, Manitoba R3Y 1G4, Canada.