The crystal structure of stichtite, re-examination of barbertonite, and the nature of polytypism in MgCr hydrotalcites

Stichtite, ideally Mg(6)Cr(2)CO(3)(OH)(16)center dot 4H(2)O, from Stichtite Hill, Tasmania, Australia, and barbertonite, also ideally Mg(6)Cr(2)CO(3)(OH)(16 center dot)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 R (3) over barm, with unit-cell parameters a = 3.09575(3) and c = 23.5069(6) angstrom, V= 195.099(6) angstrom(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) angstrom, and V= 129.731(8) angstrom(3) and a = 3.09646(6), c = 15.627(1) angstrom V = 129.76(1) angstrom(3), and Z = 1/4. 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