Stichtite

Abstract: Abstract Stichtite, ideally Mg6Cr2CO3(OH)16∙4H2O, from Stichtite Hill, Tasmania, Australia, and barbertonite, also ideally Mg6Cr2CO3(OH)16∙4H2O, 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 R3̄m, with unitcell parameters a = 3.09575(3) and c = 23.5069(6) Å, V = 195.099(6) Å3, with Z = 3/8. Barbertonite from the Kaapsehoop asbestos mine crystallizes in the hexagonal space group P63/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) Å3 and a = 3.09646(6), c = 15.627(1) Å V = 129.76(1) Å3, 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 CO2. At Mount Keith, the amount of CO2 mined as stichtite could exceed 45 000 metric tons per year, while exchange of Cl for CO3 could fix in excess of 40 000 metric tons CO2 per year if end-member iowaite is reacted to form pyaroaurite.

Abstract: Stichtite and reevesite are built of positively charged brucite-like layers and contain charge-compensating CO~ 2anions and water molecules in interlayers, which identify them as members of the hydrotalcite-like group of minerals. In the literature, authors have failed to agree about stichtite and reevesite polytypism. White et al. (1967) described reevesite and Taylor (1973) described stichtite in terms of a three-layer rhombohedral cell with a = 3 It. De Waal and Viljoen (1971) found additional reflections in the XRD pattern of reevesite and proposed a new cell with a doubled a-parameter and number of layers. For the same reason, the XRD data of stichtite given in ASTM card (PDF) # 14-330 and reported by Tatarinov et aL (1985) was also interpreted in terms of a similar double cell. The present work is a further study ofstichtite and reevesite polytypism in an attempt to clarify whether six-layer polytypes of hydrotalcite-like minerals exist.

Abstract: Mountkeithite [(Mg,Ni)9(Fe3+,Cr,Al)3(OH)24]3+[(CO{in3,SO4)1.5(Mg,Ni)2(SO4)2(H2O)11]3− occurs with pyrite, magnetite, magnesite, hexahydrite, and morenosite in a low-temperature hydrothermal vein system in the Mount Keith disseminated nickel sulphide deposit, Western Australia. Electron-microprobe analyses gave the following average composition: MgO 31.7, NiO 6.1, CuO 0.1, Cr2O3 6.1, Fe2O3 8.3, Al2O3 2.6, SO3 14.7; microanalysis for C and H gave CO2 3.9, H2O 30.6; total 104.1. The X-ray powder diffraction pattern has the following strongest lines: 11.30(10), 5.63(8), 4.63(2), 3.765(6), 2.645(4), 2.545(4), 1.554(5), and 1.505(3); these can be indexed on a hexagonal cell with a = 10.698 A, c = 22.545 A. Mountkeithite occurs as friable aggregates and rosettes composed of soft, pearly to translucent, pale pink to white flakes with perfect basal cleavage. Optical properties are uniaxial negative with ɛ = 1.51 and ω = 1.52; weakly dichroic from colourless to palest pink. Specific gravity is 2.12 (calc. 1.95). Mountkeithite is soluble, with efferves-cence, in dilute HCl. Mountkeithite changes to a pyroaurite-like phase, with a 7.8.A basal spacing when immersed in water, but can be changed back to an 11.3 A phase by immersion in 1M MgSO4; mountkeithite also collapses to a 7.8 A basal spacing under vacuum. Phases similar to mountkeithite have been synthesized by immersing grains of stichtite in 1M MgSO4, at 20, 50, 105, and 200°C. The structure of mountkeithite is interpreted as consisting of positively charged brucite-like hydroxyl layers, containing both divalent and trivalent cations, separated by about 7 A-thick, negatively charged interlayers containing carbonate and sulphate anions, magnesium sulphate, and water. Carrboydite, motukoreaite, and hydrohonessite are minerals with similarly large basal spacings of 10–11 A, and are also believed to have pyroaurite-related structures with expanded interlayers.