Received 23 November 2011
Pyrosmalite-(Fe), ideally FeII8Si6O15(OH,Cl)10 [refined composition in this study: Fe8Si6O15(OH0.814Cl0.186)10·0.45H2O, octairon(II) hexasilicate deca(chloride/hydroxide) 0.45-hydrate], is a phyllosilicate mineral and a member of the pyrosmalite series (Fe,Mn)8Si6O15(OH,Cl)10, which includes pyrosmalite-(Mn), as well as friedelite and mcgillite, two polytypes of pyrosmalite-(Mn). This study presents the first structure determination of pyrosmalite-(Fe) based on single-crystal X-ray diffraction data from a natural sample from Burguillos del Cerro, Badajos, Spain. Pyrosmalite-(Fe) is isotypic with pyrosmalite-(Mn) and its structure is characterized by a stacking of brucite-type layers of FeO6-octahedra alternating with sheets of SiO4 tetrahedra along . These sheets consist of 12-, six- and four-membered rings of tetrahedra in a 1:2:3 ratio. In contrast to previous studies on pyrosmalite-(Mn), which all assumed that Cl and one of the four OH-groups occupy the same site, our data on pyrosmalite-(Fe) revealed a split-site structure model with Cl and OH occupying distinct sites. Furthermore, our study appears to suggest the presence of disordered structural water in pyrosmalite-(Fe), consistent with infrared spectroscopic data measured from the same sample. Weak hydrogen bonding between the ordered OH-groups that are part of the brucite-type layers and the terminal silicate O atoms is present.
For pyrosmalite-(Fe), see: Zambonini (1901); Vaughan (1986); Pan et al. (1993). For other minerals of the pyrosmalite series, see: Frondel & Bauer (1953); Stillwell & McAndrew (1957); Takéuchi et al. (1963, 1969); Kashaev & Drits (1970); Kashaev (1968); Kato & Takéuchi (1983); Kato & Watanabe (1992); Ozawa et al. (1983); Abrecht (1989); Kodera et al. (2003). Correlations between O-H streching frequencies and O-HO donor-acceptor distances were given by Libowitzky (1999). The presence of H2O in the pyrosmalite series was proposed by Kayupova (1964).
Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: publCIF (Westrip, 2010).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2570 ).
The authors gratefully acknowledge support of this study by the Arizona Science Foundation.
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