Pyrosmalite-(Fe), Fe(8)Si(6)O(15)(OH,Cl)(10).

Pyrosmalite-(Fe), ideally Fe(II) (8)Si(6)O(15)(OH,Cl)(10) [refined composition in this study: Fe(8)Si(6)O(15)(OH(0.814)Cl(0.186))(10)·0.45H(2)O, octa-iron(II) hexa-silicate deca-(chloride/hydroxide) 0.45-hydrate], is a phyllosilicate mineral and a member of the pyrosmalite series (Fe,Mn)(8)Si(6)O(15)(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 FeO(6)-octa-hedra alternating with sheets of SiO(4) tetra-hedra along [001]. These sheets consist of 12-, six- and four-membered rings of tetra-hedra 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.