Received 4 September 2013
The crystal structure of tamarugite [sodium aluminium bis(sulfate) hexahydrate] was redetermined from a single crystal from Mina Alcaparossa, near Cerritos Bayos, southwest of Calama, Chile. In contrast to the previous work [Robinson & Fang (1969). Am. Mineral. 54, 19-30], all non-H atoms were refined with anisotropic displacement parameters and H-atoms were located by difference Fourier methods and refined from X-ray diffraction data. The structure is built up from nearly regular [Al(H2O)6]3+ octahedra and infinite double-stranded chains [Na(SO4)2]3- that extend parallel to . The Na+ cation has a strongly distorted octahedral coordination by sulfate O atoms [Na-O = 2.2709 (11) - 2.5117 (12) Å], of which five are furnished by the chain-building sulfate group S2O4 and one by the non-bridging sulfate group S1O4. The [Na(SO4)2]3- chain features an unusual centrosymmetric group formed by two NaO6 octahedra and two S2O4 tetrahedra sharing five adjacent edges, one between two NaO6 octahedra and two each between the resulting double octahedron and two S2O4 tetrahedra. These groups are then linked into a double-stranded chain via corner-sharing between NaO6 octahedra and S2O4 tetrahedra. The S1O4 group, attached to Na in the terminal position, completes the chains. The [Al(H2O)6]3+ octahedron (<Al-O> = 1.885 (11) Å) donates 12 comparatively strong hydrogen bonds (OO = 2.6665 (14) - 2.7971 (15) Å) to the sulfate O atoms of three neighbouring [Na(SO4)2]3- chains, helping to connect them in three dimensions, but with a prevalence parallel to (010), the cleavage plane of the mineral. Compared with the previous work on tamarugite, the bond precision of Al-O bond lengths as an example improved from 0.024 to 0.001 Å.
For the previous structure determination of tamarugite, see: Robinson & Fang (1969). For mineralogical data of tamarugite, see: Anthony et al. (2003). For the mineralogy of three sulfate deposits of northern Chile including Mina Alcaparrosa, see: Bandy (1938). For the recently described new sulfate mineral alcaparrosite, see: Kampf et al. (2012). For crystal structures of the related aluminium sulfate hydrates mendozite [NaAl(SO4)2·11H2O], sodium alum [NaAl(SO4)2·12H2O], alunogen [Al2(SO4)3·17H2O] and apjohnite [MnAl2(SO4)4·22H2O], see: Fang & Robinson (1972); Cromer et al. (1967); Menchetti & Sabelli (1974, 1976).
Data collection: SMART (Bruker, 1999); cell refinement: SAINT (Bruker, 1999); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2012); software used to prepare material for publication: SHELXL97 and publCIF (Westrip, 2010).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: PJ2005 ).
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