Acta Crystallographica Section E

Structure Reports Online

Volume 69, Part 10 (October 2013)

inorganic compounds

Acta Cryst. (2013). E69, i63-i64    [ doi:10.1107/S1600536813025154 ]

Redetermination of tamarugite, NaAl(SO4)2·6H2O

K. Mereiter

Abstract: 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 [001]. 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 (O...O = 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.

CCDC reference: 960270

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