Redetermination of tamarugite, NaAl(SO(4))(2)·6H(2)O

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(H(2)O)(6)](3+) octa­hedra and infinite double-stranded chains [Na(SO(4))(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 S2O(4) and one by the non-bridging sulfate group S1O(4). The [Na(SO(4))(2)](3−) chain features an unusual centrosymmetric group formed by two NaO(6) octa­hedra and two S2O(4) tetra­hedra sharing five adjacent edges, one between two NaO(6) octa­hedra and two each between the resulting double octa­hedron and two S2O(4) tetra­hedra. These groups are then linked into a double-stranded chain via corner-sharing between NaO(6) octa­hedra and S2O(4) tetra­hedra. The S1O4 group, attached to Na in the terminal position, completes the chains. The [Al(H(2)O)(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(SO(4))(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.