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

Abstract

The crystal structure of tamarugite [sodium aluminium bis-(sulfate) hexa-hydrate] 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+) octa-hedra and infinite double-stranded chains [Na(SO4)2](3-) that extend parallel to [001]. The Na(+) cation has a strongly distorted octa-hedral 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 octa-hedra and two S2O4 tetra-hedra sharing five adjacent edges, one between two NaO6 octa-hedra and two each between the resulting double octa-hedron and two S2O4 tetra-hedra. These groups are then linked into a double-stranded chain via corner-sharing between NaO6 octa-hedra and S2O4 tetra-hedra. The S1O4 group, attached to Na in the terminal position, completes the chains. The [Al(H2O)6](3+) octa-hedron (〈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.