Refinement of the alum structures. II. X-ray and neutron diffraction of NaAl(SO4)2.12H2O, γ alum

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

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.

A case of polytypism in hydrated oxysalts: the crystal structure of Mg3(P04)2 22 H20-II

Mg3(P04)2 · 22H20-II crystallizes in space group P−, with a = 6.937(3), b = 6.932(3), c = 16.132(5)Å, α = 82.15(3), β = 89.72(3), γ = 119.49(3)°, Z = 1. The structure was solved (direct methods) and refined anisotropically (least squares) to R = 0.042, using 1826 observed reflections measured on a single-crystal diffractometer (MoK α radiation). All hydrogen atoms were located from difference maps and included in the refinement. (001) layers formed by Mg(H20)6 and P04 polyhedra and by lattice water molecules are linked by hydrogen bonds and stacked with the… ABB′A… scheme. A structure previously studied by other authors has been recognized as a polytype, Mg3(P04)2 · 22 H20-I, where the pair of layers AB is nearly identical as in II, but the B′ layer is oriented differently with respect to B. The two orientations are related by a mirror pseudo-symmetry of the B layer. Differences of the hydrogen bonding scheme in the two cases are pointed out.

The role of OH and H2O in oxide and oxysalt minerals

Hydrogen plays an extremely important role in the structure and chemistry of the oxide and oxysalt minerals. The characteristics of this role can be profitably analyzed in a simple and intuitive fashion using bond-valence theory. For any crystal structure, the structural unit may be defined as the strongly bonded part of the structure; structural units are linked together by interstitial species, usually alkali or alkaline earth cations and (H2O)0 groups that are involved in much weaker bonding. This scheme gives a binary representation of even the most complex structure. The interaction between the structural unit and the interstitial species can be quantitatively evaluated using the valence-matching principle (Brown, 1981).

As components of the structural unit, (OH)− and (H2O)0 play a major role in dictating the dimensional polymerization of the structural unit because of the very asymmetric nature of the donor-hydrogen and hydrogen…acceptor interactions. As an interstitial component, (H2O)0 can play three different roles. Interstitial (H2O)0 may bond to an interstitial cation, essentially forming a complex cation. In this role, (H2O)0 acts as a bond-valence transformer, moderating the Lewis acidity of the interstitial cations such that it matches the Lewis basicity of the structural unit and the valence-matching principle is satisfied. Interstitial (H2O)0 need not bond to an interstitial cation to occupy well-ordered atomic positions; a stable hydrogen-bonded network can occur in the interstitial regions between structural units. The role of such (H2O)0 is to satisfy the bond-valence requirements of H atoms that are part of the structural unit, propagating the bonding across the interstitial space to other parts of the structural unit. Occluded (H2O)0 may occur in some minerals. Such (H2O)0 is not bonded to interstitial cations and does not participate in a static ordered hydrogen-bond network. However, this type of (H2O)0 will still affect many of the physical properties of a mineral.