Modular crystal chemistry of sulphosalts and other complex sulphides
Published:January 01, 1997
Crystal structures of a large number of complex sulphides can be described as composed of rods, blocks or layers of simple, archetypal structures that are joined (recombined) in various ways by the action of one or more structure-building operators, such as the various kinds of unit-cell twinning. The resulting structures were denoted as the recombination structures by the Subcommittee on the Nomenclature of Inorganic Structure Types (Lima-de-Faria et al., 1990). It is this kind of complex sulphides we deal with in the present review; those with complex chemistry but simple structural principles (e.g., the substitution-, interstitial- or omission derivatives of archetypal structures) are not included.
The principal sulphide family with recombination structures are the so-called “sulphosalts”. They are defined as complex sulphides (rarely also selenides and tellurides) AxBySz with A = Pb, Cu, Ag, Hg, Tl, Fe, Mn, Sn2+, Sn4+ and other metals; these may include alkali metals, alkaline earths, lanthanides and various organic cations in some synthetic sulphosalts; B = formally trivalent cations As, Sb and Bi in non-planar, (fundamentally) threefold coordination BS3; and S = S2−, Se2− and Te2−. Sn2+ and Te4+ have the same coordination properties as B3+ and it is difficult to draw a borderline between their sulphides and the sulphosalts sensu stricto.
Structurally very close to sulphosalts are (a) the complex sulphides of (primarily) octahedral In with Pb, Sn, Bi and other metals, (b) the layer misfit sulphides, based on a combination of layers that contain large cations (La to Sn, Y, Sn2+, Pb and Bi3+)
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Modular Aspects of Minerals
Since the first beginning of the crystal chemical study of the inorganic compounds, a simple modular approach was developed, by looking at the crystal structures as built up through the assembling of simple polyhedral units. This approach was no only useful for a vivid and insightful description of the complex atomic arrangements of natural and synthetic compounds, but, through the use of simple and powerful rules for assembling polyhedral units, it displayed an extraordinary heuristic power, suggesting reliable models for many complex structural assemblages. The polyhedral approach also laid the basis for meaningful classifications which were applied to all the classes of inorganic compounds.