Bull. Minéral. (1981), 104, 536-547.

Complex crystal structures related to glaserite, K3Na(S04)2 : evidence for very dense packings among oxysalts

Structures cristallines complexes apparentées à la glasérite K3Na(SO�)2 : mise en évidence d'empilements très compacts dans les oxosels.

Introduction

The glaserite, K3Na(S04)2, structure type is one of the most versatile arrangements for mineralogical crystal chemists. Generously exploited by over 100 compounds, in particular silicates, phosphates and sulfates, extensive structure genealogies can be effected from the parent glaserite structure type. Mathew et al. (1977), Dickens and Brown (1971) and Calvo and Gopal (1975) have exploited it in discussion of complex Ca — Mg-phosphate structures; Moore (1973, 1976) and Moore and Araki (1976) explored its topology and geometry in considerable detail. In these structures, a central column of cation-oxygen coordination polyhe-dra linked by circumjacent anionic tetrahedra occur. It was shown that the structures could be conceived as local slabs of close-packed polyhedra and that the tetrahedral apices could point either up (u) or down (d). The classification then reduced to a combinatorial problem, of determining how many distinct «brace¬ let » arrangements could occur. In the structures discussed, slabs of close-packed polyhedra could be considered orthogonal to the polyhedral columns, or more technically rods. Alternatively, the structures can be considered the hexagonal packing of rods, the

colorings of the rod segments dictating the repeat unit along the rod direction. More specifically, they can be considered rod groups normal to planes of finite thickness (Shubnikov and Koptsik, 1974), the rod groups and plane groups being «black-white » or two-color groups. The simple u-d disposition of the circum¬ jacent tetrahedral apices and the complementarity from slab to its adjacent slabs dictate a repeat of two slabs within a rod axis segment. In the silicate and phosphate structures with alkali and alkaline earth counterions, the rod segment repeat distance is ca. 6.5-7.5 A, with an average around 7.0 A, or a 3.5 A distance between adjacent cations in a rod. Seven  is an important number because it assists in sorting out potential members of this large family. Perhaps most fascinating is the fact that the glaserite arrangement exploits the packings of octahedra (M), tetrahedra (T), icosahedra (I) and 10-coordinate hexagonal pyramidal polyhedra (Y) with maximal point symmetry {3 m} (Moore, 1973). In this paper I shall use the symbol X for any cation or cation-oxygen polyhedron with coordi¬ nation number greater than 4.

Two problems immediately arise : (1) what if the u-d symbolism and a coordinating tetrahedral apex breaks down and (2) what if the X : T ratio deviates