CRYSTAL-STRUCTURE OF MINEHILLITE - TWINNING AND STRUCTURAL RELATIONSHIPS TO REYERITE

The crystal structure of minehillite from Franklin, New Jersey, (K,Na) (2)Ca28Zn5Al4Si40-O-112(OH)(16), was solved and refined in space group P ($) over bar 3c1, a = 9.777(2), c = 33.293(2) Angstrom, to R = 0.02 2 for 1510 unique reflections collected from a twinned crystal. Two tw inned portions are related to each other by a twofold axis coincident with the ($) over bar 3 axis, which can result in apparent hexagonal s ymmetry for minehillite. The structure consists of a stacked sequence of three types of layer units: (1) an infinite sheet of edge-sharing C a-(O,OH) polyhedra, (2) a single sheet of SiO4 tetrahedra connected in oval and pseudohexagonal-shaped six-membered rings, and (3) a complex slab built of SiO4 tetrahedra and Al0(6) octahedra into which alkali elements and Zn are accommodated. The first two units are identical to those found in the reyerite structure. The third is analogous to the double layer in reyerite but with significant differences. The SiO4 te trahedra in the complex slab in minehillite form two sheets of isolate d pseudohexagonal six-membered rings that are connected by AlO6 octahe dra centered on the threefold axis, which are unusual features among l ayered silicate minerals. An average of five Zn atoms per cell partial ly occupy a highly distorted tetrahedral site.