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The crystal structure of minehillite from Franklin, New Jersey, (K,Na) 2 Ca 28 Zn 5 Al 4 Si 40 - O 112 (OH) 16 , was solved and refined in space group P3̅&gt;c1, a = 9.777(2), c = 33.293(2) Å, to R = 0.022 for 1510 unique reflections collected from a twinned crystal. Two twinned portions are related to each other by a twofold axis coincident with the 3̅ axis, which can result in apparent hexagonal symmetry for minehillite. The structure consists of a stacked sequence of three types of layer units: (I) an infinite sheet of edge-sharing Ca-(O,OH) polyhedra, (2) a single sheet of SiO 4  tetrahedra connected in oval and pseudohexagonal­shaped six-membered rings, and (3) a complex slab built of SiO 4  tetrahedra and AlO 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 SiO 4  tetrahedra in the complex slab in minehillite form two sheets of isolated pseudohexagonal six-membered rings that are con­nected by AlO 6  octahedra centered on the threefold axis, which are unusual features among layered silicate minerals. An average of five Zn atoms per cell partially occupy a highly distorted tetrahedral site.

Crystal structure of minehillite: Twinning and structural relationships to reyerite