New wurtzite polytypes from Joplin, Missouri

Smali, hemimorphic hexagonal crystals implanted on botryoidal zinc sulfide from the zigzagMine, Joplin, Missouri, have been studied by crystallographic and r-ray diffraction techniques. They are identified as a new wurtzite polytype, wurtzite-l0H. The unit cell data are: space group) P6sme; a:3.824 fr, c:3l.ZO A; cell contents, 10ZnS. Comparison of difiraction intensity data indicates that the published structure of SiC-10FI (stacking sequence 3223) is different from that of wurtzite-10H. Calculation of intensities for various models shows that the stacking sequence for wurtzite-l0H is 55. Crystals of wurtzite from Joplin described by A. F. Rogers (1904) were evidently wurtzite-l0H. powder diffraction data revealed the presence o-f another polytype, wurtzite-8H, with a hexagonal unit cell: space group PQmc; a:3.82 A, c:24.96 L; cell contents 8ZnS. Wurtzite-6H was also found at t}'e zigzagMine The wurtzite polytypes evidently form a homologous series (zH,4lF,, 6H, 8H, 10H) resulting from growth phenomena based on screw dislocations. OccunnBncB During the course of a study of the Zig Zag Mine in north Joplin, If issouri, by one of us (E.T.N{cK.), specimens of an unusual stalactite formation oI zinc sulfide were collected. optical examination of the radiating structure of these stalactit ic forms in thin section showed that the mass consisted of about 90 per cent isotropic sphalerite and 10 per cent of an anisotropic zinc sulfide phase. On the surface of some of the stalactites small wurtzite-like pyramidal crystals were found implanted on their points, in a manner resembling prickly-pear fruit. This resemblance is most apt for the imperfect, rounded crystals which greatly outnumber those with well-defined crystal faces. As shown in the photograph in Fig. 1, a secondary crystal is occasionaliy found growing out of the base of the larger crystals. When clean, the crystals are brown to Iight brown, but their color is often obscured by a black stain covering the surface of the stalactites. None of them ranges over 2 mm. in size. These crystals have been proved by crystallographic and r-ray difiraction study to be a previously unreported polytype of wurtzite. CnysrerrocRApHy The crystal habit is characterized by a negative base and a steep positive pyramid. The faces of the best crystals gave poor signals on the goniometer, but good enough to indicate an inclination angle (p) of about 77f'. This angle corresponds to the form l[5052] as l isted in palache, Berman and Frondel (1944, p. 227). Frcndel and Palache (1950) found in * Publication approved by the Director, U. S. Geological Survey. t210 NEW WURTZITE POLVTYPES 12IT Frc. 1. Wurtzite-lOH crystals in vug of botryoidal sphalerite ftomZigZagMine' Joplin, Missouri. Bar at top indicates one mm. Photograph by J' A' Denscn' hexagonal lattice. The resulting data for the Joplin wurtzite are given in Table 1. No polytype oI zinc sulfide of these dimensions has been previously reporled in either natural or artificial specimens (Frondel and Palache, 1950; Strock and Brophy, 1955). Also, the Joplin wurtzite' which in the notation of Frondel and Palache will be designated wurtzite-