NCHWANINGITE, MN22-CENTER-DOT-H2O, A NEW PYROXENE-RELATED CHAIN SILICATE FROM THE NCHWANING MINE, KALAHARI MANGANESE FIELD, SOUTH-AFRICA(SIO3(OH)(2))

Nchwaningite, Mn22+SiO3(OH)(2) . H2O, is an orthorhombic chain silicat e [space group Pca2(1), Z = 4, a = 12.672(9), b = 7.217(3), c = 5.341( 2) Angstrom], occurring as aggregates shaped like pin cushions, togeth er with calcite, bultfonteinite, and chlorite at N'chwaning mine, loca ted in the Kalahari manganese field, northern Cape Province, South Afr ica. The aggregates consist of light brown, transparent needles, which average 1.0 x 0.1 x 0.05 mm in size. Nchwaningite is named after the mine N'chwaning II, where it was found first. Nchwaningite is biaxiall y negative with the refractive indices alpha = 1.681(2), beta = 1.688( 2), gamma = 1.690(2), 2 V-x = 54.4(4)degrees. The optical orientation is X = b, Y = a, Z = c. Nchwaningite has two perfect cleavages paralle l to (010) and(100). The calculated density is 3.202 g/cm(3). The chem ical composition, as determined by electron microprobe analyses, indic ates minor substitutions of Mg for Mn. The crystal structure, includin g H positions, was solved and refined from X-ray single-crystal data t o R = 2.14%, R(w) = 2.91%. The nchwaningite structure consists of doub le layers of laterally linked so-called truncated pyroxene-building un its formed by a double chain of octahedra, topped with a Zweier single chain of Si tetrahedra. Symmetry-equivalent units are linked laterall y but turned upside down. This yields a double-layer structure with H bridges linking the layers. A striking feature of the structure is tha t one MnO6 corner is formed by a H2O molecule. The tetrahedral chain a nd octahedral distortion of the new mineral is compared with pyroxenes having Mn2+ in M1 [synthetic MnSiO3 (P2(1)/c clinopyroxene) and johan nsenite CaMn(SiO3)(2)]. To test a complete Ca and Mg substitution for Mn in the new nchwaningite structure type, distance least-square refin ements were performed. It was found that Ca and Mg analogues would als o yield reasonable interatomic distances and polyhedra.