engleski

Nchwaningite, Mn2+2Si03(OH)2'H20, a new pyroxene-related chain silicate from the N'chwaning mine, Kalahari manganese field, South Africa

Nchwaningite, Mn~+ Si03(OH)2' H20, is an orthorhombic chain silicate [space group Pea2" Z = 4, a = 12.672(9), b = 7.217(3), e = 5.341(2) A], occurring as aggregates shaped like pin cushions, together with calcite, bultfonteinite, and chlorite at N'chwaning mine, located in the Kalahari manganese field, northern Cape Province, South Africa. 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 biaxially negative with the refractive indices ex= 1.681(2), fJ = 1.688(2), 'Y= 1.690(2), 2 ~ = 54.4(4)°. The optical orientation is X = b, Y = a, Z = c. Nchwaningite has two perfect cleavages parallel to (010) and (100). The calculated density is 3.202 glcm3. The chemical composition, as determined by electron microprobe analyses, indicates minor substitutions of Mg for Mn. The crystal structure, including H positions, was solved and refined from X-ray singlecrystal data to R = 2.14%, Rw = 2.91%. The nchwaningite structure consists of double layers of laterally linked so-called truncated pyroxene-building units formed by a double chain of octahedra, topped with a Zweier single chain of Si tetrahedra. Symmetry-equivalent units are linked laterally but turned upside down. This yields a double-layer structure with H bridges linking the layers. A striking feature of the structure is that one Mn06 comer is formed by a H20 molecule. The tetrahedral chain and octahedral distortion of the new mineral is compared with pyroxenes having MnH in Ml [synthetic MnSi03 (P2/e clinopyroxene) andjohannsenite CaMn(Si03)2]' To test a complete Ca and Mg substitution for Mn in the new nchwaningite structure type, distance least-square refinements were performed. It was found that Ca and Mg analogues would also yield reasonable interatomic distances and polyhedra.

Nchwaningite; Kalahari manganese field; South Africa; chain silicate

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