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Olmiite, CaMn[SiO3(OH)](OH), the Mn-dominant analogue of poldervaartite, a new mineral species from Kalahari manganese fields (Republic of South Africa)

  • P. Bonazzi (a1), L. Bindi (a2), O. Medenbach (a3), R. Pagano (a4), G. I. Lampronti (a1) and S. Menchetti (a1)...


Olmiite, ideally CaMn[SiO3(OH)](OH), is a newly identified mineral from the N’Chwaning II mine of the Kalahari manganese fields (Republic of South Africa), which occurs as a product of hydrothermal alteration associated with poldervaartite, celestine, sturmanite, bultfonteinite and hematite. The mineral occurs as wheat-sheaf aggregates consisting of pale to intense reddish pink minute crystals. Olmiite is transparent with vitreous lustre, and exhibits deep-red fluorescence under short-wave UV-light. The mineral is brittle, with irregular fracture. Streak is white and Mohs hardness is 5–51/2. No cleavage was observed. The measured density (pycnometer method) is 3.05(3) g/cm3. The calculated density is 3.102 g/cm3 or 3.109 g/cm3 using the unit-cell volume from single-crystal or powder data, respectively. Olmiite is biaxial positive, with refractive indices α = 1.663(1), β = 1.672(1), γ = 1.694(1) (589 nm), 2Vmeas = 71.8(1)º, 2Vcalc = 66(8)º. The optical orientation is X = a, Y = c, Z = b and dispersion: r > v, distinct. Pleochroism is not observed. Chemical analysis by electron microprobe yielded the chemical formula (Ca2–xMnxFey)[SiO3(OH)](OH), with 0.84 ≤ x ≤ 0.86, and y ≤ 0.01. Olmiite is orthorhombic, space group Pbca, with a = 9.249(3), b = 9.076(9), c = 10.342(9) Å , V = 868(1) Å 3 and Z = 8. The strongest five powder-diffraction lines [d in Å , (I/Io), hkl] are: 4.14, (45), 021; 3.19, (100), 122; 2.807, (35), 311; 2.545 (35), 312; 2.361, (40), 223. Single-crystal structure refinement (R1 = 2.74% for 1012 observed reflections) showed that the atomic arrangement of olmiite is similar to that of poldervaartite, with all Mnordered onthe M2 site. Significant variations in bond distances and angles are related to the pronounced difference in the Mn content. Olmiite, therefore, is the Mn-dominant analogue of poldervaartite. The name poldervaartite should be reserved for samples having Ca dominant at the M2 site.


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Albee, A.L. and Ray, L. (1970) Correction factors for electronprobe analysis of silicate, oxides, carbonates, phosphates, and sulfates. Analytical Chemistry, 48, 1408–1414.
Bence, A.E. and Albee, A.L. (1968) Empirical correctionfactors for the electronmicroan alysis of silicate and oxides. Journal of Geology, 76, 382–403.
Bonazzi, P., Menchetti, S. and Reinecke, T. (1996) Solid solution between piemontite and androsite-(La), a new mineral of the epidote group from Andros island, Greece. American Mineralogist, 81, 735–743.
Brese, N.E. and O'Keeffe, M. (1991) Bond-valence parameters for solids. Acta Crystallographica, B47, 192–197.
Brown, I.D. and Altermatt, D. (1985) Bond-valence parameters obtained from a systematic analyses of the Inorganic Crystal Structure Database. Acta Crystallographica, B41, 244–247.
Cairncross, B., Beukes, N. and Gutzmer, J. (1997) The Manganese Adventure: The South African Manganese Fields. Associated Ore and Metal Cooperation Limited, Marshalltown, Johannesburg 2107, Republic of South Africa, 236 pp.
Cairncross, B., Gutzmer, J. and Park, A. (2002) Spektakulä rer Neufund: Poldervaartit aus der N'Chwaning II – Mine, Kalahari, Südafrika. Lapis, 5, 30–34.
Dai, Y., Harlow, G.E. and McGhie, A.R. (1993) Poldervaartite, Ca(Ca0.5Mn0.5)(SiO3OH)(OH), a new acid nesosilicate from the Kalahari manganese field, South Africa: Crystal structure and description. American Mineralogist, 78, 1082–1087.
Downs, R.T., Bartelmehs, K.L., Gibbs, G.V. and Boisen, M.B. Jr. (1993) Interactive software for calculating and displaying X-ray or neutron powder diffractometer patterns of crystalline materials. American Mineralogist, 78, 1104–1107.
Ibers, J.A. and Hamilton, W.C., editors (1974) International Tables for X-ray Crystallography, vol. IV, 366 pp. Kynoch, Birmingham, UK.
Mandarino, J.A. (1981) The Gladstone-Dale relationship. IV The compatibility concept and its application. The Canadian Mineralogist, 19, 441–450.
Marsh, R.E. (1994) A revised structure for α-dicalcium silicate hydrate. Acta Crystallographica, C50, 996–997.
North, A.C.T., Phillips, D.C. and Mathews, F.S. (1968) A semiempirical method of absorptioncorrection. Acta Crystallographica, A24, 351–359.
Nyfeler, D., Armbruster, Th., Dixon, R. and Bermanec, V. (1995) Nchwanigite, Mn2+ 2 SiO3(OH)2·H2O, a new pyroxene-related chain silicate from N'chwaning mine, Kalahari manganese field, South Africa. American Mineralogist, 80, 377–386.
Robinson, K., Gibbs, G.V. and Ribbe, P.H. (1971) Quadratic elongation; a quantitative measure of distortion in coordination polyhedral. Science, 172, 567–570.
Sheldrick, G.M. (1997) SHELXL-97. A program for crystal structure refinement. University of Göttingen, Germany.
Yano, T., Urabe, K., Ikawa, H., Teraushi, T., Ishizawa, N. and Udagawa, S. (1993) Structure of α-dicalcium silicate hydrate. Acta Crystallographica, C49, 1555–1559.


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Olmiite, CaMn[SiO3(OH)](OH), the Mn-dominant analogue of poldervaartite, a new mineral species from Kalahari manganese fields (Republic of South Africa)

  • P. Bonazzi (a1), L. Bindi (a2), O. Medenbach (a3), R. Pagano (a4), G. I. Lampronti (a1) and S. Menchetti (a1)...


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