Cerchiaraite and Ca-bearing noe´lbensonite from Woods mine, New South Wales, Australia

Authors: Coombs, Douglas S.; Kawachi, Yosuke; Miura, Hiroyuki; Chappell, Debra

Source: European Journal of Mineralogy, Volume 16, Number 1, 1 January 2004 , pp. 185-189(5)

Publisher: E. Schweizerbart'sche Verlagsbuchhandlung

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Abstract:

Cerchiaraite occurs as tiny blocky crystals and sheafs of irresolvable fibres in the Woods mine in northern New South Wales, Australia. It forms veinlets from <10 to ~90 µm wide in fractures cutting serandite-quartz-namansilite-ungarettiite assemblages. It also replaces these minerals in the rock matrix. It is accompanied by and locally replaces noélbensonite, but in much smaller amount. These two Ba-bearing minerals are among the latest Mn³⁺-rich phases that locally replace Mn²⁺-rich assemblages Ba₄Mn3+₄O₃(OH)₃(Si₄O₁₂)(Si₂O₃)(OH)₄Cl with some substitution of (Ca,Sr,Mn2+,Na) for Ba, Al for Mn³⁺, and (OH) for Cl. It contains about 1000 ± 500 ppm F. The mineral is tetragonal or, as suggested by optical observation, pseudotetragonal. A tetragonal I4/mmm unit cell and X-ray powder diffraction data give: a = 14.219(8) Å, c = 6.118(5) Å, V = 1237.0(1.7) Å³; Z = 2.

EMP analyses of noélbensonite of both blocky and radial fibrolamellar habit coexisting with the cerchiaraite show a range of Ca : Ba from Ca below detection limit to 0.52 : 0.48. This suggests the possibility of a Ca-dominant species amongst the Mn³⁺ analogues of lawsonite. It is not known whether the observed limit of approximately 50% Ca for Ba substitution results from limited available Ca or structural features. Ion probe data show about 18 ppm Cl in the noélbensonite. Late-stage hydrothermal alteration at the Woods Mine was effected by complex interactions with solutions of changing composition. in the quarry. EMP analyses supported b y ion-probe conform with end-member composition

Keywords: CERCHIARAITE; NOELBENSONITE; LAWSONITE GROUP; MANGANESE MINERALS; WOODS MINE

Document Type: Research article

DOI: http://dx.doi.org/10.1127/0935-1221/2004/0016-0185

Publication date: 2004-01-01

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