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Cerchiaraite and Ca-bearing noélbensonite from Woods mine, New South Wales, Australia

¹ Geology Department, University of Otago, Dunedin 9001, New Zealand
² Department of Earth and Planetary Science, Graduate School of Science, Hokkaido University, Japan

^* Corresponding author, e-mail: doug.coombs{at}stonebow.otago.ac.nz

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 in the quarry. EMP analyses supported by ion-probe conform with end-member composition 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) ų; 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.

Key-words: cerchiaraite, noélbensonite, lawsonite group, manganese minerals, Woods mine.