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

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.