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 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) 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.

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