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 3+ -rich phases that locally replace Mn 2+ -rich assemblages in the quarry. EMP analyses supported by ion-probe conform with end-member composition Ba 4 Mn 3+ 4 O 3 (OH) 3 (Si 4 O 12 )(Si 2 O 3 )(OH) 4 Cl with some substitution of (Ca, Sr, Mn 2+ , Na) for Ba, Al for Mn 3+ , and (OH) for Cl. It contains about 1000 ± 500 ppm F. The mineral is tetragonal or, as suggested by optical observation, pseudotetragonal. A tetragonal I 4/ mmm unit cell and X-ray powder diffraction data give: a = 14.219(8) A, c = 6.118(5) A, V = 1237.0(1.7) A 3 ; Z = 2. EMP analyses of noelbensonite 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 Mn 3+ 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 noelbensonite. Late-stage hydrothermal alteration at the Woods Mine was effected by complex interactions with solutions of changing composition.