A CALCIAN RALSTONITE-LIKE MINERAL FROM THE CLEVELAND MINE, TASMANIA, AUSTRALIA

A calcium-rich ralstonite, forming colourless octahedral crystals up to 2 mm across, occurs in a F-rich assemblage at the Cleveland Mine, northwestern Tasmania. Other minerals present include well-crystallized morinite, gearksutite, vivianite, siderite, K-rich feldspar (adularia), fluorite and quartz. The F-rich assemblage probably formed during greisenization when Na and F-rich magmatic brines reacted with enclosing carbonate host rocks to produce hydrothermal Na-Ca-Mg-bearing solutions. Chemical analysis of the ralstonite-like mineral gave a formula (Na1.47Ca0.52) (Mg1.49Al0.55P0.04) F6 [(OH)0.43O0.36F0.21)]. This differs from the 'pyrochlore' formula, due to the substitution of Ca + (OH,O,F) for H2O, in addition to the coupled substitution of Na + Mg for Al in 'normal' ralstonites. The Cleveland Mine ralstonite has the highest recorded Ca, Na and Mg contents. While the mineral is likely to be a new species on chemical grounds, single crystal X-ray photographs indicate the structure is disordered, possibly composed of small compositional domains.