A Calcian Ralstonite-like Mineral from the Cleveland Mine, Tasmania, Australia

W. D. Birch and A. Pring
Department of Mineralogy and Petrology, Museum of Victoria, 285 Russell Street, Melbourne, Victoria, 3000
Department of Mineralogy, South Australian Museum, North Terrace, Adelaide, South Australia, 5000

Abstract: 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.

Keywords: ralstonite • Cleveland Mine • Tasmania • Australia

Mineralogical Magazine; December 1990 v. 54; no. 377; p. 599-602; DOI: 10.1180/minmag.1990.054.377.11
© 1990, The Mineralogical Society
Mineralogical Society (www.minersoc.org)