Formation of cryolite and other aluminofluorides: A petrologic review

Abstract

Petrologic reviews are provided for 17 confirmed localities of cryolite, and for occurrences of 19 other aluminofluorides. Four environments can be recognised: granitic, carbonatitic, volcanic and aqueous. Cryolite, partly replaced by other aluminofluorides, forms at the post-magmatic stage of granite evolu­tion in alkali granite pegmatites, albitised riebeckite granites and hydrothermal bodies. The associated magmatic granites are post- or non-orogenic, late intrusions often associated with regional faulting. They are rich in Na and F, poor in Ca, and low in normative anorthite, acmite and sodium silicate. High contents of Rb, Zr, Nb, REE and Li are characteristic. They equilibrated at 1-2 kb P H20 in agreement with their epizonal setting. Experiments in the granite-HlO-HF system develop aluminofluorides in coexisting dense vapours. Equivalent fluids cooled to multi-phase inclusions in the Volyn pegmatites, Ukraine, and contain cryolite and elpasolite. Post-magmatic autometasomatism extracts and then redeposits F, Na and other elements probably via fluids containing fluoriferous complex ions. Quartz-feldspar metasomatites in E Siberia are related to regional metamorphism and deep faulting. Rare carbonatites and silico-carbonatites with high contents of Na concentrate Fin late hydrothermal solutions and produced cryolite at Goldie, Colorado and Francon quarry, Montreal. Ralstonite is widely reported, and elpasolite and weberite. rarely reported, from · fumaroles and HF-metasomatised lavas of basic to intermediate volcanoes. Deposition occurred at 300-100°C. Thermal springs and surface runoff froin volcanoes carry relatively high F contents and occasionally form gearksu­tite and creedite by reaction with aluminosilicate materials. Cryolite is a rare authigenic mineral in sediments of the Green River Formation, W USA. F-rich interstitial brines, developed as in the modem Lake Magadi, Kenya, probably reacted with the host aluminosilicate sediments. Gearksutite and creedite, plus five other recently discovered aluminofluorides, occur within or just. below the oxidation zones, or in the weathering crusts, of some F-rich mineral deposits. Low-temperature precipitated aluminofluorides have been produced synthetically but are un­known in nature. The only economic aluminofluoride, cryolite, was present as several million tons at Ivigh1t, S Green­land. Fluorine was concentrated immediately below the sealed roof of a narrow granite pipe.