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Alunite formation within silica stalactites from the Sydney Region, South-eastern Australia

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Author(s): Wray Robert A.L.

Journal: International Journal of Speleology
ISSN 0392-6672

Volume: 40;
Issue: 2;
Start page: 109;
Date: 2011;
Original page

Keywords: alunite | natroalunite | opal-A | speleothem | stalactite

ABSTRACT
This paper presents X-ray diffraction and SEM evidence for the formation of alunite, and possibly small quantities of natroalunite, withinopal-A stalactites formed on quartz sandstone near Sydney in south-eastern, Australia. Alunite has been reported as a speleogenetic mineralfrom sediments within a number of caves around the world, but this is believed to be the first report of speleothemic alunite in opalinesilica speleothems. Individual alunite crystals have not been visually identified, but SEM X-ray element mapping suggests the alunite hasformed amongst kaolinite clay. Sedimentary alunite and natroalunite formation is usually associated with the reaction of sulphuric acidon illite, smectite and kaolinite clay materials. In this location groundwater sulphate levels are not high, but evaporative concentration ofstalactite drip-water containing small amounts of sulphuric acid generated by oxidization of pyrite might lower the pH to a level sufficientlyacidic for conversion of kaolinite or illite to alunite. The ferrolysis of hydrous Fe2+-oxides, or the biochemical activities of bacteria or othermicro-organisms, also provide conceivable pathways for the generation of pH sufficiently low to contribute to alunite formation. The occurrenceof alunite in these silica stalactites, whilst unusual, is consistent with the normal silica stalactite-forming process in this region, and inaccord with observations of the authigenic formation of alunite and groundwater opal in weathering profiles elsewhere
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