"This paper presents X-ray diffraction and SEM evidence for the formation of alunite, and possibly small quantities of natroalunite, within opal-A stalactites formed on quartz sandstone near Sydney in south-eastern, Australia. Alunite has been reported as a speleogenetic mineral from sediments within a number of caves around the world, but this is believed to be the first report of speleothemic alunite in opaline silica speleothems. Individual alunite crystals have not been visually identified, but SEM X-ray element mapping suggests the alunite has formed amongst kaolinite clay. Sedimentary alunite and natroalunite formation is usually associated with the reaction of sulphuric acid on illite, smectite and kaolinite clay materials. In this location groundwater sulphate levels are not high, but evaporative concentration of stalactite drip-water containing small amounts of sulphuric acid generated by oxidization of pyrite might lower the pH to a level sufficiently acidic for conversion of kaolinite or illite to alunite. The ferrolysis of hydrous Fe(2+)-oxides, or the biochemical activities of bacteria or other micro-organisms, also provide conceivable pathways for the generation of pH sufficiently low to contribute to alunite formation. The occurrence of alunite in these silica stalactites, whilst unusual, is consistent with the normal silica stalactite-forming process in this region, and in accord with observations of the authigenic formation of alunite and groundwater opal in weathering profiles elsewhere."