Weathering sequence of rhyolitic minerals: the kaolin deposit of Torniella (Italy)

The Torniella mine (Southern Tuscany, Italy) represents one of the most important Italian deposit of kaolin, formed by hydrothermal weathering of primary rhyolites. Raw material is currently exploited for the ceramic industry. The mining area is heterogeneous, ranging from preserved rhyolites to completely weathered rocks; all the different lithologies have been sampled and investigated by XRPD, XRF and SEM-EDS techniques, to reconstruct the overall weathering sequence. Early weathering, characterized by low fluid/rock ratios, first affects plagioclase and biotite, replaced by crystalline kaolinite (possibly associated with poorly-crystalline halloysite) along cleavage planes and crystal edges; later, weathering affects the glassy groundmass, which is replaced by a microgranular association of silica and kaolin. The last weathering stage, characterized by high fluid/rock ratios, pervasive fluid circulation and fast reactions, produces a nearly-pure kaolin material, consisting of kaolinite and halloysite, in variable proportions; kinetically-favoured, fluid-enhanced halloysite occurs only in most weathered samples. Sanidine is completely replaced, whereas quartz relics may persist. Fine-grained pseudocubic crystals of alunite are commonly associated to feldspars weathering; alunite occurrence and whole-rock sulfur content (increasing with weathering extent) suggest circulation of sulfur-rich hydrothermal fluids.