Viti,C., Lupieri,M. and Reginelli,M.(2007): Weathering sequence of rhyolitic minerals: the kaolin deposit of Torniella (Italy). N. Jb. Miner. Abh., 183/2, 203-213.

『流紋岩を構成する鉱物の風化順序:トルニエラ(イタリア)のカオリン鉱床』


Abstract
 The Torniella mine (Southern Tuscany, Italy) represents one of the most important Italian deposits of kaolin, formed by hydrothermal weathering of primary rhyolites. The 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 XRD, 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 feldspar weathering; alunite occurrence and whole-rock sulfur content (increasing with weathering extent) suggest circulation of sulfur-rich hydrothermal fluids.

Key words: hydrothermal weathering; rhyolite; kaolinite; halloysite; alunite; ceramic material』

Introduction
Methods
Geological setting and samples
Petrographic description
Whole-rock composition and mineralogy: XRF and XRD data
Determination of microtextures and mineral compositions by SEM-EDS
Discussion and conclusions
 The weathering sequence
 Glassy vs. microgranular groundmasses: late magmatic crystallization or low-T weathering?
 Nature of fluids responsible for kaolinization and S-bearing phases distribution
 Economic importance of the Torniella kaolin deposit
Acknowledgements
References


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