『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