『Abstract
Mount Amiata is a volcano that has been extinct for 200 ka. It
is located in southern Tuscany, central Italy, and is composed
largely of trachydacitic rocks. The soils of Mt. Amiata show a
dark, carbon-rich A horizon that overlies a slightly structured
Bw horizon or, directly, a very thick saprolite, which at a depth
of about 3 m still maintains some ellipsoidal cores of coherent
material. Mineral weathering in two of these soils, one under
beech and the other under chestnut, was studied in detail. The
mineralogy of the parent rock consists predominantly of volcanic
glass with phenocrysts of feldspar (principally K-feldspar), biotite
and pyroxene. The main weathering products are an embryonic form
of halloysite and gibbsite, while allophane is usually absent
or occurs in minor amounts. SEM evidence indicates that the clay
minerals form directly from the weathering of volcanic glass in
an environment where Si is preferentially depleted in relation
to Al. In the saprolite an anomalous situation is observed where
optically fresh ferromagnesian minerals are thickly coated by
clay particles whereas the surfaces of the glass and feldspar
grains are relatively much cleaner. However, the ferromagnesian
minerals are also much more heavily colonised by fungal hyphae
and it is proposed that this results in the preferential deposition
or flocculation of colloidal suspensions which translocate from
higher positions in the weathering profile.
Keywords: Volcanic soils; Halloysite; Gibbsite; Mineral weathering;
Saprolite; Volcanic glass』
1. Introduction
2. Materials
3. Methods
4. Results
4.1. Mechanical and chemical properties
4.2. Optical microscopy
4.3. X-ray diffraction (XRD)
4.3.1. Bulk material
4.3.2. Clay fraction
4.4. Infrared spectroscopy (IR)
4.5. Scanning electron microscopy (SEM)
4.6. Transmission electron microscopy (TEM)
5. Discussion
5.1. Occurrence of clay mineral constituents
5.2. Weathering of primary mineral constituents
5.2.1. Weathering of glass
5.2.2. Weathering of biotite
5.2.3. Weathering of pyroxene
5.2.4. Fungal weathering
6. Conclusions
Acknowledgements
References