Mineral weathering in trachydacitic-derived soils and saprolites involving formation of embryonic halloysite and gibbsite at Mt. Amiata, Central Italy

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.