The bentonites of Sardinia: occurrences, geology, mineralogy and properties

The main Italian reserves of bentonite occur in the Oligocene-Miocene calc-alkaline volcanic rocks of Sardinia. The most important occurrences, mainly located in Gallura, Logudoro, Nurra, Sarcidano and Sulcis sub-areas, are: I) in the ignimbrite sequences, as interlayered bodies, II) in single lenses, originally formed from volcanic ash, interlayered in sedimentary Miocene sequences. The ignimbrite depositional sequence, hosting the I) deposits, has a prevalent rhyolitic-rhyodacitic composition. Several Units form this sequence generally consisting of a lower welded part, rich in glass and poor in crystals and lithics, and an upper part, where pumice and volcanic ash prevail. The mineralization is mainly located in the upper part of the Units where the pumice and ash-rich lauers occur. The II) deposits, formed by alteration of single cineritic lenses, are far from the volcanic centers. The mineralogical composition of Sardinian bentonites varies in the different deposit. Generally, the smectite group minerals prevail and the subordinate minerals are quartz, cristbalite, K-feldspar, plagioclases and zeolites. Chemical analyses show that Sardinian bentonites are of calcium-magnesium type. For the type I) deposits, a deuteric alteration seems to be the most probable hypothesis to explain their genesis, supported by the observed preservation of textures of the parent rocks. The type II) deposits were formed by alteration of volcanic ash in fresh waters and the alteration processes were favoured by the porosity of the cineritic volcanic lenses. The bentonite deposits of Ittiri, Villanovatulo, Trinità d'Agultu, Uri and Neoneli, are economically interesting for both the reserves and mineral-tecvhonological properties, which allow intensive exploitation for various industrial uses (e.g., bonding clay for foundry uses, drilling muds).