PHYSICAL AND CHEMICAL-PROPERTIES OF BENTONITE DEPOSITS OF MILOS ISLAND, GREECE

The bentonite deposits of Miles Island, Aegean, Greece, which formed f rom low-temperature alteration of pyroclastic rocks, have been investi gated to determine physical and chemical properties and group them acc ordingly. These properties include the cation exchange capacity, swell ing capacity, pH, rheological properties and some important foundry pr operties, such as the green and dry compressive strengths, shatter ind ex, wet tensile strength, compactability and permeability. In principl e, deposits with abundant opal-CT develop poor physical properties. Th e rheological and swelling properties are influenced mainly by the ben tonite grade (smectite content) and the degree of disaggregation of th e smectite quasi-crystals. The latter might have been caused by such f actors as the nature of the interlayer cation, the smectite layer char ge and its localization, the degree of oxidation of iron and the type of smectite-opal-CT interaction. Deposits in which opal-CT forms aggre gates separated from smectite develop viscous suspensions, but their g rade is low. Only deposits of high grade that are free of opal-CT deve lop good foundry properties. Both the foundry and the rheological prop erties are influenced by swelling, since only bentonites of high swell ing capacity form viscous suspensions and develop good bonding propert ies. Moreover, viscous fluids result after the smectite flakes have sw ollen beyond a certain degree. Hydrothermal alteration is detrimental to the physical and chemical properties of bentonites in that it decre ases the smectite content and the pH and changes the smectite layer ch arge and the lattice dimensions.