More than 20 bentonite deposits crop out on the islands of Milos, Kimolos and Chios, Aegean Sea, Greece many of which are currently under exploitation. The bentonite deposits have been formed at the expense of volcaniclastic rocks, probably pyroclastic flows in the majority of the deposits, under subaqueous conditions. The presence of abundant authigenic K-feldspar in most deposits suggests that alteration was diagenetic and took place at very low temperatures.\ud Alteration of the glass involves mobilization of alkalis, and uptake of Mg and S. Al and Ti are essentially immobile while the behaviour of Si and Ca is controlled by both the nature of the parent rock and the composition of smectite. Fe displays small scale migration controlled by the prevailing redox conditions. Zr, Nb, Cr, Ni, V and the LREE are essentially immobile, while Ba, Sr, Rb, Zn, Y and the HREE are mobile. The behaviour of Th is controlled by the phase which hosts the LREE. The mobility of chemical elements has caused variation in the chemistry of the microenvironment in which smectites formed, resulting in large variations in the chemistry of smectites. Smectites might have been formed through an Ostwald Ripening-like process affecting the pore fluid chemistry and thus the chemistry of other phases like zeolites. Beidellites coexist with Cheto- but not with Wyoming-type montmorillonites. The crystal chemistry of smectites is affected by the nature of the parent rock, but the conditions prevailing during alteration might modify this "inherited" factor, as indicated by 1000 EPMA microanalyses which complement XRF, XRD, IR, DTA/TG, SEM, TEM and HRTEM data.\ud Almost all deposits have been affected by a later hydrothermal alteration which has effectively "diluted" the original smectite content either by conversion of smectite to illite/smectite, kaolinite/halloysite and/or alunite or by precipitation of new phases (carbonates, phosphates, sulphides). Illitization of smectite probably proceeds by an Ostwald ripening-like mechanism characterized by low supersaturation conditions.\ud The Greek bentonites have good swelling properties after Na-activation, and cation exchange capacity which ranges from 40 to 105meq/100gr. These properties are closely related in bentonites containing smectites with similar crystal chemistry. The bentonites with low CEC contain abundant opal-CT. The rheological properties vary and are probably related with the degree of disaggregation of the smectite tactoids. Low grade bentonites might be suitable for the drilling industry.\ud Acid activation increased the surface area of the bentonites up to 5 times and rendered them suitable for decolourization of crude edible oils. However, the maximum bleaching efficiency is not associated with maximum surface area. Mg-smectites are activated easier, but their decolourization properties deteriorate faster than Al-smectites. High grade Greek bentonites have foundry properties comparable to those of commercial products after Naactivation.\ud This treatment increases the wet tensile strength but its effect on the other foundry properties is unpredictable.\ud The properties of the bentonites have been degraded by the superimposed hydrothermal alteration, in general, although in some cases limited kaolinization of smectite seems to have a positive effect on the rheological properties
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