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The origin and control of colour of white bentonites from the Aegean islands of Milos and Kimolos, Greece

Christidis Georgios, P.W. Scott

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Some of the Lower Pleistocene bentonites of Milos and Kimolos islands, Greece, are valued for their white colour and physicochemicalproperties. They contain opal-CT and, sometimes, zeolite along with smectite, and have been derived from the alteration ofrhyolitic volcanic rocks. The Miloan white bentonites contain Tatatilla-type montmorillonite and beidellite. The Kimolianones have Chambers-type montmorillonite. The alteration process involved removal of alkalis and uptake of Mg, probably fromsea water. Si is either redistributed or partially removed. The Kimolian white bentonites have higher brightnesses, L* and whiteness index values, and lower yellowness index and ΔE*ab values compared with the Miloan ones. The variations in white colour are inversely related to the abundance of Fe oxidesand anatase, the occurrence of Fe in the smectite structure and its oxidation state. The presence of silica minerals is notan important factor affecting colour, but is undesirable since it imparts high abrasiveness in commercial products. The formationof white bentonites of high quality requires the removal of alkalis and silica during alteration of acid volcanics in orderto avoid crystallization of zeolites and opal-CT. Fe needs to be incorporated into the smectite structure. Such conditionsare rarely attained.

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