Greece is a major bentonite producer, with Milos Island being one of the largest bentonite mining centres in the world. In addition to the bentonite deposits which are well known, various other occurrences exist in the islands of Chios and Samos, Eastern Aegean and in some areas of Thrace, in NE Greece. These bentonites are associated with volcanic activity and their age ranges from Lower Oligocene in Thrace, to Lower–Middle Miocene in Chios and to Upper Miocene in Samos. Although some of these materials have been utilized since ancient times (e.g. the Samian Earth), these bentonites have been mined only at a local scale. In this study the first data on the mineralogical and geochemical characteristics of these bentonites are presented.
The Samos bentonites of Upper Miocene age are medium–low grade (40–70% smectite) and crop out in the SE margin of the Karlovasi Basin formed at the expense of acidic pyroclastic flows mainly in a subaerial-lacustrine environment. They consist mainly of Ca-rich medium–high-charge montmorillonite, opal-CT and sanidine with minor quartz and locally mordenite. The Chios bentonites of Lower–Middle Miocene age were formed at the expense of trachyandesitic tuffs in a lacustrine environment in the Neogene Basin at the SE part of the island. They consist mainly of Ca,Mg-rich high-charge montmorillonite (33–75%), similar to the SAz-1, and opal-CT, minor plagioclase, chlorite, plagioclase, carbonates and traces of talc and serpentine. In Thrace, Lower Oligocene pyroclastic flows, tuffs and lavas of acidic-intermediate composition were altered to zeolites and bentonites in the sedimentary basins of the Feres-Pefka, Skaloma and Sappes areas. The parent pyroclastic rocks were altered to Ca-rich and in places, Na-rich, dioctahedral montmorillonite or Fe-rich beidellite (Sappes area) and with minor opal-CT, mordenite, quartz, plagioclase and, in places, illite. The bentonites are medium–high grade (50–58% smectite in Feres-Pefka, 74–86% in Skaloma and 29–40% in Sappes).
The bentonites have moderate cation exchange capacity and moderate/low swelling index and viscosity and they are not suitable for the drilling and foundry industries. However, preliminary results show that after processing, most of the deposits might be utilized in high added-value applications such as bleaching earths or in the synthesis of clay-based polymer nanocomposites.