The mineralogy, geochemistry, physical properties and possible industrial applications of volcanic zeolitic tuffs from Santorini and Polyegos Islands, Greece.

The microcrystalline vitric mass of Pliocene volcanic tuffs of Santorini and Polyegos islands, in the South Aegean Volcanic Arc, has been replaced by authigenic zeolites and clays. The Santorini tuffs are dominated by heulandite type 2 and 3, the classification based on thermal tests, illite/smectite phases, and mordenite, opal-CT, cristobalite, feldspar, quartz and amphibole. The composition of the heulandite minerals can range from (Na2.12K2.56 Ca0.40Mg0.19) (Si29.97A16.08O72) 24H2O and (Na2.60K2.28Ca0.48Mg0.27) (Si29.71 A16.20O72) 24H2O, where Na and K dominate in different proportions, to more calcic members (Na1.56K1.26Ca1.19Mg0.58) (Si29.35A16.71 O72) 24H2O where Ca and Mg dominate. Microprobe analyses classify the heulandites as type 3. Heulandite classification, then, requires the combined use of thermal tests and microprobe analyses. The heulandites show a platy crystal habit and they form, by the mean of dissolution, pseudomorphs after the inner parts of glass shards. The tuffs from the north part of Polyegos island are dominated by mordenite and illite, rarely illite/smectite, along with heulandite type 3, opal-CT, kaolinite, quartz and feldspar. The composition of the mordenite includes, as well as the typical Na-Ca members, with analyses within the known field of mordenite composition, mordenites with unusual high K values. So the formula can range from a (Ca0.68Mg0.49Na4.05K0.52) (Si41.06Al6.94O96) 24H2O where Na and Ca dominate and the Na/K ratio is 7.79 to (Ca1.20Mg0.20Na2.31K2.39) (Si40.61A17.41O96) 24H2O where the Na/K ratio is 0.97. The mordenites show a fibrous habit and replace the inner part of lapillus pumice fragments. They were probably formed by the circulation of hydrothermal fluids through the volcanic tuffs, followed by the emplacement of rhyolitic lavas in the area. The hydrothermal alteration event led to the formation of numerous baryte and galena veins and flats. The examination of other crystal phases present in Santorini tuffs indicates that not all of the products in the Santorini volcanic suite were the result of the involvement of more fertile asthenosphere, as was believed. Volcanic products, like the tuffs examined, show mineral chemistry similarities with their counterparts from the two ends of the volcanic arc. Therefore, the maximum crustal thinning under the central part of the South Aegean Volcanic Arc, which is responsible for this different source for the generation of magmas in this area of the Arc, had not been completed before the Upper Pliocene, at the very earliest. The examination of geochemistry reveals that the parent materials were dacites to rhyodacites in Santorini and dacites-rhyodacites to trachyandesites-trachytes in Polyegos. The volcanic tuffs from both areas are the product of a fractional crystallisation process with most distinctive evidence found in Polyegos. Santorini shows evidence of magma mixing process also. The traditional cation exchange capacity methods require modification when used with tuffs rich in zeolites regarding the saturation time and the ammonia measurement techniques. Nevertheless, the zeolitic tuffs exhibit excellent cation exchange properties higher than it was expected. Their great ability to combine with lime, in water saturated environments, and act as pozzolanic materials, improves the performance of concrete mixtures. Mixtures which are made by replacing ordinary Portland cement, for calcined zeolitic materials and in various proportions, are able to reach and maintain compressive strength values, higher than standard concrete cubes which are made of 100% Portland cement as the binding agent. Finally, using relatively cheap mineral separation techniques, like dry sieving, fractions rich in zeolite minerals can be easily obtained.