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A 33 metre thick pyroclastic-rich zone of the Mezardere formationof Oligocene age is exposed in the Keşan region of Thrace, Turkey.In this zone, vitreous tuffs of dacitic composition have altered primarilyto zeolites, including mordenite, heulandite–clinoptilolite andanalcime. Silicification and alteration to clay minerals are common.Zeolite minerals have developed from volcanic glass, whereas somemordenites have formed from dissolution of heulandite-group zeolites.Although authigenic mineral paragenesis does not vary laterally,there is a marked vertical variation, particularly in zeolites. Mordenite(+heulandite–clinoptilolite) and analcime do not coexist and haveformed in different stratigraphic levels. This suggests that their chemicalenvironment is controlled by different hydrologic systems.Whole rock composition shows the relationship between chemistryand secondary mineralogy. For example, whole rock trace elementgeochemistry indicates the natural selectivities of zeolites. There isalso stratigraphic control on the chemistry and texture of mordenites.Specifically, (Na+K)/(Ca+Mg) ratios of mordenites decrease fromthe lower to the upper levels. Mordenites of the lower level show afibrous habit while the upper level mordenites are needle-like inshape. The average Si/Al ratio in mordenites is 3.90, in heulanditegroupminerals 3.95, and in analcimes 2.34.
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