Zeolitization of Tuffaceous Rocks in the Keşan Region, Thrace, Turkey

Fahri Esenli,Ö Işık Ece,Vildan Esenli,Bektas Uz,Işık Kumbasar,Fikret Suner

doi:10.4154/gc.2005.08

Abstract

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.

Highlights

The pyroclastic rocks have extensive zeolite reserves in nature
A 33 metre thick pyroclastic-rich zone of the Mezardere formation of Oligocene age is exposed in the Keşan region of Thrace, Turkey
The crystallization sequence of minerals and reaction products during rock–solution interaction depends on the chemical composition of the starting material, contact of solution with volcanic glass and minerals of the pyroclastic rocks, rock–solution interaction time, temperature, pressure, pH, geological environment and hydrological systems (BOLES, 1988; BARTHWIRSHING & HOLLER, 1989)

Summary

Introduction

The pyroclastic rocks have extensive zeolite reserves in nature This is due to their high volcanic glass content and porosity. The crystallization sequence of minerals and reaction products during rock–solution interaction depends on the chemical composition of the starting material, contact of solution with volcanic glass and minerals of the pyroclastic rocks, rock–solution interaction time, temperature, pressure, pH, geological environment and hydrological systems (BOLES, 1988; BARTHWIRSHING & HOLLER, 1989). The Si/Al ratio of mordenites ranges from 4.2 to 5.9, and their cation contents per unit cell are 1.6–2.5 for Ca, 2.0–5.0 for Na and 0.1–0.8 for K (PASSAGLIA, 1975). The cation contents in heulandites and clinoptilolites are highly variable and Ca-, Na, K- and Sr-heulandites and Ca-, Na- and K-clinoptilolites are commonly known (COOMBS et al, 1997)

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