Mineral and Chemical Composition of Rudist Valves from Upper Cretaceous Limestones of Southern Istria, Croatia

Abstract

Turonian to late Santonian limestones of southern Istria are rich inrudist remains. The main aim of this study was to determine themineral and chemical composition of 22 samples of rudist valvesbelonging to the genera Durania, Praeradiolites, Radiolites, Gorjanoviciaand Vaccinites, and to explain any observed differences.X-ray diffraction and chemical analyses showed that in all the analysedsamples, the primary constituents of rudist shells, aragonite andlow-Mg calcite, were transformed into diagenetic low-Mg calcite.The strontium concentrations of all the analysed shells correspond topelagic bulk rock samples altered by diagenesis and are significantlylower than those for biological calcite. Observed differences in thechemical composition of diagenetically altered rudist shells belongingto different rudist genera, namely a slightly higher content of Srin specimens of Gorjanovicia and Durania and lower concentrationsof Mg in shells of Vaccinites are most probably the result of differentshell structures and original mineral constituents.