engleski

Mineralogy of altered tephra layers in the Upper Jurassic Lemeš deposits near Maovice (Dalmatia, Croatia)

Upper Jurassic Lemeš deposits in the External Dinarides represent sedimentary succession comprised of light coloured platy limestones and/or dolomitic limestones and dolomite (BRAUN, 1991 ; VELIĆ et al., 1995) with chert intercalations. Two bentonite layers near Maovice village in the Dalmatian hinterland intercalate within typical Lemeš deposits (BRAUN, 1991). The material for investigations has been sampled from bentonite layers on the localities Gornji Ždanj and Bunarić. The XRD analyses of samples revealed smectite mineral as the principal mineral in both samples accompanied by calcite, cristobalite, plagioclase, quartz, biotite and augite in the Bunarić sample and calcite, quartz, zeolite and cristobalite in Gornji Ždanj sample. Dioctaedrical smectite mineral was determined on oriented samples on the basis of maxima with d(001)≈ 14.83 &Aring ; , d(001)≈ 15.02 &Aring ; (air dried), d(001)≈ 16.68 &Aring ; , d(001)≈ 16.62 &Aring ; (etilenglycole saturated), d(001) ≈ 9.79 &Aring ; , d(001) ≈ 9.58 &Aring ; (heated) for Bunarić and Gornji Ždanj samples, respectively. To determine the heulandite group zeolite procedure after ALIETTI, 1972 was used. X-ray powder patterns were recorded on the original Gornji Ždanj sample (d(020)≈ 8.98 &Aring ; ), and on samples heated at 400°C for 24 hours and at 550°C for additional 24 hours. The observed contraction(d(020)≈ 8.35 &Aring ; ), and destruction of the lattice indicate heulandite type 1. SEM photographs of samples, coated with carbon, were made using TESCAN Scaning Electron Microscope equipped with EDS. Alteration of tephra to the assemblage consisting exclusively of montmorillonite and heulandite suggests diagenesis at around pH~8, typical for the relatively deep marine setting within the carbonate platform. Different fossils, which still have to be described, are abundant in both samples.

bentonite; altered tephra layers; Upper Jurassic; Lemeš deposit; Maovice; Dalmatia; Croatia

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