Mg-Phyllosilicates crystal cremistry of the Andrici sepiolite deposit (Serbia)

Trabajo presentado en la 2nd European Mineralogical Conference (EMC), celebrada en Rimini (Italia) del 11 al 15 de septiembre de 2016The sepiolite deposit Andri i is located in the Pranjani Basin, in the western part of Serbia. The studied section of Andri i deposit represents the sepiolite-bearing succession characterized by rhythmic alternation of clay-rich and dolomite-rich intervals. The section started with claystone dominated by detrital material derived from the surrounding paleorelief of ultramafic rocks. In this basal part smectites are inherited but smectites deposited from solution are also present. In dolomicrite marls, sepiolite occurs as interwoven fibres which coat and fill pores between dolomite crystals while, in the clay-rich layers, sepiolite consists of fibres forming planes. The texture in these layers indicates that sepiolite crystallized directly from solution. The chemical composition of individual particles was obtained using analytical electron microscope (AEM) with TEM, using a JEOL 3000 FX Field Emission Microscope at an acceleration voltage of 300 kV that incorporates an OXFORD ISIS EDX spectrometer. In these materials, different Mg-phyllosilicates have been found and it is remarkable that all these phyllosilicates are rich in Fe. In the basal detrital material, Fe-rich chrysotile and two types of smectites appear. From the structural formula obtained, the two smectites can be classified as Fe-rich saponite. In the rest of the section, sepiolite is the main mineral and it can occur with trioctahedral smectites, with dolomite, or almost pure. The crystal chemistry of these three groups is slightly different: i) in dolomitic marls sepiolite has the octahedral content close to the ideal, with very low amounts of VIAl and Fe3+ and also small amounts of tetrahedral substitutions. It is remarkable that Fe3+ appears as tetrahedral cation in the mean structural formulae in most samples, which is not very common in sepiolite. ii) The sepiolites that appear with smectites are characterized by the high amount of tetrahedral substitution, while the octahedral content is close to the ideal sepiolite. In these samples, as in the dolomitic levels Fe3+ is needed to fill the tetrahedral positions indicating the low availability of Al in the sedimentary ambience. iii) Finally, the purest sepiolite level presents a composition relatively close to the ideal sepiolite whit only small amounts of Al as tetrahedral and octahedral cation.Peer reviewe