EVIDENCE OF A PRECURSOR IN THE NEOFORMATION OF PALYGORSKITE - NEW DATA BY ANALYTICAL ELECTRON-MICROSCOPY

The rocks of the palygorskite deposit at Bercimuel (Segovia, Spain) ha ve been studied by transmission electron microscopy and microanalysis. These rocks correspond to the zone of convergence of two alluvial fan systems that have filled the small basin of the River Riaza and would originally have been composed of illite and quartz silts. Among the a ccumulations of palygorskite it is possible to observe surrounded part icles of micromicas that have undergone dissolution and opening thereb y giving rise to disordered illite-smectite mixed-layer clays. This pr ocess continued up to the individualization of structural relics forme d of units of 1-5 layers. At the same time, the chemical composition w as modified with a loss of K and Al and a relative increase in Si and Mg, progressively evolving towards the composition of palygorskite. Th e paleogeographic position of the deposits, and the climatic condition s (arid environment), appear to be the dominant factors in the neoform ation of palygorskite by alteration of the original sediments. The bas ic mineralogical process could be referred to as 'early diagenesis' in the formation of calcretes.