Publication: Clay mineral genesis and chemical
evolution in the Miocene sediments of
Somosaguas, Madrid Basin, Spain
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Publication Date
2007
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Blackwell Scientific publications
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Abstract
A mineralogical and microtextural study of Somosaguas Miocene deposits, located
in the Madrid Basin (western Madrid, Spain), was carried out using X-ray diffraction, scanning
electron microscopy, transmission electron microscopy and optical microscopy, whereas crystal
chemistry data were obtained by analytical electron microscopy-transmission electron microscopy
and electron icroprobe analysis. Four stratigraphic sections were studied, compising detrital rocks
representing intermediate and distal facies from alluvial fan deposits. The predominant source area of
these sediments was the granitic rocks of the Spanish Central System with a lesser contribution of
metamorphic rocks. Clayey arkoses are the most abundant rocks of these sections, typical of granite
alteration under warm, semi-arid climates. The mineralogy is characterized by phyllosilicates,
followed by feldspars and quartz. The data obtained reveal mineral mixtures of detrital (quartz,
feldspars, kaolinite, micas and chlorite), transformed (illite and beidellite) and neoformed
(montmorillonite) origin. Clay minerals resulted from interactions between detrital minerals and
meteoric waters. Two trends of degradation of micas are detected. The first shows a transition from
muscovites and dioctahedral illites, to beidellites. The other trend is defined by the biotite
degradation to beidellites with different layer charge and octahedral Fe content. Montmorillonites
were neoformed from the hydrolysis and weathering of primary minerals (feldspars and muscovite).
Magnesian clay minerals such as sepiolite, palygorskite and trioctahedral smectites, extremely
abundant in the centre of the basin, were not detected in Somosaguas sediments.