13Silica stromatolites occur in a number of modern hydrothermal environments, but their
Palygorskite in Miocene mudstones, palustrine limestones and calcretes from the Esquivias locality (Madrid Basin, Spain) has been analyzed by X-ray diffraction, infrared spectroscopy, scanning electron microscopy, transmission electron microscopy and analytical electron microscopy to determine its characteristics and chemical composition. Other palygorskites from the literature are used as references. The mean structural formula obtained from the analysis of isolated particles is (Si7.8 7Al o. 1 3 )020(All.04 Fe6:' 2o Mg 3 .11 DO.6S)(OH)2(OH2)4(CaO.02Ko.osNao.08). This palygorskite has the largest Mg content reported in the literature, and it seems that, chemically, it fills the 'compositional gap' existing between sepiolite and palygorskite. Infrared spectroscopy reveals the absence of trioctahedral Mg and therefore the possibility of the existence of magnesic clusters in the ribbons is discounted. An homogeneous distribution of the octahedral cations (AI, Fe 3 + and Mg) along the ribbons is proposed.
RESUMENLos depósitos distales de los abanicos aluviales arcósicos y también los carbonatos palustres del área de Paracuellos de Jarama presentan rasgos muy específicos debidos a los procesos edáficos y diagenéticos que han sufrido. Los procesos que han afectado a estos sedimentos, y su intensidad, estuvieron controlados por: el ambiente sedimentario, la litología y textura/estructura inicial del material y también por el ambiente pedogéni-co-diagenético superficial en el que se han desarrollado las transformaciones. Así, en las lutitas de las zonas distales de los abanicos (perfil PEL) se forman inicialmente láminas de carbonato edáfico y sepiolita, posteriormente tiene lugar la silicificación y, a continuación, la formación de dolomías esencialmente esferoidales. En ambientes lacustres (perfil UPC), la intensa desecación y la actividad biológica conducen a la formación de calizas palustres que posteriormente se silicifican conservando la textura y estructura. La silicificación es, en gran parte, un reemplazamiento pseudomórfico que origina principalmente ópalo, mientras que el cuarzo se forma por cementación y también por envejecimiento del ópalo. En ambos ambientes, cuando la silicificación se produce sobre arcillas, probablemente sepiolíticas, o sobre carbonatos muy ricos en sepiolita, se generan ópalos atípicos de microestructura fibrosa. Este proceso puede liberar magnesio, que posteriormente puede ser utilizado para la formación de dolomita de posible origen bacteriano sobre el sílex opalino (perfil PEL). Todos estos procesos tuvieron lugar en ambiente freático somero o incluso vadoso.Palabras clave: caliza palustre, sílex opalino, dolomía esferoidal, silicificación de sepiolita,freáti-co-vadoso. ABSTRACTThe distal aluvial fan and the palustrine deposits of the Paracuellos de Jarama area display a variety of specific pedogenic and diagenetic features. The processes affecting these sediments were controlled by: the sedimentary environment, the lithology and initial texture of the sediments and by the pedogenic-diagenetic environment in which the transformations occurred. Thus, in the distal alluvial areas (PEL profile) the first processes were the formation of thin carbonate laminae and sepiolite, further, these deposits were silicified and later, a spheroidal dolomite is formed on the opaline chert. In lacustrine environments (UPC profile), drying and root activity led to the formation of palustrine carbonates that were silicified, and the initial texture and structure were preserved. The silicification consisted on a pseudomorphic replacement that produced mainly opal whereas quartz was formed mostly by cementation and ageing of previous opa!. In both environtnents, when the silicification occurs on sepiolite, or micrite with sepiolite, atypical microfibrous opal is produced. This replacement can lead to the release of Mg, favouring a later formation of dolomite on the opaline chert, with evidence of bacterial activity. All these processes occurred in a shallow phreatic or even vadose environment.
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