Arcillas fibrosas anómalas en encostramientos y sedimentos superficiales: características y génesis (Esquivias, Cuenca de Madrid)

Abstract

Se estudian silcretas, calcretas/calizas palustres y niveles arcillosos de zonas continentales superficiales con objeto de caracterizar la mineralogía de las arcillas fibrosas, su composición química y las condiciones de formación. La caracterización mineralógica, petrológica y geoquímica se realiza con difracción de Rayos X (DRX), microscopía óptica, microscopía electrónica de barrido (MEB) y de transmisión (MET) que llevan asociados espectrómetros de energías dispersivas (EDS). En los niveles arcillosos puros y en los encostramientos siliceos las arcillas fibrosas son paligorskita y sepiolita, mientras que en las calcretas/calizas palustres normalmente solo aparece paligorskita. Ambas arcillas presentan anomalías en su composición, siendo las sepiolitas más ricas en Al que lo usual y las paligorskitas más magnésicas. En los niveles arcillosos la presencia de vestigios de esmectitas puede indicar que los minerales fibrosos resultan de ellas por disolución y posterior precipitación, mientras que en los encostramientos, la ausencia de esmectitas, puede indicar que se forman directamente a partir de fluidos intersticiales. En las calcretas/calizas palustres parte de la paligorskita es posterior a la calcita y su proceso de formación estaría favorecido por procesos de disolución de la calcita dentro de la calcreta /caliza palustre. Las silcretas están formadas por un proceso de silicificación de las calcretas/calizas palustres. La paligorskita es un relicto de la roca original, mientras que la sepiolita es un mineral neoformado junto con el ópalo durante el proceso de silicificación, o algo después, durante el envejecimiento de un gel sílico-magnesiano. Palabras clave: Sepiolita, paligorskita, silicificación, ambiente superficial, geles siliceos. Uncommon fibrous-clays in duricrust and near surface sediments: features and genesis (Esquivias, Madrid basin)Mineralogy, chemical composition and textures of silcretes, calcretes/palustrine limestones and argillaceous beds are studied to consider the features and the genesis of fibrous-clay minerals. The mineralogy, petrology and geochemical studies were carried out by X-ray diffraction, optical microscopy, and, scanning (SEM) and transmission (TEM) electron microscopy with dispersive analysis (EDS). Sepiolite and palygorskite were found in the argillaceous beds and silcretes while only palygorskite was found in the calcretes/palustrine limestones. The chemical compositions of sepiolite and palygorskite are anomalous. The sepiolite shows Mg/Al relations lower than the standard, and the palygorskite higher than standard. Smectites occurs in the argillaceous beds, and the dissolution of these smectites can generate appropriated cations for the formation of sepiolite and palygorskite. The absence of smectites in the silcretes and calcretes/palustrine limestones could suggest that the fibrous clay minerals were formed by direct precipitation from interstitial water of these rocks. Part of the palygorskite included in the calcretes/palustrine limestones formed latter than calc...

Spanish palygorskites: geological setting, mineralogical, textural and crystal-chemical characterization

“…This small deposit is located in the centre of the Iberian Peninsula. Occurrence of palygorskite in this area has been reported repeatedly (Leguey et al, 1985;Pozo et al, 1985;García-Romero, 1988;Bustillo & García-Romero, 2003;García-Romero et al, 2004;García-Romero & Suárez, 2010), in the lacustrine and distal alluvial fans deposits of the Intermediate Unit (Miocene) of the Madrid Basin, part of the Tajo Basin. To the northeast of the town of Esquivias, thin argillaceous beds are interlayered with calcrete/palustrine limestones.…”

“…To the northeast of the town of Esquivias, thin argillaceous beds are interlayered with calcrete/palustrine limestones. Palygorskite occurs together with calcite and other minerals, mainly sepiolite, smectites and opal (Bustillo & García-Romero, 2003). The origin of this palygorskite is from direct precipitation of interstitial waters present in the calcrete/palustrine limestones.…”

Authigenic Silicate Minerals – Sepiolite-Palygorskite, Zeolites and Sodium Silicates

“…Other types of precursors include opaline material (Blank & Fosberg, 1991) and unidentified spherical aggregates (Martin de Vidales et al, 1987). A relationship between palygorskite formation and calcite dissolution has also been suggested (Bustillo & García Romero, 2003). TEM observations of replacement of palygorskite or sepiolite by another phase are reported by Vogt and Larqué (1998), who illustrate the replacement of sepiolite by opal.…”

“…Nahon et al (1975) describe domains of neoformed palygorskite, formed at the expense of palygorskite-rich marls with a unistrial to crystallitic b-fabric. Bustillo and García Romero (2003) describe relict fibrous textures in silcretes, inherited from palygorskite-bearing calcretes and possibly enhanced by sepiolite neoformation. Silicification of sepiolite deposits can be marked by inheritance of a striated b-fabric and a granular microstructure (Bustillo & Alonso-Zarza, 2007).…”

Silcrete

“…; Ballesteros et al (1997); Gomez-Gras et al (2000); Bustillo (2001); Parcerisa et al (2001); Bustillo and García Romero (2003).…”