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Here, the microstructure and mechanical properties of a basalt with three alteration degrees (low, medium, and high) are studied, and these properties are compared to the alteration degree of the basalt. The representative samples of each alteration degree have been studied both in raw state as after a short ageing treatment with ethylene glycol. The properties studied are density, P-wave velocity (VP) and uniaxial compression strength (UCS). The microstructure has been studied via optical and scanning electron microscopy and X-ray computed microtomography (microCT), which also provides quantitative data on connected and non-connected porosity. The representative samples of the three alteration degrees show clear differences and Vp and UCS values are progressively lower as higher the alteration. The loss of the good properties of low altered basalt was due to smectites filling a dense network of micro- and nano-fractures that crossed the full rock. Smectites as alteration products of the basalt explained the behaviour of these rocks. With only three swelling–shrinking cycles of smectites, the porosity grew ∼500% in the most altered sample, which explains the cohesion loss of the rock and its lower UCS. Statistical analysis showed the negative correlation among the connected porosity and the mechanical properties of these basalts., Ministerio de Ciencia e Innovación, Depto. de Mineralogía y Petrología, Fac. de Ciencias Geológicas, TRUE, pub

Biswas et al. (2016) have recently reported “Influence of thermally modified palygorskite on the viability of polycyclic aromatic hydrocarbon-degrading bacteria”. This topic is very interesting because is related to a clay-mediated bioremediation technology and the authors investigated the compatibility of heat treated palygorskite products (100–900 °C) with a polycyclic aromatic hydrocarbons -degrading soil bacterium Burkholderia sartisoli. Nevertheless, the discussion of the results shows a lack of knowledge of the structure and thermal behaviour of palygorskite, and consequently, the conclusions reached are not correct.

[ES]Las arcillas magnésicas son un grupo de minerales de gran importancia económica a nivel mundial. Son unos recursos escasos que se encuentran restringidos a algunas cuencas sedimentarias como la Cuenca de Polatli (Turquía) y la Cuenca del Tajo (España). Este grupo de arcillas presenta una variabilidad composicional y estructural muy grande, incluyendo tanto filosilicatos laminares del grupo de las esmectitas, más concretamente saponita – estevensita, como filosilicatos modulados (sepiolita – palygorskita). Esta Tesis Doctoral se ha realizado mediante el formato de compendio de artículos, con 4 objetivos claramente diferenciados entre sí que pretenden ampliar el conocimiento sobre algunos aspectos concretos de las arcillas magnésicas: la realización de un estudio de detalle de la estructura de la palygorskita, la realización de un estudio genético de las bentonitas magnésicas de la Cuenca del Tajo, un estudio preliminar para determinar la viabilidad del uso de la espectroscopía VNIR – SWIR para la identificación de las arcillas magnésicas a través de técnicas de teledetección y la caracterización mineralógica y de las propiedades de superficie de sepiolita – palygorskita en relación con su composición química, textura y estructura., El estudio en detalle de la estructura de la palygorskita se realizó utilizando difracción de Rayos-X con radiación de sincrotrón tanto para polvo como para fibra orientada. Estos resultados experimentales se compararon con simulaciones teóricas de los patrones de difracción de polvo y de fibra orientada, en 2D y 1D, poniendo de manifiesto la existencia de unas diferencias significativas entre los resultados experimentales y los datos teóricos. Estas diferencias, que no pueden ser justificadas por la posible presencia de impurezas ni por el tratamiento de los datos realizado, indican que la estructura de la palygorskita no ha sido aún resuelta de una manera definitiva. La Cuenca del Tajo presenta una de las mayores concentraciones de arcillas magnésicas del mundo, pero presenta una problemática especial respecto a su génesis. El registro fósil y sedimentario de la cuenca indica unas condiciones de pH en las cuales no se daría la precipitación de estos minerales, al igual que las posibles áreas fuente tienen una composición granítica o metapelítica, de manera general, siendo en cualquier caso ricas en aluminosilicatos que no justifican el aporte de magnesio necesario para la formación de estos minerales. Esta problemática llevó a realizar un estudio genético de las bentonitas de la Cuenca del Tajo, obtenidas en una cantera en las proximidades de Esquivias, combinando el estudio mineralógico y geoquímico clásico con el análisis de biomarcadores realizado por primera vez sobre estos materiales. Las muestras se clasificaron en 4 asociaciones minerales de acuerdo a los datos de difracción de Rayos-X: bentonítica (incluyendo tanto arcillas verdes como arcillas rosas), carbonatada, illítica y arenosa. La geoquímica indica que tanto las asociaciones arenosa, illítica y las arcillas verdes presentan un carácter detrítico mientras que la asociación carbonatada y las arcillas rosas presentan un carácter autigénico. El análisis de los biomarcadores permite observar que dentro d, Con el objetivo de determinar la viabilidad de la aplicación de estas técnicas en la prospección de arcillas magnésicas en la Cuenca del Tajo, se ha realizado un estudio preliminar de la respuesta espectral de muestras superficiales provenientes de las proximidades de la localidad de Esquivias en el rango VNIR – SWIR en el laboratorio. Las muestras se clasificaron en diferentes grupos y subgrupos de acuerdo a la morfología general de sus espectros y estos grupos resultaron corresponder a distintas asociaciones minerales. Así pues, la clasificación permite la asignación de los distintos grupos y subgrupos a las diferentes unidades estratigráficas muestreadas de acuerdo a su mineralogía promedio y posibilita la realización de una cartografía del área en términos de composición mineralógica mediante técnicas de teledetección. Es especialmente interesante remarcar que, pese a la pequeña cantidad que representan las arcillas magnésicas en superficie, estas se pueden identificar claramente en los espectros, demostrando la viabilidad de prospectarlas mediante teledetección. El interés económico de las arcillas magnésicas es debido a sus propiedades físico – químicas, principalmente su elevada superficie específica, y es especialmente interesante estudiarlas en relación con su composición química, textura y estructura. El estudio de estas propiedades en relación con los factores descritos se ha realizado con muestras de sepiolita – palygorskita, siendo las sepiolitas estudiadas de origen turco, concretamente de la Cuenca de Polatli, y las palygorskitas provenientes de diferentes puntos de la geografía española. El estudio de estos materiales muestra que las propiedades de superficie no dependen únicamente de la presencia de diferentes impurezas. La textura juega un papel importante puesto que puede aportar microporosidad textural, pero más allá de eso, la presencia de intercrecimientos de palygorskita dentro de sepiolita y viceversa, deducible tanto a través de los difracto

Evidence of crystallization by particle attachment in synthetic materials is described in numerous contributions. However, efforts to establish the contribution of the particle attachment mechanism to inorganic crystallization in natural environments have barely begun. Here, we show, for the first time, evidence that confirms oriented particle attachment as a crystal growth mechanism that is relevant in sedimentary environments. In these natural settings, oriented particle attachment operates during the formation of highly anisotropically structured clay minerals, which constitute one of the most extensively distributed groups of minerals in the Earth's crust. High-resolution transmission electron microscopy images show that the clay minerals aggregation process occurs in different manners. Smectites aggregate by semi-oriented attachment, while kaolinite, sepiolite and palygorskite aggregate by oriented attachment.

The Tajo Basin is one of the richest in Mg-clays known around the world. Mg-bentonites, kwon as pink clays and green clays, alter in the Intermediate Unit of the Miocene sediments. In this work, a new approach to the genesis of these bentonites is performed by studying for the first time the biomarkers present in these clays in relation to the mineralogy and geochemistry, as well as using discriminatory criteria between green and pink clays. Samples were collected at a quarry of Mg-bentonites, in the proximities of Esquivias (Toledo, Spain). Mineralogical characterization and semi-quantification (X-ray diffraction (XRD)) allowed a well-defined classification according to the mineralogical content of the samples to be established, differentiating four associations. Geochemical analyses are clearly linked to the mineralogy and provide criteria to differentiate the genesis of the materials studied. In this regard, green clays are interpreted as having a more detrital character than pink clays, which present a more authigenic character. Biomarkers (n-alkanes and n-alkanoic acids) were studied, not showing a clear link with the mineralogy as in the case of the geochemistry, but providing interesting information about the origin and degradation of the organic matter. Pink clays have higher contents in biomarkers than green clays, providing a discriminative criterion.

41 soil samples were collected at the Tajo Basin (Spain), in an area where Mg-rich clays are benefitted, whit the aim of studying their spectral response in the Visible, Near Infrared (VNIR) – Short Wave Infrared (SWIR) range (350–2500 nm) in terms of mineralogical composition and exploring the possibility of using these data as the basis of a geological mapping through hyperspectral imaging in this wavenumber interval in future research. The samples, belonging to nine different stratigraphic units, were characterized by X-Ray diffraction and VNIR – SWIR laboratory reflectance spectroscopy. The mineralogical associations are formed by complex mixtures of carbonates, gypsum, quartz, feldspars, illite, and smectites in variable proportions depending on the stratigraphic unit. The samples were classified into different groups and subgroups according to their spectral response. The resulting groups allow to extrapolate certain type-spectra to different mineralogical associations corresponding to the stratigraphic units sampled within the area of study. This work is of upmost importance for future works through remote-sensing techniques using VNIR – SWIR imaging of the area. The classification of the samples in different groups, according to their spectral response, and their attribution to the different stratigraphic units sampled, according to their mineralogical content, could help improve the geological mapping of the area of study as well as detecting deposits of Mg-rich clays of economic interest.

A representative group of palygorskites from the six most important Spanish deposits and six other occurrences were studied comparing their mineralogy, textural features, crystal-chemistry, and surface properties. These palygorskites have different geological origin: (1) sedimentary in large Tertiary continental basins, (2) hydrothermally altered volcanic rocks in the southern Spanish Peninsula, also Tertiary in age; and (3) fault-hosted pure palygorskites. The most common impurities are quartz and carbonates, both calcite and dolomite, but also feldspar, illite, smectite, and sepiolite have been identified. The structural formulae obtained show great variability between samples, as well as with respect to the theoretical formula for palygorskite, as they range from magnesian to aluminous palygorskites. The most common fibre length ranges between 1 and 10 µm but the palygorskites of hydrothermal origin are much longer, with lengths over 10 µm. The shorter fibres give rise to tightened textures, while longer fibres usually generate more open textures. These properties, along with the degree of compaction, condition the porosity of the sample, which in turn influences its surface properties. The specific surface areas obtained range between 30 m2 g−1 and 263 m2 g−1, depending on the impurities content and on the texture and crystal-chemistry; the highest values correspond to Mg-rich palygorskite.