Geochemistry and Biomarker Analysis of the Bentonites from Esquivias (Toledo, Spain)

Abstract

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.

Spanish Bentonites: A Review and New Data on Their Geology, Mineralogy, and Crystal Chemistry

“…In addition, as shown by the XRD patterns, the Green Clays had higher impurity contents (illite, quartz, and feldspar), while the Pink Clay samples usually were pure smectite, almost devoid of other minor minerals. This is related to the inherited character of the Green Clays versus the autigenic origin of the Pink Clays formed in the sedimentary basin, as indicated by Garcia-Rivas et al [77].…”

“…On the other hand, the Pink Clays are pure smectites of stevensitic composition without detrital minerals that correspond to levels of authigenic clays newly formed in the sedimentary basin, which appear interbedded in the Green Clay unit [71]. García-Rivas et al [77] (from the geochemistry and biomarker criteria) have concluded that the Green Clays have a more detrital character than the Pink Clays do, which present a more authigenic character. De Santiago et al [78] (from TEM observations) have concluded that Pink smectites have a cellular texture that resembles descriptions of primitive clay precursors that were formed from glasses, gels, or the weathering of previous minerals.…”

On the structural formula of smectites: a review and new data on the influence of exchangeable cations

“…All trioctahedral samples were collected at the Tajo Basin (Spain) and they belong to the same unit, the Pink Clays Unit, so their chemical compositions should be similar. These clays have been studied by different authors and have been characterized as stevensite (Cuevas et al, 1993(Cuevas et al, , 2003de Santiago Buey et al, 2000;García-Rivas et al, 2018), kerolite or interstratified kerolite/stevensite (Martin de Vidales et al, 1991;Pozo et al, 1992Clauer et al, 2012), and as a fine-grained interstratification of turbostratic talc and saponite (Steudel et al, 2017). The lack of agreement on the classification of Pink Clays is easily understood by considering the chemical compositions and structural formulae obtained from our natural samples.…”

“…Three samples (ESB6, RESQ and ROS) were collected at the Tajo Basin, located in the centre of the Iberian Peninsula. They are sedimentary clays belonging to the Pink Clays Unit (Martin de Vidales et al, 1991;Pozo et al, 1992;Cuevas et al, 1993;de Santiago Buey et al, 2000;Cuevas et al, 2003;García-Rivas et al, 2018;García-Romero et al, 2019). Tajo Basin is particularly interesting because it is one of the richest basins for Mg clays in the world, with high economic value.…”

“…Firstly, the assumption that all negative charge comes from oxygens and hydroxyl groups can be erroneous, because a part of the negative charge can derive from F À substituting the hydroxyl groups of the octahedral sheet. Different amounts of F À have been found in smectites, ranging from 0.02-0.45% for saponites from the Spanish Tajo Basin (Pozo et al, 2014;García-Rivas et al, 2018) to more than 5% for hectorite (Thomas et al, 1977). A small amount of F À can influence the final fit, though the main problem in having F À is that if it is not possible to fix the negative charge, then the proportion of the cations cannot be normalized with respect to any other element.…”