Varied crystallization conditions for Neogene sepiolite and associated Mg-clays from Madrid Basin (Spain) traced by oxygen and hydrogen isotope geochemistry

Clauer, Norbert; Fallick, Anthony E.; Galán, E.; Pozo, Manuel; Taylor, Christopher

doi:10.1016/j.gca.2012.07.016

Cited by 15 publications

(9 citation statements)

References 52 publications

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“…The role of aqueous CO 2 in the formation of Mg-silicates under evaporative conditions is quite relevant because its effect on pH [22,23]. Mg-clay formation can occur in a wide range of salinities, widely controlled by Mg/Si concentration and pH, which supports previous results on suitable conditions for the crystallization of kerolite-stevensite, saponite and sepiolite by means of isotopic analysis (δ 18 O, δ D) [122].…”

Section: Pathwaysupporting

confidence: 84%

An Overview of Authigenic Magnesian Clays

Pozo

Calvo

2018

Minerals

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Clay authigenesis mostly concerns: (a) the formation of clays by direct precipitation from solution, called "neoformation" and (b) development of clays by transformation of precursor minerals. Precipitation from solution implies that a new mineral structure crystallizes, so that a prior mineral structure is not inherited. Transformation of precursor detrital minerals, a process also termed "neoformation by addition", can be conducted whether throughout precipitation on pre-existing natural surfaces or transformation and reaction on pre-existing surfaces. Both processes have been recognized as effective mechanisms in the formation of Mg-clays, which mostly include 2:1 clay minerals, such as talc-kerolite and Mg-smectites, as well as fibrous clays (sepiolite, palygorskite). Authigenic Mg-clay minerals occur in both modern and ancient marine and non-marine depositional environments, although formation of these clays in hydrothermal continental and seafloor settings must be also outlined. Most favourable conditions for the formation of Mg-clays on earth surface are found in evaporitic depositional environments, especially where parent rocks are enriched in ferromagnesian minerals. In these settings, Mg-clays are important constituent of weathering profiles and soils and can form thick deposits of significant economic interest. Based on this review of authigenic clay deposits, we propose three geochemical pathways, mainly related to continental environments, for the origin of authigenic Mg-clays: formation of Al-bearing Mg-clays (pathway 1), formation of Al-free Mg clays (pathway 2) and formation of sepiolite from other Mg-clay minerals (pathway 3).

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Section: Pathwaysupporting

confidence: 84%

An Overview of Authigenic Magnesian Clays

Pozo

Calvo

2018

Minerals

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“…Because minerals formed in this way often record primary solution composition and its fluctuation over time, Mg-silicates have played a prominent role in the construction of facies models in modern and recent lacustrine settings, epicontinental and inland seas and lakes, and marine environments (e.g. Millot, 1970;Jeans, 1971;Weaver & Beck, 1977;Stoessell & Hay, 1978;Bodine, 1983;Jones & Weir, 1983;Callen, 1984;Bertani & Carozzi, 1985;Jones, 1986;Webster & Jones, 1994;Calvo et al, 1999;Chahi et al, 1999;Deocampo, 2004;Deocampo et al, 2009;Jones & Conko, 2011;Bristow et al, 2012;Clauer et al, 2012). In fact, Hardie & Eugster (1970) and Eugster & Jones (1979) recognized early on that Mg-silicates act as early evaporative precipitates creating critical ''chemical divides'' in the chemical evolution of closed basin brines, but considered the relative timing of these reactions problematic.…”

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confidence: 99%

Chemical controls on incipient Mg-silicate crystallization at 25°C: Implications for early and late diagenesis

2014

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AB ST R ACT : Mg-silicate minerals (e.g., stevensite, kerolite, talc, sepiolite) play an important role in the construction of facies models in lacustrine and peri-marine environments because they are sensitive to changes in solution chemistry. However, the response of Mg-silicate mineralogy to changing aqueous chemistry is only broadly understood because the mechanisms underpinning the coprecipitation of Mg 2+ and SiO 2 (aq) from surface water, and subsequent Mg-silicate crystallization, are unclear. Here we describe the results of experiments designed to systematically examine the effects of pH, Mg/Si and salinity of the parent solution on the nature of initially precipitated products. Structural interrogation of the products with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR) and thermal analysis (TGA/DTA) allow comparison of synthetic products with naturally occurring crystalline counterparts. In general, Mg 2+ and SiO 2 (aq) co-precipitation and nucleation of Mg-silicate layer structures first involves the rapid formation of 2:1 layers with trioctahedral occupancy and a mean coherent X-ray scattering domain between 1À2 unit cells with respect to the c axis. Well defined but diffuse hk reflections indicate two-dimensional growth, turbostratic stacking and highly variable interlayer hydration. Diffuse reflectance FTIR shows numerous structural similarities with stevensite, kerolite and sepiolite. However, TGA/DTA analysis indicates the presence of variable kerolite/stevensite interstratification not readily detectable through XRD analyses, as well as a significant degree of surface and interlayer hydration (e.g. 15À20 wt.%).We observe a number of clear trends in the products with respect to solution chemistry. For example, at low salinity, kerolite-like products dominate at high Mg/Si and high pH, whereas sepiolite-like products are formed at lower pH and lower Mg/Si. At high salinity and high Mg/Si, stevensite-like products are favoured at high pH and kerolite-like products dominate at lower pH, whereas a decrease in Mg/Si of the solution leads to sepiolite-like products at low pH and only stevensite-like products at high pH. Higher pH leads to an increase in octahedral vacancies which favour stevensite-like products; this may result from a higher rate of two-dimensional tetrahedral sheet expansion relative to the octahedral sheet, as inferred from studies of silica oligomerization and brucite growth kinetics.Together, our results indicate that the neoformation of Mg-rich silicates from solution may often begin with the rapid nucleation of hydrated 2:1 layers. Subsequent dehydration leads to progressive layer stacking order and could occur in response to wetting/drying cycles, prolonged exposure to high salinity solutions, or burial and heating. The surface and interlayer water associated with these products is undoubtedly an important source of diagenetic water in Mg-silicate-bearing successions, and the chemistry of this water upon later diagenesis should be a focus of future inves...

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“…Their origins differ, being the green clays formed in a lacustrine environment while the pink ones correspond to palustrine environments [22]. Several geochemical studies have been performed on these clayey materials, both regarding the smectites [22][23][24] and the sepiolite-palygorskite deposits [25][26][27], establishing different correlations between certain elements and the probable origin of the clays in this. On the other hand, no biogeochemical analyses from these deposits have been found, although they are thought to reveal important information referring to the paleoenvironments in which they were formed.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2018

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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.

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Varied crystallization conditions for Neogene sepiolite and associated Mg-clays from Madrid Basin (Spain) traced by oxygen and hydrogen isotope geochemistry

Cited by 15 publications

References 52 publications

An Overview of Authigenic Magnesian Clays

An Overview of Authigenic Magnesian Clays

Chemical controls on incipient Mg-silicate crystallization at 25°C: Implications for early and late diagenesis

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

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