Precipitation of kerolite and sepiolite associated with Mg-rich carbonates in a cave environment

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Martín-Pérez, Andrea and La Iglesia, Á. and Almendros, Gonzalo and González Pérez, José Antonio and Alonso-Zarza, Ana María (2021) Precipitation of kerolite and sepiolite associated with Mg-rich carbonates in a cave environment. Sedimentary Geology, 411 (105793). ISSN 0037-0738

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Official URL: https://www.sciencedirect.com/science/article/pii/S0037073820302086



Abstract

In Castañar Cave (Cáceres, Spain), Mg-Si phases forming fibres and films occur associated with aragonite, magnesite, huntite and spheroidal dolomite in moonmilk, coatings and crust speleothems. A detailed study of bulk and carbonate-removed samples allowed us to identify the Mg-Si phases as kerolite (Mg3Si4O10(OH)2·nH2O) and sepiolite Mg4Si6O15(OH)2·6(H2O) of variable “crystallinity” and very pure composition. Under the scanning electron microscope, kerolite appears as gelatinous, smooth masses or films, showing some desiccation features. Sepiolite mostly occurs as isolated fibres 50 to 200 nm thick and up to 50 μm long and mats of interwoven fibres of several shapes. Analytical pyrolysis (Py-GC/MS) of the organic matter associated with these minerals in the moonmilk speleothems shows a compound assemblage lacking carbohydrates, nitrogen-bearing molecules and fatty acids, indicating no microbial activity.

Kerolite and sepiolite most probably formed abiotically by direct precipitation in the cave waters, in conditions of high silica and magnesium activity and relatively high pH. These conditions were attained by local CO2 degassing taking place in extremely slow drips and capillary waters in speleothems with a large surface area, in a dynamic system where the renovation of infiltrated waters and diagenetic transformations of carbonates and silicates also contributed to significant hydrochemical variability at the microscale.


Item Type:Article
Uncontrolled Keywords:Mg-clays, Dolomite, Huntite, Speleothems, Moonmilk, Analytical pyrolysis
Subjects:Sciences > Geology > Mineralogy
Sciences > Geology > Petrology
ID Code:63603
Deposited On:15 Jan 2021 17:18
Last Modified:18 Jan 2021 10:44

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