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

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 (MgSiO(OH)·nHO) and sepiolite MgSiO(OH)·6(HO) 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 CO 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.This work received financial support from the Junta de Extremadura through FEOGA-ORIENTACIÓN FEDER funds, the Spanish Ministry of Economy and Competitiveness (MINECO) (Project CGL2014-54818-P), and the Slovenian Research Agency (Programme P-0008). The work of AMP has also been supported by Slovenian Research Agency projects Z1-7247 and J1-9185