The role of biomineralization in the origin of sepiolite and dolomite

Abstract

Sepiolite is an important industrial mineral whose mechanism of formation and detailed origin are still not clear. Sepiolite is frequently found with dolomite, Mg-smectites or palygorskite in the Intermediate Unit of Miocene sediments of the Madrid basin. Herein, we describe mineralogy, texture and mineral chemistry of sepiolite-rich layers (up to 90% concentration) in marl-clay sediments, either with gypsum, chert (micro-crystalline quartz), or dolomite, that are up to 2 to 3 m thick. The materials studied were formed in shallow lake environments or mudflats. These sediments contain dolomite aggregates resembling mineralized microorganisms (biomorphs) with ovoid morphologies and spherical to tubular internal voids. The regular size and relatively uniform shape of the aggregates are likely controlled by the shape and size of the microorganisms. The paper describes the presence of organic debris, silicification processes and potassium silicate depletion related to the basal sediments that preceded sepiolite rich layers. Potassium depletion, as a nutrient, in detrital components and the occurrence of organic and reduced sediment (U and sulfide concentration) at the interface with sepiolite also can be linked to biosignatures related to the origin of sepiolite. In addition, the occurrence of dolomite with carbon that has δ¹³C PDB values of < −7.5 permil confirms the possibility of an organic carbon source. High concentrations of sepiolite are observed where the amount of dolomite is small. The remaining dolomite in virtually pure sepiolite layers is present as partially dissolved biomorphs or fully recrystallized isolated crystals. Thus, a general process for dolomite dissolution and recrystallization is associated with sepiolite differentiation. In addition, the formation of sepiolite related to the biomineralization of dolomite is likely. The potential role of biomineralization in the formation of these high-grade sepiolite deposits depends on two factors: (1) specific organic-inorganic interactions and (2) the highly porous nature of the sepiolite precursor dolomite during the diagenetic evolution of these sediments. The confirmation of these processes awaits the future development of sepiolite synthesis strategies that incorporate the appropriate biogeochemical reactions.