Resistant cellulose-derivative biopolymer templates in natural sepiolite

Abstract

Sepiolite is a fibrous sheet silicate with a composition of Si12Mg8 (H2O)4(Si6O15)2(OH)4 · 8H2O and has significant physicochemical and rheological properties due to the presence of zeolitic channels in its structure. The highest concentrations of sepiolite worldwide are found in the Madrid Miocene Basin within the edges of saline lakes and are associated with dolomitic and siliceous microbialites. It is remarkable that monomineral sepiolite sediments up to 6 m thick are composed of 6 nm crystallite-sized fibers. The aim of this study is to gather information concerning the organomineralization of sepiolite. The mineral paragenetic sequences associated with this mineral were characterized by XRD in representative samples. The texture was studied by polarized light microscopy and by SEM-, TEM- and HRTEM-EDX. The remains of mineralized biomass are commonly found mixed with sepiolite fibers. Lipidic biomarkers reveal the prevalence of non-branched derivatives of short-chain fatty-acids (C9 to C18), which are characteristic of cyanobacteria and microalgae and support the presence of microorganisms within the studied sediments. The textural organization of sepiolite fibers is similar to the fabric of cellulose fibers produced by microorganisms. There are two types of fiber associations: short and straight fibers ( 10 μm length) forming thin tissues that mimic cellulose and other exopolymers produced by bacteria (for example, acetobacter, cyanobacteria). The similarity is extended to the nanometric scale. Sepiolite is composed of 6 to 10 nm-thick fibers that form mesocrystals (oriented nanocrystal aggregates) that are 50 to 200 nm in length. Based on an XRD analysis of the crystal size distribution, this average fibril thickness is the same as that of cellulose. The growth of sepiolite is limited to the size of cellulose crystallites. The cellobiose unit size is similar to the size of the zeolitic channels of sepiolite (1.04 nm) in the β-[1.4] linkage propagating direction, in which a 180° rotation is produced, as well as to the SiO4 tetrahedra in sepiolite. Its formation is controlled by the orientation and repeating distance of polar groups (OH and derivatives), with numerous defects and disruptions that coincide with amorphous phases of cellulose. The maintenance of polar or ionic characteristics in the organic reticulum and its orientation is essential for the nucleation of sepiolite. This fact makes its recrystallization difficult, which, in turn, proves the high geochemical stability of sepiolite in spite of its nanometric nature.