Nanostructure of Biomineral Apatite of Modern Oceanic Phosphorites

Abstract

Calcium phosphates of biogenic nature and their synthetic analogs, as a rule, are composed of primary particles of the nanometer-size range. In the present work, the structure and morphology of oceanic phosphorites formed in the shelf zone were investigated using a set of methods including transmission and scanning electron microscopy in combination with energy dispersive spectroscopy. The materials under investigation were diversely lithified phosphorite contractions and coproliths of sea lions from modern diatomaceous silt from the inner shelf of Namibia. Along with the investigation of micro- and nanostructures of these formations, experiments on the synthesis of calcium phosphate in aqueous solutions at low calcium and phosphorus concentrations (Са2+ = 1.33 mM/L; Р5− = 1.5 mM/L) with the participation of organic biopolymer (protein: albumen) were carried out. The result of the synthesis is the mineral–organic gel (MOG) in which the mineral part is represented by hydroxyapatite single crystals. In the samples under investigation, calcium phosphate is represented by the particles of irregular, globular or elongated shape of micrometer size, composed of aggregated particles 10–1000 nm in size. Especially interesting are spindle-shaped particles (1–1.5 μm) that are often localized on the silicate shucks of diatoms. Many authors consider them to be mineralized bacteria; however, the results of the present work contradict this view. According to the high-resolution electron microscopy results, coarse globules that comprise the major part of coproliths are composed of thin-plate nanocrystals of different sizes (10–1000 nm) with apatite structure. Bacteriomorphous particles are represented by thin-plate apatite nanocrystals 10–50 nm in size. In calcium phosphate particles, apatite nanocrystals are oriented in one direction with the off-orientation angle not more than 5°. It follows from the results of the dark-field analysis that the rectilinear texture of the orientation of nanocrystals in bacteriomorphous particles is conserved along the whole length. Coarse particles have numerous pores and are composed of mesacrystals. The results of the present investigation showed that the formation of the particles of calcium phosphate in the medium with the participation of organic residues occurs from the MOG. The shape of particles formed in these interactions in the MOG is determined mainly by two factors: anisotropy of nanocrystals and the orienting effect of biopolymers participating in the process of their formation. Lithification of sediments causes consolidation of the substance with a decrease in the size and number of pores in the mesacrystals of calcium phosphate.