Micro-Scale Physical and Chemical Heterogeneities in Biogenic Materials - A Combined Micro-Raman, Chemical Composition and Microhardness Investigation

摘要

The ultrastructure and chemical composition of calcitic shells of the modern brachiopod specimen Magellania flavescens (Linnaeus) - order: Terebratulida - was investigated with μ-Raman spectroscopy, Vickers microhardness indentation and laser-ablation-inductively-coupled-plasma-mass-spectrometry. The shells contain a thin outer, nanocrystalline primary layer, which is followed by an inner, much softer, secondary layer composed of inorganic/organic fibre composite material. We observed significant chemical and structural inhomogeneities within the shells. The calcite A_(1g) Raman mode was slightly reduced from 1084 cm~(-1) at the hinge (lock) down to 1083.5 cm~(-1) towards the tip. This is accompanied by a variation of some chemical impurity concentrations (e.g. Mg, Sr). A strong decrease in microhardness and distinct changes in chemical composition from the primary or the outermost part of the secondary layer towards the innermost portion of the secondary shell layer can be observed. Thus, our measurements show that chemical and structural inhomogeneities occur in modern brachiopods and not only between the primary and the secondary shell layer, but also within the secondary layer of the shell.