A study of the aragonite-calcite transformation using Raman spectroscopy, synchrotron powder diffraction and scanning electron microscopy

Abstract

The thermal transformation of aragonite to calcite in synthetic and biogenic aragonite (mussel nacre) is studied using Raman spectroscopy, high temperature synchrotron powder diffraction and scanning electron microscopy (SEM). Anomalous thermal behaviour in the form of plateau regions in the lattice parameter expansion of both the progenitor aragonite and transformed calcite phases is observed. This is attributed to the effect of increasing internal pressure caused by gas inclusions trapped within the structure during crystallisation. Biogenic aragonite exhibits a similar pattern of behaviour, except the plateau regions for each phase occupy a wider range of temperature which is likely due to the presence of intra-crystalline organic molecules intercalated in the atomic structure. An increase in micro-strain as a function of temperature is observed in the synthetic aragonite and calcite phases as the transformation progresses. This is attributed to the preservation of the original aragonite crystal morphology as the larger aragonite crystallites transform into smaller calcite crystals, with no intermediate phase being observed. For the biogenic sample this is not observed, and the effect of thermal treatment is to reduce the intrinsic strain through the degradation of the macromolecules forming the organic matrix of the nacre.