Nucleation and growth of aragonite in a calcite single crystal

Abstract

Abstract Direct observations by optical microscopy of nucleation and growth of aragonite in calcite single crystals were made in conjunction with investigations into the transformation kinetics under varying temperature and pressure. Techniques used for the detection of regions transformed into aragonite and crystal imperfections included (a) etching before transformation; (b) staining after transformation and (c) staining and etching after transformation. Etch-pit counts on sample surfaces before transformation indicate highly perfect crystals with an average dislocation density of 7 × 102 cm−2. Etching after transformation shows that the samples underwent considerable plastic deformation by glide and twinning processes precursory to, and concomitant with, the transformation. Transformed regions were observed by staining in Feigl's solution, and independently also by x-ray diffraction topography. The observations indicate that in the very early stage of the transformation the transformation rate is dominated by nucleations at surface sites near slip lines, twins and deformation bands. Most of the later stages of the transformation are determined by homogeneous surface nucleations with a low rate of bulk volume nucleations. On the scale of individual nucleations and that of larger agglomerations of nuclei, growth appears to be largely homogeneous as required for the application of simple nucleation and growth theory. A complete term for the nucleation rate, separating nucleations at line imperfections, surfaces and in the bulk volume, may be used to account formally for the dominance of the observed nucleations in different stages of the transformation process.