Epitaxial growth of otavite on calcite observed in situ by synchrotron X-ray scattering

Synchrotron X-ray reflectivity and X-ray diffraction techniques were used to characterizean otavite (CdCO₃) overgrowth during its precipitation from an aqueous solution onto a calcite (101¯4) cleavage surface. X-ray reflectivity was used to measure the otavite thickness and the roughness of the calcite/otavite and otavite/fluid interfaces. Specular and off-specular X-ray diffraction were used to measure the crystallographic orientation and long-range atomic order of the otavite overgrowth. The otavite grew coherently with a (101¯4) growth plane oriented parallal the calcite (101¯4) cleavage surface. The average growth rate of the otavite for the first 9 hours was 15 Å · h^-. During the early growth stage (≤50 Å), the otavite (101¯4) lattice spacing (d-value) was compressed by as much 2.2% in the direction perpendicular to the calcite cleavage surface. As the otavite thickness increased, this d-value approached that of bulk otavite. At a thickness of 443 Å, the otavite was determined to be of single-crystal quality (0.4° mosaic) and epitaxial with calcite. This study demonstrates a new and accurate approach for measuring in situ precipitation rates and growth mechanisms in mineral-fluid systems.