EPITAXIAL-GROWTH OF OTAVITE ON CALCITE OBSERVED IN-SITU BY SYNCHROTRON X-RAY-SCATTERING

Synchrotron X-ray reflectivity and X-ray diffraction techniques were u sed to characterize an otavite (CdCO3) overgrowth during its precipita tion from an aqueous solution onto a calcite (1O $$($) over bar 14) cl eavage surface. X-ray reflectivity was used to measure the otavite thi ckness and the roughness of the calcite/otavite and otavite/fluid inte rfaces. Specular and off-specular X-ray diffraction were used to measu re the crystallographic orientation and long-range atomic order of the otavite overgrowth. The otavite grew coherently with a (10 $$($) over bar 14) growth plane oriented parallel to the calcite (10 $$($) over bar 14) cleavage surface. The average growth rate of the otavite for t he first 9 hours was 15 Angstrom.h(-1). During the early growth stage (less than or equal to 50 Angstrom), the otavite (10 $$($) over bar 14 ) lattice spacing (d-value) was compressed by as much 2.2% in the dire ction perpendicular to the calcite cleavage surface. As the otavite th ickness increased, this d-value approached that of bulk otavite. At a thickness of 443 Angstrom, the otavite was determined to be of single- crystal quality (0.4 degrees mosaic) and epitaxial with calcite. This study demonstrates a new and accurate approach for measuring in situ p recipitation rates and growth mechanisms in mineral-fluid systems.