OTAVITE-CALCITE SOLID-SOLUTION FORMATION AT THE CALCITE-WATER INTERFACE STUDIED IN-SITU BY SYNCHROTRON X-RAY-SCATTERING

Synchrotron X-ray scattering measurements were performed in situ durin g the formation of thin (50-600 Angstrom) overgrowths of otavite-calci te solid-solutions at the (<10(1)over bar 4>) cleavage surface of sing le- crystal calcite. These solid-solutions were precipitated from EDTA -bearing aqueous solutions having varied initial saturation states of otavite and calcite. From repetitive X-ray diffraction scans, the Cd/( Ca + Cd) ratios and the effective thicknesses (average domain size per pendicular to the calcite cleavage surface) of the solid-solutions wer e determined as a function of time. Additional in-plane X-ray diffract ion scans were done to further characterize the relationship between t he solid-solutions and the calcite cleavage surface. The solid-solutio n phase grew epitaxially with a(<10(1)over bar 4>) growth plane orient ed parallel to the calcite (<10(1)over bar 4>) cleavage surface, The c ompositions of the solid-solutions evolved with time, while their grow th rates (increases in effective thickness) remained fairly constant ( 10-54 Angstrom/hr). In each experiment, the coverage of the initial su rface by the solid-solution (calculated from the difference between th e initial and final Cd concentrations in the aqueous solution) was abo ut 20%. Glancing-incidence X-ray reflectivity scans were also monitore d as a function of time. From these scans, we determined that the soli d-water interface did not become significantly rougher during the nucl eation and growth of the solid-solution phase, These observations indi cate that the solid-solution grew by layer spreading and that most gro wth may have occurred preferentially at macrostep faces produced durin g cleavage. Copyright (C) 1997 Elsevier Science Ltd.