X-ray standing wave study of arsenite incorporation at the calcite surface

The location and orientation of arsenite incorporated at the CaCO3 (10 (1) over bar 4) cleavage surface from a dilute aqueous solution was examined with the X-ray standing wave (XSW) technique. The high coherent fractions measured for As on the (10 (1) over bar 4) and (0006) Bragg reflections indicate that the arsenite was well ordered in registration with the calcite surface lattice. The As coherent positions show that arsenite was located at the carbonate site. The XSW analysis is consistent with a structural model in which the pyramidal-shaped arsenite ion as oriented with its oxygen base coincident with the carbonate plane, and its As apex pointing outward from the (10 (1) over bar 4) surface. The structure observed here for arsenite, and previously for selenite, suggests that other pyramidal trioxyanions could be incorporated at the calcite (10 (1) over bar 4) surface with the same geometry. Copyright (C) 1999 Elsevier Science Ltd.