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 wi th the X-ray standing wave (XSW) technique. The high coherent fractions mea sured for As on the (10 (1) over bar 4) and (0006) Bragg reflections indica te that the arsenite was well ordered in registration with the calcite surf ace lattice. The As coherent positions show that arsenite was located at th e carbonate site. The XSW analysis is consistent with a structural model in which the pyramidal-shaped arsenite ion as oriented with its oxygen base c oincident with the carbonate plane, and its As apex pointing outward from t he (10 (1) over bar 4) surface. The structure observed here for arsenite, a nd previously for selenite, suggests that other pyramidal trioxyanions coul d be incorporated at the calcite (10 (1) over bar 4) surface with the same geometry. Copyright (C) 1999 Elsevier Science Ltd.