Calcium L2,3-edge XANES of carbonates, carbonate apatite, and oldhamite (CaS)

Abstract

The local electronic structure and stereochemistry of calcite, aragonite, dolomite, ferroan dolomite, manganoan calcite, synthetic carbonate hydroxylapatite (CHAP), and CaS (synthetic oldhamite) have been studied using Ca L 2,3 -edge X-ray absorption near-edge structure (XANES) spectroscopy. The XANES spectra of the calcite- and dolomite-structure carbonates are identical within error of measurement, confirming the local nature of X-ray absorption at the L 2,3 edge of Ca 2+ . The Ca L 2,3 -edge XANES spectrum of aragonite is distinct and indicates a weak crystal-field splitting of positive 10 Dq . Separate Ca1 and Ca2 sites are resolved in the XANES of hydroxylapatite and CHAP: Ca1 appears to have a very weak crystal field of negative 10 Dq , and Ca2 has a weak crystal field of positive 10 Dq . The Ca L 2,3 -edge XANES spectrum of CaS reflects both Ca and S unoccupied 3 d states, and is used to show progressive oxidation of the sulfide on exposure to air. The L 2,3 X-ray absorption edge of 3 d 0 cations is associated with the 2 p 5 3 d 1 excited electronic state. It is, therefore, a novel technique for studying the crystal field of K + , Ca 2+ , Sc 3+ , and Ti 4+ , which do not have populated 3 d orbitals in their ground state.