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 L2,3-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 L2,3 edge of Ca2+. The Ca L2,3-edge XANES spectrum of aragonite is distinct and indicates a weak crystal-field splitting of positive 10Dq. 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 10Dq, and Ca2 has a weak crystal field of positive 10Dq. The Ca L2,3-edge XANES spectrum of CaS reflects both Ca and S unoccupied 3d states, and is used to show progressive oxidation of the sulfide on exposure to air. The L2,3 X-ray absorption edge of 3d0 cations is associated with the 2p53d1 excited electronic state. It is, therefore, a novel technique for studying the crystal field of K+, Ca2+, Sc3+, and Ti4+, which do not have populated 3d orbitals in their ground state.

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