Calcium L_2,3-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 Ca²⁺. The Ca L_2,3-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 L_2,3-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 L_2,3 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.