Mode of incorporation of Sr 2+ in calcite: Determination by X-ray absorption spectroscopy

Abstract

By probing the local atomic environment of strontium coprecipitated with natural and synthetic calcites, X-ray absorption spectroscopy (XAS) reveals that the strontium is incorporated in the calcite by substitution at Ca 2+ structural sites, forming a dilute solid solution. Both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) demonstrated that the local structural environment of Sr 2+ in natural and synthetic calcites was similar to that of Ca 2+ in calcite and quite different from that of Sr 2+ in strontianite. The Sr 2+-O 2− distance derived from EXAFS, 2.58 ± 0.03Å, is consistent with the sum of the radii of these two ions in six- and three-fold coordination, respectively, of 2.57 Å. The XANES and EXAFS further eliminate such modes of incorporation as adsorption, occlusion, and the presence of trace amounts of strontianite or other Sr 2+-rich phase. However, evidence of a possible relict Sr 2+-rich aragonite was encountered in a diagenetic calcite, indicating that XAS may prove a sensitive tool for detecting incomplete mineralogical alteration in carbonates. Because Sr 2+ is bound at Ca 2+ lattice sites, the record of biomineralization, diagenesis and age encoded in Sr 2+ Ca 2+ ratios, and strontium isotopes in geologic calcites is secure; likewise, 90Sr sequestered in natural calcite should not undergo significant preferential leaching.