Reliability of experimental partition coefficients in carbonate systems: Evidence for inhomogeneous distribution of impurity cations

Electron spin resonance (ESR) spectroscopy, thermal analysis and X-ray diffraction (XRD) analysis are used to study the mode of incorporation of a wide range of impurity ions (up to 1% Ni²⁺, Sr²⁺, Cd²⁺, Mg²⁺, Cu²⁺, Fe²⁺, Zn²⁺, Pb²⁺, Mn²⁺ and Ba²⁺) in calcites synthesised in aqueous solutions. It is shown that when certain impurity ions are incorporated in these calcites they become concentrated, together with other trace elements such as Mn²⁺, into hitherto unsuspected trace phases. The important conclusion is drawn that unless the scavenging of trace elements by these phases can be avoided or corrected for, any calcite—liquid partition coefficients measured may have limited significance. ESR spectroscopy has revealed the intense strain common in calcites grown by different methods, whilst thermal analysis suggested that such strain may significantly alter the thermodynamic properties of these calcites. ESR spectroscopic analysis of aragonites synthesised by methods previously reported in the literature has revealed the common occurrence of trace calcite at concentrations below the XRD detection limit and proves the tendency of this calcite to scavenge trace Mn. Using ESR spectroscopy to monitor trace calcite levels and structural imperfections in synthetic aragonites, the urea hydrolysis technique is modified to provide a method for the synthesis of high-purity strain-free aragonite available for experimental studies.