Radius Effect of the Alkaline Earths on the Rate of Inversion of Aragonite to Calcite

Abstract

The effect of magnesium, strontium, and other alkaline earths on the formation and persistence of metastable carbonates in the natural environment was investigated to determine the nature of the controlling mechanism. Barium and beryllium were studied to evaluate the effect of ionic radius; magnesium and strontium, in order to determine if the results correlate with the usual order of stability for complexes and adsorbed species. Known weights of aragonite were placed in contact with solutions of beryllium, magnesium, calcium, strontium, and barium. Samples were covered and periodically both pH and percent composition of aragonite determined; supernatant liquids and precipitates were analyzed for cation concentrations by atomic absorption spectroscopy and titrimetric methods. Results indicated that the order of effectiveness of alkaline earth metals in inhibiting recrystallization is : Be > Mg > Sr > Ba. This is the expected order of effectiveness for both surface and solution effects. A solution effect (i.e., sequestration of bicarbonate ions) is strongly suggested by the chemical behavior of each cation.