Spontaneous movement of ions through calcite at standard temperature and pressure

Abstract

At the resolution limits of traditional geochemical techniques, there is little evidence to challenge the common assumptions that, under the Earth's ambient surface conditions, dry calcite is static and that the bulk mineral behaves as a closed system. Solid-state diffusion has been recognized at elevated temperatures^1,2,3, but ithas always been assumed that diffusion in carbonate minerals is negligible under standard conditions⁴,⁵. There is, however, some evidence to the contrary. More than 30 years ago, the ⁴⁵Ca diffusion coefficient was estimated to be ∼8 × 10⁻²⁰ cm² (ref. 6) and, more recently, we have demonstrated movement of adsorbed Cd²⁺ and Zn²⁺ into bulk calcite at rates of tens of nanometres over weeks to months (refs 7, 8). Here we present evidence thatmonovalent ions, Na⁺, K⁺ and Cl⁻, originating from fluid inclusions, accumulate in crystallites on the surface of calcite. This process is spontaneous at the Earth's surface conditions, in air. The results show that calcite under standard conditions does not always behave as a closed system, which is a critical assumption in the use of isotope ratios, trace-element distribution and fluid-inclusion composition for interpretations of palaeoclimate, geochronology or petrogenesis. Moreover, calcite's uptake capacity for contaminants in environmental systems is probably higher than current models predict, because surface sites are constantly renewed by ionic mobility.