Surface analytical techniques applied to calcite: evidence of solid-state diffusion and implications for isotope methods

Surface analytical techniques have become common tools for physicists and materials scientists for making direct observations at the molecular scale but they can also provide Earth Scientists with new information about mineral/fluid interfaces. High resolution surface analysis compliments our traditional methods for analysing the bulk of solids and solutions which can give us, for example, a more complete picture of the geochemical processes that affect the mineral grains in sediments. Stable isotope methods have become widely used for dating and palaeoclimate studies but they rely on the assumption of a closed system. Artifacts resulting from carbonate mineral recrystallization can be avoided by careful sampling but ion by ion replacement can significantly alter the composition of a solid without leaving visible traces. Calcite has long been known to take up substituting ions into its atomic structure. Recent evidence gained from surface analysis shows Cd²⁺ and Zn²⁺, that had been adsorbed or precipitated on calcite surfaces, moves into calcite at the rate of nanometers per month, apparently by solid-state diffusion. No fractures that could serve as conduits from surface to bulk have ever been observed at the micrometer to nanometer scale on the single crystals used for these experiments. Mixing within the top few surface layers by reprecipitation during exposure to the humidity in air can account for some incorporation, but the evidence collected so far does not explain the exchange of position for ions beyond about ten calcite monolayers. With similar rates of movement, other trace components, such as K⁺, Na⁺, Cl^- and F^-, have been observed to move out of bulk calcite and to accumulate in discrete crystallites on surfaces exposed to air. Such mobility may be particular to these near-perfect, Iceland spar crystals and the ions investigated, but if O and C also move into and out of bulk calcite at similar rates, the integrity of isotope ratios from carbonate minerals, even from non-diagenetic environments, may be questionable.