Dissolution-Reprecipitation of Amorphous and Crystalline (Ca)-Mg-Carbonates in Thin Water Films.

Ca-Mg-carbonate minerals crystallized from amorphous precursors have complex morphologies and unique material properties. The transformation of amorphous Ca-, Ca-Mg-, and Mg-carbonate (ACC, ACMC, and AMC) phases is sensitive to relative humidity (RH) of the surrounding air. Here, we measure the effective thicknesses of water films on amorphous carbonates from 10 to 98% RH using a dynamic vapor sorption analyzer and specific surface areas from N2 adsorption. We find that water sorption increases exponentially starting at 76-84% RH for ACMC phases (21 ≤ mol % Mg ≤ 100). We supplement these data with static sorption experiments at 98% RH and constrain sorption rates and effective equilibrium water film thicknesses to 2-6 nm. In long-term experiments with fixed humidity, ACMC and its transformation products show evidence of continued recrystallization over a period of seven months at 98% RH. Diverse crystal morphologies suggest distinct microenvironments of recrystallization. Finally, recrystallization of mixed Ca-Mg-carbonate phases to discrete Ca- and Mg-carbonate minerals suggests a process of ion transport in the thin films. We discuss how Ostwald ripening, phase metastability, and ion diffusion may operate as drivers of recrystallization in thin film environments.