Amorphous-to-Crystalline Transition of Ca–Mg-Carbonates as a Function of Composition, Time, and Temperature

Calcium and magnesium carbonate minerals play an important role in regulating Earth’s climate, and they are among the most common biominerals. Growing evidence suggests that some natural biotic and abiotic calcium and magnesium carbonate minerals precipitated from amorphous precursors. Here, we synthesized amorphous carbonates with different molar ratios of Ca and Mg (n = 6 compositions from 0–100 mol % Mg). We then examined their crystallization under water-limited conditions (i.e., in dry air) as a function of time and at two temperatures (23 and 75 °C). Experiments were performed under dry conditions to slow the amorphous-to-crystalline transition so that it could be studied in detail. Our results show that the mol % Mg in amorphous calcium magnesium carbonates controls the rate of crystallization in air and the resulting complex mineral assemblages. We found that considerable amorphous material remains after more than 1 year, particularly for intermediate compositions near 50 mol % Mg. Room temperature crystallization in air is faster for Ca-bearing amorphous phases. Our results provide new insights that are applicable to biomineralization, the complex mineralogy of carbonate sediments and in developing methods to sequester CO₂ in minerals, particularly under water-limited conditions.