Effect of Strontium Ions on Crystallization of Amorphous Calcium Carbonate

Abstract

In biomineralization, amorphous precursors enable formation of complicated crystal shapes and incorporation of organic and inorganic impurities. In some cases the concentrations of the inorganic impurity atoms greatly exceed the thermodynamical (equilibrium) solubility limits. To shed additional light on this, crystallization experiments with Sr-rich amorphous calcium carbonate (ACC) are carried out. The Sr-concentration in ACC, Sr/(Ca+Sr) 0.3, is more than 60 times larger than its solubility limit in calcite. Crystallization of ACC is accomplished using two different pathways: i) in a high humidity environment at room temperature, and ii) upon heating at 300°C. In both cases, X-ray diffraction measurements revealed the formation of particular crystal phases with an ability to take up substantial amounts of Sr. Crystals formed via the humidity route comprised spatially separated regions of almost pure rhombohedral calcite and orthorhombic aragonite/strontianite solid solution with a Sr/(Ca+Sr) ratio equal nearly 0.5. Crystallization of ACC upon heating resulted in the appearance of a new calcite-like phase with reduced space symmetry, R3m, instead of R3c for regular calcite. Symmetry reduction in this Sr-rich R3m-phase is due to the rotational disorder of flat carbonate groups about the c-axis, which is responsible for the enhanced lattice capacity of Sr incorporation.

Additives Control the Stability of Amorphous Calcium Carbonate via Two Different Mechanisms: Surface Adsorption versus Bulk Incorporation

“…[ 3 ] In some organisms ACC can be stabilized for many years. [ 4 ] Previous studies have shown that the high stability of biogenic ACC could be attributed to the presence of soluble inorganic and organic additives, such as magnesium (Mg 2+ ) ions, [ 5 ] strontium (Sr 2+ ) ions, [ 6 ] silicate (SiO 4 4− ) ions, [ 7 ] phosphate (PO 4 3− ) ions, [ 4,8 ] organic molecules, [ 9 ] phosphorylated proteins, [ 8a ] and intrinsically disordered proteins, [ 10 ] etc. Because crystallization of ACC in solution typically proceeds via a dissolution and recrystallization mechanism, [ 11 ] the high stability of ACC in the presence of additives is often ascribed to their ability to inhibit the dissolution of ACC or the formation of crystalline phases.…”

Effect of catalyst preparation and storage on chemical looping epoxidation of ethylene

“…In both Fig.6 a) and b), the composition was checked with EDX (Section 11 in the Supplementary Information), confirming that the species at the surfaces contain Sr and O (but no Ag or Fe). The amorphous form of the carbonate and hydroxide is possible[31,32], although neither is thermally stable. In contrast, the surface of the sample calcined at 650ºC, Fig.6 c) and d), was free of impurities.…”

Thermodynamic Control of Amorphous Precursor Phases for Calcium Carbonate via Additive Ions

“…For the initial systems, 50–200 additive ions were replaced by Ca 2+ ions (e.g., x = 0.25, 0.5, 0.75, and 1.0). Note that these high fractions of additive ions (i.e., x ≥ 0.25) have been reported to be experimentally feasible in the previous literature. ,, Next, according to the previously reported dehydration scheme by Wallace et al, we removed water molecules until the water-to-cation ratio ( n ) became zero. It was achieved through a total of 18 sequential NPT MD simulations; each simulation contained 200 (i.e., H 2 O/Ca 2+ ratio = ∼2–9) or 40 (i.e., H 2 O/Ca 2+ ratio = ∼0–2) fewer water molecules than the previous step.…”