Calcified minerals in biogenic materials often play a utilitarian role, such as structural supports in bone, teeth, and shells, where the crystals are arranged in well-ordered arrays (1, 2). The ability of organisms to produce single crystalline scaffolds and hierarchical architectures with unique features, such as bent or spheroidal shapes, has long fascinated scientists. The mechanisms of biomineral nucleation and growth are complex and not fully understood, while the ability to mimic these processes in vitro has proven challenging. Much attention has been given to studying the skeletal sections of sea urchins and other organisms (e.g., mollusks, echinoderms, calcisponges, and corals) that contain curved surfaces and various convoluted shapes (3). From numerous studies of these materials, which are predominantly composed of calcium carbonate minerals, it has become evident that crystallization originates from the initial formation of amorphous calcium carbonate (ACC) (4, 5). The pathways by which ACC transforms into crystalline CaCO₃ polymorphs has been a topic of ongoing investigation, although many fundamental details remain elusive. In PNAS, Liu et al. (6) report the time-resolved evolution of ACC to crystalline calcium carbonate using in situ transmission electron microscopy to show that the presence of inorganic ions leads to the direct transformation of the amorphous phase to calcite, all the while preserving the morphology of the original precursor.

ACC has become a recognized and frequently investigated form of calcium carbonate owing to its importance in biomineralization. Six additional crystalline forms of calcium carbonate (1) include calcite, aragonite, vaterite, monohydrocalcite, calcium carbonate hexahydrate (ikaite), and the most recently discovered hemihydrate (CaCO₃·1/2H₂O) (7). ACC is structurally complex (i.e., not a single mineral phase) with varying degrees of hydration. For example, in sea urchin larval spicules it has been shown that crystallization involves three distinct stages: an initial (short-lived) hydrated ACC phase, an intermediate (transient) dehydrated form of ACC, and finally crystalline calcite (8). This sequence is consistent with thermodynamic data from Navrotsky and coworkers (9) who showed that the amorphous-to-crystalline transition occurs in the following order (energetically downhill): ACC (more metastable hydrated) → ACC (less metastable hydrated) → ACC (anhydrous) → vaterite → aragonite → calcite. Chemical analysis of ACC phases extracted from biominerals also reveals the presence of inorganic ions, such as Mg²⁺ (4, 10, 11), and organic macromolecules that can facilitate the stabilization of transient ACC phases (8, 12).

In PNAS, Liu et al. (6) elegantly show that the presence of Mg²⁺ ions alters the ACC-to-calcite transition pathway in a concentration-dependent manner. While it has been known that Mg²⁺ incorporation in ACC leads to increased water content (13), Liu et al. confirm this using molecular dynamics to show that the presence of hydrogen-bonded networks between H₂O and CO₃²⁻ allow for more rapid ion rearrangements in ACC (14). Introduction of these unstable sites putatively enables the direct transformation of ACC into calcite wherein nucleation occurs within the amorphous precursor. This process is in stark contrast to calcite nucleation in solution involving the dissolution–reprecipitation of amorphous precursors. Liu et al. suggest that the presence of Mg²⁺ ions in solution inhibit this pathway by decreasing the supersaturation with respect to calcite. This, in turn, allows more time for ACC phases to directly transform into Mg-calcite by virtue of the incorporated structured water. This process is illustrated in Fig. 1 where the spherical shape of ACC particles infused with Mg²⁺ ions is preserved throughout calcite nucleation and growth. During this process, water is expelled from the ACC phase during concomitant densification without any noticeable change in particle morphology. Given the presence of Mg²⁺ ions in numerous biominerals, it is conceivable that this pathway may offer an explanation for the mechanism by which organisms produce exquisite structures where the ACC phases function as “molds” to preserve shape during mineralization.

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Fig. 1.

Illustration of the shape-preserving amorphous-to-crystalline transformation in magnesium-containing precursors of calcium carbonate, which is one of the most abundant near-surface biominerals and a structural component of sea urchin spicules and coccolith exoskeletons (both depicted in the blueprint as bioinspired designs). This in vitro process involves two-step nucleation wherein the spheroidal morphology of amorphous calcium carbonate (ACC) is maintained throughout crystallization of Mg-calcite. The role of Mg²⁺ ions is generally twofold: These additives inhibit nucleation in the solution phase, and they also bring excess water into the precursors to facilitate solute rearrangement within the solid phase, thus allowing calcite to retain the original shape of the amorphous particles.

The nucleation of calcite in (or on) the amorphous solid is characteristic of a two-step process that was first observed for proteins (15) but has since become a ubiquitous mechanism in the field of crystallization. The physical state of amorphous precursors can be quite diverse, ranging from liquid-like droplets and clusters to gels or nanoparticles. The impact of amorphous species in crystallization is increasingly more evident given the significant increase in literature citations emphasizing their roles as precursors for nucleation as well as growth units during crystallization by particle attachment, which both fall under the moniker of nonclassical pathways (16). There are additional examples, such as zeolites (nanoporous aluminosilicates) (17), where amorphous gel-like precursors undergo direct transformation to crystals; however, unlike the evolution of Mg-ACC precursors, the spherical morphology of zeolite amorphous gels is not preserved during their disorder-to-order transition, resulting in faceted cubes, analogous to the morphology of calcite in the absence of inorganic additives, as reported in PNAS. The unique observation by Liu et al. may reflect the ability of Mg²⁺ ions to act as inhibitors of calcification, either on the surfaces of growing crystals (18) or by their incorporation in calcite leading to defects that increase mineral solubility (19).

ACC has been observed as a precursor to aragonite [55,56], a polymorph of calcite; however, the role of amorphous phases in natural systems extends beyond ACC to diverse materials such as silica in plants and animals (opal), the skeletons of single-celled organisms (diatoms), or multicellular tissues (sponges) (1, 20, 21). Additional examples include magnetite formed from a disordered ferrihydrite precursor (22) and the amorphous calcium phosphate (ACP) phase detected in the fins of zebrafish (23). Indeed, there is increasing in vivo and in vitro evidence that ACP in collagen matrices plays an important role in bone formation (24, 25). Analogous to ACC, it is hypothesized that ACP acts as a transient (precursor) phase during the growth of hydroxyapatite. This further highlights the important role of direct amorphous-to-crystalline transformations in many natural and biological processes. Studies such as those by Liu et al. provide insights into the mechanisms of biomineralization. Moreover, the discovery of controlled transformation of amorphous phases to crystals has practical implications in the manufacturing of new materials. For instance, Tang and coworkers (26) previously demonstrated the ability to mold calcite into shapes (e.g., stars) and patterned arrays from cross-linked ionic oligomers using organic additives. The ability to accomplish similar processes in ACC using magnesium, an inexpensive earth-abundant resource, holds considerable promise for designing facile, efficient methods to tailor single crystalline materials with unprecedented morphologies that cannot be easily achieved through conventional routes.