Role of metastable phases in the spontaneous precipitation of calcium carbonate

Abstract

This paper reports a study of the spontaneous precipitation of calcium carbonate from aqueous solutions which are very supersaturated with respect to calcite, both in the absence of inhibitors, and in the presence of triphosphate as inhibitor. The sequence of events during precipitation is governed strongly by the initial supersaturation, t h e temperature, and t h e presence or absence of triphosphate.At high supersaturation, the first-formed phase is an amorphous calcium carbonate. It is observed only above a well defined ionic activity product, is homogeneously nucleated, and its formation is not inhibited by triphosphate. Over the temperature range 289-333 K, this amorphous phase has a solubility product, K , , defined by the equation : log K1 = (1247.0/T) -10.224At concentrations insufficient to produce the amorphous phase, t h e first-formed solid nucleates heterogeneously. In t h e absence of triphosphate it is calcite. In the presence of triphosphate, the first solid is calcium

A thermodynamic solution model for calcium carbonate: Towards an understanding of multi-equilibria precipitation pathways

“…The calculation methods are based on thermodynamic equilibria of all species in the solution, including the activity correction in or-der to approach their exact composition. More often than not, solubility calculations are only applied to calculate new dissociation constants [6][7][8][9][10] or initial solution compositions [11][12][13][14][15][16][17]. Thus, solubility calculations are only applied in general to equilibrated systems.…”

Transient precursor amorphous phases in biomineralization.In the footsteps of Heinz A. Lowenstam

“…ACC is highly unstable in vitro [11,12]. The discovery that echinoderm larvae form their skeleton through an amorphous precursor phase was followed by a series of studies which showed that echinoderm adults also adopt the same strategy [13], as do larval mollusks [14,15] and mature mollusks [16].…”

“…There is a so-called "confusion principle" in glass physics that relates to this process [57]. It has been shown in vitro that Mg [58], phosphate [12] and silicate [59] stabilize ACC. It has also been shown that certain proteins that are extracted from calcium carbonate-containing mineralized tissues can stabilize ACC in vitro either transiently or permanently [17,25,60,61].…”

Now You See Them

“…Calcium carbonate is highly polymorphic in that it can exist in six different crystal structures. The first polymorph formed after nucleation is often amorphous calcium carbonate (ACC), which subsequently crystallizes (6,7). ACC has no long-range order, but it often has short-range structural order that appears to determine the lattice structure after crystallization (8).…”