Formation of Scales of Calcium Carbonate Polymorphs: The Influence of Magnesium Ion and Inhibitors
Authors: Nancollas G.H. , Sawada K. 
Published: December 2007

Summary
The crystallization of calcium carbonate on calcite and aragonite at 70 deg. C has been investigated by use of a highly reproducible seeded growth technique. In both cases the rates of reaction are proportional to the square of the relative supersaturations and are surface controlled. The presence of low levels of magnesium ion inhibits calcite growth with the formation of magnesian calcite, while at higher concentrations the spontaneous precipitation of aragonite takes place. Hydroxyethylidene 1,1 diphosphonic acid (HEDP), a potential scale inhibitor, markedly reduces the growth rate of both calcite and aragonite but has little effect on the crystallization of vaterite, the least thermodynamically stable calcium carbonate polymorph. The exclusive growth of vaterite on vaterite seed at 70 deg. C under these conditions strikingly demonstrates the possibility of the formation of intermediate metastable precursors during scale formation.

Introduction
The precipitation and dissolution of calcium carbonate is important in a wide variety of fields. Most subsurface waters produced with oil and gas contain relatively high concentrations of calcium ions. Moreover, since the concentration of CO2 in these water, is usually considerably greater than that at the surface, calcium carbonate is one of the more important scale-forming minerals in oil and gas production. In thermal flood operations, the reinjection of waste water into subsurface formations frequently introduces the problem of calcium carbonate scale formation. Although this often can be controlled by lowering the pH of the solution contacting the surfaces, the problem is made more acute by a decreasing solubility with increasing temperature and because the salt can crystallize from aqueous solution in at least three forms - calcite, aragonite, and vaterite. Although calcite has the greatest thermodynamic stability at ambient conditions, the thermodynamically less stable aragonite and/or vaterite phases may be stabilized under certain conditions of temperature or in the presence of other ions. Many surface waters contain appreciable quantities of magnesium ion, and its influence on the mineralogy and morphology of calcium carbonate has been investigated intensively. Thus, it is well established that the magnesium ion favors the precipitation of aragonite rather than calcite. In addition, this additive markedly inhibits the nucleation and crystal growth of calcite and sometimes induces formation of needle-like magnesian calcites, MgCal. In contrast, the crystal growth of aragonite is scarcely affected by the presence of magnesium ion. The inhibition of calcite growth by magnesium ion has been attributed to various factors. Reversible adsorption at active growth sites on the calcite crystals may prevent crystallization; the magnesium ion therefore behaves as a simple surface poison, allowing the aragonite, which precipitates more rapidly, to be stabilized kinetically. Another view is that the incorporation of magnesium ion into the growing crystalline phase during the formation of magnesian calcite introduces strain into the lattice with concomitant increase in solubility of the solid phase. The resulting decrease in the degree of super-saturation therefore leads to an apparent retardation of calcite growth caused by the decrease in driving force. The incorporation of magnesium ion having a smaller ionic radius compared with the calcium ion produces appreciable lattice strain on the surface of the calcite, inhibiting further crystal growth. This model also accounts for the observed formation of needle-like crystals during the growth of calcite in the presence of magnesium by preferentially poisoning crystal growth perpendicular to the c axis.
JPT
P. 645^