How Short-Lived Ikaite Affects Calcite Crystallization

R Besselink,L G Benning,T M Stawski,D J Tobler,J D Rodriguez-Blanco

doi:10.1021/acs.cgd.7b00743

Abstract

The pathways of CaCO3 crystallization are manifold, often involving one or several metastable amorphous or nanocrystalline intermediate phases. The presence of such intermediates is often overlooked, because they are short-lived and/or occur at small molar fractions. However, their occurrence does not just impact the mechanisms and pathways of formation of the final stable CaCO3 phase, but also affects their crystal size, shape, and structure. Here we document the presence of a short-lived intermediate through in situ and time-resolved small and wide-angle X-ray scattering combined with high resolution electron microscope observations. When ikaite forms concomitant with the dissolution of amorphous calcium carbonate (ACC) but prior to calcite formation, fairly large glendonite-type calcite crystals grow despite the presence of citrate ligands that usually reduce crystal size. These were ideal seeding crystals for further crystallization from supersaturated ions in solution. In contrast, in the absence of ikaite the crystallization of calcite proceeds through transformation from ACC, resulting in fine-grained spherulitic calcite with sizes ∼8 times smaller than when ikaite was present. Noteworthy is that the formation of the intermediate ikaite, although it consumes less than 3 mol % of the total precipitated CaCO3, still clearly affected the calcite formation mechanism.

Highlights

CaCO3 minerals are widespread in nature where they play a pivotal role in biomineralisation processes and in the carbon cycle.[1]
For the tested CIT/Ca ratios, amorphous calcium carbonate (ACC) formed within the first 30 seconds after solution mixing as indicated by the sharp increase to ∝ q-4 in the SAXS intensity in the low regime
The so formed ACC remained stable for approximately 20 minutes prior to crystalline CaCO3 formation

Summary

Introduction

CaCO3 minerals are widespread in nature where they play a pivotal role in biomineralisation processes and in the carbon cycle.[1] They are important to industry where they are used for the production of paint, ceramics, paper, drugs, food supplements, abrasives etc.[2] In many natural systems CaCO3 crystallizes as the thermodynamically most stable polymorph calcite. CaCO3 crystallization pathways are diverse and complex because they are sensitive to many parameters, including temperature, pressure pH and the presence of impurities.[3,4,5] From a kinetic point of view, it is very difficult to form highly symmetrical and completely dehydrated calcite through a simple reaction pathway. Metastable CaCO3 phases are often more formed, either reduced interfacial energy of metastable phases[6] or smaller degree of dehydration required for formation.[7]

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Conclusion