Aragonite growth in water-alcohol mixtures: Classical or nonclassical crystallization?
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Aragonite growth in water-alcohol mixtures: Classical or nonclassical crystallization? K. K. Sand,*,1 J. D. Rodriguez-Blanco,2 E. Makovicky,3 L. G. Benning,2 and S. L. S. Stipp.1 1
Nano-Science Center, Department of Chemistry, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen, Denmark. 2 School of Earth and Environment, University of Leeds, United Kingdom. 3 Department for Geography and Geology, University of Copenhagen, Denmark. ABSTRACT Aragonite can grow from calcium carbonate solutions as the favored phase, at ambient conditions, in the presence of 1:1 volume % water:ethanol. Its form is single and branched needles, with pseudohexagonal symmetry. Morphological evidence demonstrates that all precipitated aragonite is twinned. The recently popularized hypothesis of nonclassical growth by nanocrystal self assembly cannot describe the aragonite crystal form. Rather, its formation is effectively described as spherulitic growth, i.e. by classical crystal growth theory. INTRODUCTION Calcium carbonate has three nonhydrous crystalline polymorphs: vaterite, aragonite and calcite. Aragonite, the orthorhombic form, is not the stable phase at ambient conditions unless the temperature is >40 °C or a calcite growth inhibitor is present [1,2]. In this study, aragonite was obtained at ambient conditions, by adding ethanol to the aqueous solution [3,4]. Aragonite is a common CaCO3 biomineral, preferred by organisms because of its flexibility (e.g. as an inner shell layer in mollusks: nacre). It is well established that aragonite forms pseudohexagonal cyclic twins both in nature and in controlled experiments [5,6]. Aragonite twinning has three orientations for component individuals (1, 2 and 3), where the crystallographic a axis of each twin individual emerges perpendicular to the needle axis through the edge of the needle (Figure 1). The Individuals twin on the {110} symmetry planes by reflection on (110) with an angle between the a axes of 116.18˚. The ideal hexagonal angle is 120o (Figure 1). Nonclassical crystal growth has been much discussed in the recent literature. Both classical and nonclassical growth have been invoked to explain CaCO3 polymorph formation [7-13]. Classical nucleation theory describes the formation of critical nuclei by bonding of single molecules or ion-by-ion, so the unit cell is replicated without structural change in the bulk or at the surface. Nonclassical growth is described as the oriented aggregation of primary nanoparticles or nuclei in the presence of additives (e.g. polymers), producing a coherent crystal. This nonclassical hypothesis has become popular for describing the formation of CaCO3 polymorphs, when additives are present in growth solution. For the polymorphs, vaterite and aragonite, self assembly of CaCO3 nuclei has been proposed [14-19], even though the same crystal form is obtained regardless of the presence or absence of additives. The aim of this study was to explore the aragonite growth mechanism qualitatively, by combining mineralogical, crystallographic and morphol
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