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Structure and Carbonate Orientation of Vaterite (Caco3)

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Structure and Carbonate Orientation of Vaterite (Caco3) American Mineralogist, Volume 94, pages 380–386, 2009 Structure and carbonate orientation of vaterite (CaCO3) JIANWEI WANG * AND UDO BECKER Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, U.S.A. AB STRACT First-principles calculations and molecular-dynamics simulations are employed in this study to further our understanding of the crystal structure and orientational order of carbonate ions in vaterite, which is the least stable polymorph of calcium carbonate. The structural details of vaterite have been controversial but they are prerequisite for investigating and understanding the processes involving vaterite crystal nucleation, growth, and stabilization at a molecular level. The first-principles calcula- tions, using density functional theory with the plane-wave pseudopotential method, are carried out to calculate relative thermodynamic stabilities of proposed structures. Molecular-dynamics simulations with classical empirical potentials, larger computational cells, and fully flexible models at different temperatures are performed to investigate the orientation and order-disorder of the carbonate ions. The results show that the previously accepted structure with disordered CO3 ions, which are randomly distributed over three orientations parallel to the c axis, is only metastable. By applying a temperature annealing technique to the molecular dynamics simulations, a more stable structure with fully ordered carbonate ions is found that has a hexagonal superstructure. The space group of this newly derived vaterite structure is P6522 (no. 179) with Z = 18 and cell dimensions of √3 times in a, and 3 times in c of the previous suggested disordered structure. Comparison of experimental observations of X-ray diffraction patterns, enthalpies of transformation to calcite, and volume change by heat treatment with our theoretical calculations indicates that freshly made vaterite is often carbonate-disordered and meta- stable and can fully or partially transform to a carbonate-ordered structure by aging and heating. Keywords: Vaterite, crystal structure, order-disorder, quantum mechanical calculations, XRD data, thermodynamics INTROD U CTION vaterite is stable at temperatures up to 670 K (Turnbull 1973). Calcium carbonates occur widely in nature and are the major Although less common than the other polymorphs, vaterite and minerals in many sedimentological environments. They play an its initial nucleation, growth, and stabilization offer an oppor- important role in biological functions and processes (Heywood tunity to study the mechanism of biomineralization and also to 1994; Mann 2001; Skinner 2005; Rodriguez-Navarro et al. 2007), understand crystal nucleation and growth in general (Falini et al. and they are valuable in many industrial applications, such as 1998; Champ et al. 2000). However, its crystal structure, the basis paper, rubber, plastics, and paint production (Xiang et al. 2006). for such growth and nucleation studies, is still controversial and Vaterite is thermodynamically stable with respect to amorphous not well understood (Medeiros et al. 2007). Thus, before major calcium carbonate, but metastable with respect to the other progress can be made on understanding processes involving the two crystalline polymorphs, aragonite, and calcite. Therefore, formation and stabilization of vaterite at the atomic level, details vaterite is a rare mineral in geologic settings, though it may be of its crystal structure need to be resolved. an important precursor in several carbonate-forming processes. Vaterite has the same chemical composition as calcite (rhom- Its natural occurrence is often associated with biogenic activities bohedral) and aragonite (orthorhombic), but with a different (Ariani et al. 1993; Falini et al. 1998, 2005; Kanakis et al. 2001; crystal structure in terms of symmetry, orientation of CO3 ions, Mann 2001), as in pancreatic stone or clogging in human heart and coordination environment of Ca ions. There is general agree- valves (Falini et al. 1998; Kanakis et al. 2001), hard tissues of ment that the carbonate planes are parallel to the c axis, which fish otoliths (Falini et al. 2005), and crustacean statoliths (Ariani is consistent with its optical properties such as uniaxial positive et al. 1993). In vitro, vaterite can be easily synthesized (Plummer interference figures with nω = 1.55 and nε = 1.65 (Kamhi 1963), and Busenberg 1982; Shen et al. 2006). At ambient conditions, and that the calcium atoms are in eightfold coordination with mixing of calcium chloride and sodium carbonate salt solutions oxygen atoms. In contrast, the CO3 ions in calcite and aragonite leads first to the deposition of an amorphous calcium carbonate are perpendicular to the c-axis, and the calcium atoms have phase that, within minutes, transforms to vaterite. If vaterite sixfold coordination in calcite and ninefold in aragonite (Kamhi stays in contact with the solution, within one day, it transforms to 1963; Villiers 1971; Markgraf and Reeder 1985). For vaterite, calcite, the most stable polymorph. If kept dry, such an abiogenic its symmetry and space group, dimensions of the unit cell, and orientation and site symmetry of the CO3 ions are controversial. Meyer (1959) was the first to hypothesize a vaterite crystal struc- * E-mail: jwwang@umich.edu ture derived from single-crystal X-ray diffraction experiments 0003-004X/09/0203–380$05.00/DOI: 10.2138/am.2009.2939 380 WANG AND BECKER: STRUCTURE AND CARBONATE ORIENTATION OF VATERITE (CaCO3) 381 and he reported an orthorhombic unit cell with a space group existence of two distinct symmetric stretching modes for CO3 Pbnm and Z = 4 (Fig. 1a). In this structure model, all sites are ions (Behrens et al. 1995). Anderson’s group theory analysis fully occupied, which is not consistent with apparent disordering (Anderson 1996) favors both Kamhi’s (1963) and Lippmann’s of oxygen sites originated from multiple carbonate orientations (1973) structures. A more recent Raman study (Gabrielli et al. in the structure. Kamhi (1963) found a structure with hexagonal 2000) shows that Meyer’s structure (Meyer 1969) is consistent symmetry and space group P63/mmc using the same XRD tech- with the Raman spectra. Although there is no consensus on the nique (Fig. 1b). There is only one unique carbonate in the unit structure, some features regarding the vaterite crystal structure cell, and each atom of the CO3 ion is disordered with a partial oc- emerge: (1) the Ca ions are in a hexagonal sublattice and the unit cupancy of 1/3, which accounts for considerable disorder. Based cell of this sublattice is hexagonal; (2) CO3 ions are parallel to on numerous diffraction streaks parallel to the c direction in the the c axis; and (3) there is considerable disordering of CO3 ions X-ray diffraction pattern (Kamhi 1963), a superstructure with a in the structure. unit cell rotation of 30° and a = √3 · a′, c = 2*c′, and Z = 12 was To better understand and ultimately resolve the contro- suggested, apparently owing to partial ordering of the carbonate versy on the vaterite crystal structure, quantum-mechanical ions in the vaterite sample. However, details of the superstruc- (first-principles) calculations are used in this study to estimate the ture such as symmetry and atomic positions were not provided. relative stability of the structures proposed in the literature. Mo- Meyer (1969) later provided a structure mainly consistent with lecular dynamics (MD) simulation with temperature annealing is Kamhi’s (1963) structure, which has the same space group but used to address disordering and to find an ordered superstructure. different carbonate site symmetry. Meyer (1969) also noticed The molecular modeling reported here also demonstrates the diffuse streaks and satellite reflections in the diffraction pattern value of combining these two techniques for predicting crystal and argued that there is possible stacking disorder of single lay- structures based on calculating the energetics without any free ers of trigonal symmetry related by glide reflections and screw or fitted parameters and for investigating dynamic phenomena axes along c. Again, no details of such stacking faults were dis- by simulating large systems at finite temperatures. cussed. Lippmann (1973) developed an idealized structure from COMP U TATIONAL METHODS the vaterite-type high-temperature phase of YbBO3 (Bradley et al. 1966) with space group P6322. Spectroscopic methods have Quantum-mechanics calculations been applied to resolve the controversy on the vaterite structure Quantum-mechanical calculations are performed within the density functional and focused on space group and site symmetry. However, the theory (DFT) framework using plane-wave basis sets as implemented in the VASP results are inconsistent, often due to impurities in the samples, package (Kresse and Furthmuller 2004). The projector-augmented wave method mode assignments, and differences of group theory analysis. For (PAW) (Blöchl 1994) and exchange-correlation as parameterized by Perdew-Wang example, Sato and Matsuda’s infrared spectra (Sato and Matsuda 91 (PW91 functionals) (Perdew et al. 1992; Perdew and Wang 1992) are applied in the generalized gradient approximation (Perdew and Wang 1986) (GGA-PAW). 1969) support Kamhi’s structure (Kamhi 1963). Behrens et al. The Monkhorst-Pack scheme for integration in the Brillouin zone is adopted and (1995) argued that none of the proposed structures, P63/mmc the number of k-points is determined to be large enough in our
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