DOI: 10.1021/cg900085e
Formation of Metastable CaCO3 Polymorphs in the Presence of Oxides and Silicates
2009, Vol. 9 4634–4641
Yu Lin,†,# Qiaona Hu,†,§ Jun Chen,† Junfeng Ji,† and H. Henry Teng*,‡ †
Department of Earth Sciences, Nanjing University, Nanjing, Jiangsu, 210093, P. R. China, and Department of Chemistry, the George Washington University, Washington, DC 20052. Current address: Dept. Geological & Environ. Sci., Stanford University, Stanford, CA 94305. § Current address: Dept. Geological Sci., University of Michigan, Ann Arbor, MI 48109. ‡
#
Received January 22, 2009; Revised Manuscript Received September 24, 2009
ABSTRACT: Carbonate crystallization experiments were carried out in the presence of solid-state oxides and silicates to test the effect of inorganic substrates on the formation of CaCO3 metastable polymorphs. Experimental results unambiguously demonstrate the selective formation of aragonite and vaterite in numerous cases. Further analysis indicates that these metastable phases may be correlated with negatively charged substrate surfaces. This correlation is observed in all cases except for SiO2. We suspect the exception is possibly due to the active dissolution of SiO2 at the experimental pH conditions. Additional results from experiments using pretreated aluminum silicate on which hydrated amorphous silica surface coating is generated by acid leaching support the dissolution hypothesis. These findings suggest that inorganic surface functional groups may have the ability similar to organic ones in selectively inducing metastable carbonate polymorph crystallization.
Introduction Polymorphism results from the ability of a particular chemical substance to adopt more than one crystal structure.1 Polymorphs usually possess different thermodynamic stabilities and physiochemical properties such as solubility, mechanical strength, and even color. Accordingly, the studies of polymorphism are not only scientifically fascinating, they also bear practical and financial relevance, particularly in materials engineering and the pharmaceutical industry. A widely interesting and important polymorphism is that of calcium carbonate (CaCO3) due to the minerals’ extensive occurrence, both biological and geological, on the earth surface. Calcium carbonate has three natural polymorphic forms: calcite (rhombohedral), aragonite (orthorhombic), vaterite (hexagonal). Thermodynamically, their stability increases in the order of vaterite, aragonite, and calcite. This leads to the metastable phases usually requiring more aggressive conditions to precipitate directly from aqueous solutions. For example, it is well established that high concentrations of Mg2þ in aqueous solutions is needed to induce preferential formation of aragonite over calcite.2 In biological systems, however, the metastable species are much more frequently seen in such organisms as molluscs and coccolithophores where the minerals serve as skeletal support. The most intriguing aspect of biomineralizing CaCO3 is that organisms are able to control the polymorphic switch at will, whereas the same task is very difficult to achieve in any lab settings. It is hypothesized that biomineralization is facilitated and directed by biomacromolecules such as proteins through selective interactions between specific organic functional groups and mineral surfaces or growth units.3 Laboratory studies show the presence of organics such as amino acids,4 poly organic acids,5 and proteins6 as additives or templates is indeed capable of selectively crystallizing metastable carbonate *To whom correspondence should be addressed. E-mail: hteng@ gwu.edu. pubs.acs.org/crystal
Published on Web 10/16/2009
polymorphs. Of the variety of functional groups including phosphonates7 and sulfonates8 tested by previous workers, carboxyls and hydroxyls were found to be particularly effective.9 For instance, poly(vinyl alcohol) (PVA) adsorbed on polyamide fibers induces exclusive formation of vaterite or aragonite.10 Similar observations were made for poly(acrylic acid) (PAA) that has an identical backbone as PVA but with -COOH instead of -OH being the repeating functional groups.11 Motivated by the understanding of organic functional groups’ participation in directing CaCO3 polymorph formation, we conducted carbonate crystallization experiments in the presence of solid-state silicates and oxides to test whether inorganic surface functional groups can have a similar effect. Development of functional groups on inorganic solid surfaces can be understood in light of surface complexation reactions by which water molecules form chemical bonds with undercoordinated surface ions via chemisorption.12,13 These surface-bound water molecules can undergo a proton transfer process that shifts hydrogen ions onto neighboring surface anions. On (hydr)oxide materials, for example, this dissociative sorption of water molecules results in a hydroxylated surface on which the reactive sites are surface hydroxyl groups X(OH)0 ( denotes lattice bonds and X represents cation components of the (hydr)oxides). The surface bound hydroxyls can be further protonated or deprotonated to form charged surface species XOH2þ and XO-, respectively, depending upon the pH conditions of the environments. It was reported that hydroxylation of many metal oxide surfaces proceeds to completion in air at relative humidity levels of ,1%,14 indicating a wide occurrence of surface hydroxyls and related charge groups in aqueous environments. We hypothesize that, analogous to -OH and -COOH on organic molecules, the functional groups on the surfaces of silicates and oxides would have a similar ability to control carbonate polymorph crystallization. The goal of this study is to test the hypothesis and to compare the effect of different substrates if any positive observations are made. r 2009 American Chemical Society
Article
Crystal Growth & Design, Vol. 9, No. 11, 2009
4635
Table 1. Tested Silicate Phases and Their Corresponding Mineral Names chemical formula
mineral name
KAlSi3O8 Al2Si2O5(OH)4 K(Mg,Fe)3(AlSi3)O10(OH)2 KAl2(AlSi3)O10(OH)2 ([Mg,Fe]5Al)(AlSi3)O10(OH)8
potassium feldspar kaolinite biotite illite chlorite
Experimental Procedures Preparation of Oxides and Silicates. Both simple (SiO2, Al2O3, Mn3O4, and Fe3O4) and complex (K2O 3 Al2O3 3 6SiO2) oxides were tested in this study. SiO2 of different crystallinity (amorphous, aphanitic or cryptocrystalline, and crystalline) came from a synthetic silicon oxide, an agate, and a quartz sand sample, respectively. Al2O3 was prepared from the mineral corundum. The original silica and alumina were ground using a mortar and pestle and separated into size fractions of 95
25 ∼5
20 100 20 40
80 20 60
60
∼100 5 15 100
∼100 95 ∼100 85
diameter. As expected, the crystals grown in the control experiments were exclusively calcite as indicated by the polyhedron morphology (Figure 1) and confirmed by XRD analysis (not shown here). However, aragonite and vaterite formed extensively in a number of experiments where different substrates were present (Table 2). Morphologically,10,11,19 aragonite appeared needle-like or spiky, while vaterite, resembling a conglomerate of flakes, frequently took on a spherulitic or rossette shape with a 6-fold symmetry. Carbonate Crystallization in the Presence of Simple Oxides. 1. Calcite Dominance on SiO2 and Mn3O4. Two series of SiO2-bearing experiments were carried out to test the effects of (1) SiO2 crystallinity and (2) quartz grain size on carbonate polymorph formation. The first series included crystalline, aphanitic, and amorphous SiO2 with grain sizes
