Classical vs. non-classical pathways of mineral formation (Invited)

Recent chemical analyses, microscopy studies and computer simulations suggest many minerals nucleate through aggregation of pre-nucleation clusters and grow by particle-mediated processes that involve amorphous or disordered precursors. Still other analyses, both experimental and computational, conclude that even simple mineral systems like calcium carbonate form via a barrier-free process of liquid-liquid separation, which is followed by dehydration of the ion-rich phase to form the solid products. However, careful measurements of calcite nucleation rates on a variety of ionized surfaces give results that are in complete agreement with the expectations of classical nucleation theory, in which clusters growing through ion-by-ion addition overcome a free energy barrier through the natural microscopic density fluctuations of the system. Here the challenge of integrating these seemingly disparate observations and analyses into a coherent picture of mineral formation is addressed by considering the energy barriers to calcite formation predicted by the classical theory and the changes in those barriers brought about by the introduction of interfaces and clusters, both stable and metastable. Results from a suite of in situ TEM, AFM, and optical experiments combined with simulations are used to illustrate the conclusions. The analyses show that the expected barrier to homogeneous calcite nucleation is prohibitive even at concentrations exceeding the solubility limit of amorphous calcium carbonate. However, as demonstrated by experiments on self-assembled monolayers, the introduction of surfaces that moderately decrease the interfacial energy associated with the forming nucleus can reduce the magnitude of the barrier to a level that is easily surmounted under typical laboratory conditions. In the absence of such surfaces, experiments that proceed by continually increasing supersaturation with time can easily by-pass direct nucleation of calcite and open up pathways through all other solid phases, as well as dense liquid phases associated with a spinodal. Simulations predict that this phase boundary lies within the region of the calcium carbonate - water phase diagram accessible at room temperature. AFM and TEM analyses of other mineral systems, particularly calcium phosphate, suggest cluster aggregation can play important roles both in modifying barriers and in biasing pathways towards or away from amorphous phases. Most importantly, analysis of the energetic changes shows that barriers are only reduced if the clusters are metastable relative to the free ions and that the reduction is naturally accompanied by a bias towards formation of amorphous precursors. Finally, results from in situ TEM observations of nanoparticle interactions are used to understand the mechanisms controlling particle-mediated growth following formation of primary nuclei of either crystalline phases or disordered precursors. Measurements of the particle speeds and accelerations are used to estimate the magnitude of the attractive potential that drives particle-particle aggregation.