Mechanisms of aqueous crystallization and phase transformation in titanium oxide minerals.

摘要

The sequence of mineral phases that appear during the crystallization of titanium oxides from aqueous solutions was examined using a variety of experimental and theoretical techniques. Time-resolved X-ray diffraction experiments showed that the traditionally metastable phase anatase was the first TiO 2 polymorph to crystallize for all conditions, and anatase converted to rutile over many hours. Thus, the reversal of thermodynamic phase stability for particles with large surface area to volume ratios can be attributed to increased surface energies at the nanoscale. Quantum energy calculations on model anatase and rutile nanoparticles and surfaces revealed that the energy of under-bonded atoms on various crystallographic surfaces, and especially defects at the edges and corners of nanocrystals, were responsible for the stability reversal rather than surface-induced perturbations to the crystal structure.;Kinetic modeling of mineral abundance data from the XRD experiments, using a self-authored kinetic fitting program, demonstrated that minerals precipitate from and redissolve into solution without undergoing solid state phase transitions. Time-resolved small-angle X-ray scattering (TR-SAXS) revealed that nanoparticles nucleate and grow rapidly into broad, Gaussian-type particle size distributions, and also showed evidence of oriented aggregation. An analysis of the in situ pH data and XRD data for the co-existing particles revealed the counterintuitive conclusion that precipitation is slower at high Ti 4+ concentrations. A crystallization model is proposed based on classical nucleation theory, in which different interfacial energies between the mineral surface and the fluid cause each polymorph to have different activation energies of precipitation. These activation energies scale positively with the saturation state of the fluid.