A comparative study of unrelaxed surfaces on quartz and kaolinite, using the periodic density functional theory

To investigate surface properties of fractured silica particles, which are commonly connected to the etiology of silica toxicity, models of low-index unrelaxed surfaces of quartz and kaolinite were constructed and analyzed using the periodic density functional theory calculations. The models were used to investigate surface sites that emerge in the processes of heterolytic and homolytic cleavage of quartz. It is found that the quartz surface is stabilized by two types of interactions. One, due to a more even charge distribution of sites, was characterized by surface energies of up to 0.025 eV x A(-2) and the other, due to a more even oxygen distribution between complementary surfaces, was up to 0.036 eV x A(-2). The total specific surface energies of unrelaxed surfaces ranged from 0.161 to 0.200 eV x A(-2) for quartz and from 0.017 to 0.158 eV x A(-2) for kaolinite. For the conchoidal fracture of quartz an average specific surface energy of 0.187 eV x A(-2) was obtained. These results provide a foundation for further characterization of the surface properties of mechanically comminuted respirable silica particulate and for reduction of occupational health hazards due to pulverized silica.