No abstract

This study uses empirical experimental evidence and Material Studio simulations to explain the interaction of sodium hydroxide (NaOH) with quartz. Density functional theory (DFT) calculations were carried out using the Cambridge Serial Total Energy Package. In addition, quartz grains subjected to dissolution in NaOH were characterized using scanning electron microscopy. The so-called O-middle termination in the quartz tetrahedron structure, typified by a solitary exposed oxygen atom at the surface, is the most susceptible SiO2 terminations to NaOH attack, as it is associated with the lowest surface energy. The adsorption energy values are − 1.44 kcal/mol and − 5.90 kcal/mol for a single atom layer and five-layered atomic structure, respectively. The DFT calculation reveals intramolecular energy is the dominant adsorption energy, followed by a weak van der Waals energy. The NaOH adsorbed on quartz (001) surface constitutes a lower band gap of 0.138 eV compared to cleaved quartz (001) surface (0.157 eV). In addition, the energy range of NaOH adsorbed on quartz is wider (− 50 to 10 eV), compared to (001) quartz (− 20 to 11 eV). The dissolved quartz showed the precipitation of sorbed silicate phases due to incongruent reactions, which indicates new voids and etch pits can be created through the cleaving of the sodium silicates sorbed into the quartz surface. The adsorption energy for NaOH interactions with reservoir sandstone was significantly higher compared to the solitary crystal grains, which can be attributed to the isotropic deformation of a single crystal, and non-uniform deformations of adjacent grains in granular quartz of sandstone reservoir. It can be inferred that exposure to NaOH will affect the structure and reactivity of quartz. The quartz surface textural study indicates that dissolution of crystalline (granite) and clastic rocks (sandstone) is critical to the development of voids, which will improve permeability by providing channels and routes for the passage of hydrothermal and reservoir fluids.

No abstract

No abstract

The stability of quartz in a multi-phase system of kaolinite and Al (hydr)oxides and quartz has been addressed in quite a few studies. However, the impact of pH on these multi-mineral interactions has not been elucidated. Therefore, this study shows the different dissolution behaviour and surface morphology of quartz admixed with hydrous kaolinite and aluminium (hydr)oxides under variable pH conditions. This will enhance the predictability of silica behaviour in natural rock weathering and reservoir systems. Scanning electron microscopy characterization shows that kaolinite and Al (hydr) oxides exhibit high-quality coating of quartz surface at acidic and alkali pH by creating a surficial secondary layer that allows agglomeration of other minerals, while Fourier transform infrared analysis showed shifts in peak positions of Si-O quartz from 691 cm −1 and 692 cm −1 at pH 5 and 7 to 686 cm −1 at pH13, and broadening of characteristic hydroxide and siloxane peaks as well as the formation of new Si-O-Al bonds at pH 13, suggesting structural changes in hydrous aluminosilicates and Al hydroxides (broken bonds of Al-O-Si, Al-O-Al and Si-O-Si) and Al substitution for Si in tetrahedral sheets. The dissolved silica in quartz-kaolinite-Al (hydr)oxides interactions is relatively lower under alkali conditions compared to acidic pH. Quartz dissolution was highest at pH 7; signifying the interaction of quartz with Al (hydr)oxides and kaolinite is favourable for quartz dissolution at neutral pH range. The results indicate that quartz interaction with kaolinite and Al (hydr)oxides is dependent on pH. Quartz coating by kaolinite and Al (hydr)oxides is more effective under alkali pH conditions, given the low solubility of Al within this pH range.