Elisabete Trindade Pedrosa

Understanding mineral dissolution is relevant for natural and industrial processes that involve the interaction of crystalline solids and fluids. The dissolution of slow dissolving minerals is typically surface controlled as opposed to diffusion/transport controlled. At these conditions, the dissolution rate is no longer constant in time or space, an outcome observed in rate maps and correspondent rate spectra. The contribution and statistical prevalence of different dissolution mechanisms is not known. Aiming to contribute to close this gap, we present a statistical analysis of the variability of calcite dissolution rates at the nano- to micrometer scale. A calcite-cemented sandstone was used to perform flow experiments. Dissolution of the calcite-filled rock pores was measured using vertical scanning interferometry. The resultant types of surface morphologies influenced the outcome of dissolution. We provide a statistical description of these morphologies and show their temporal evolution as an alternative to the lack of rate spatial variability in rate constants. Crystal size impacts dissolution rates most probably due to the contribution of the crystal edges. We propose a new methodology to analyze the highest rates (tales of rate spectra) that represent the formation of deeper etch pits. These results have application to the parametrization and upscaling of geochemical kinetic models, the characterization of industrial solid materials and the fundamental understanding of crystal dissolution.

Understanding the mechanism and kinetics of the replacement of carbonates by fluorite has applications in Earth sciences and engineering. Samples of Carrara marble were reacted with an ammonium fluoride (NH₄F) solution for different reaction times and temperatures. The microstructure of the product phase (fluorite) was analyzed using SEM. The kinetics of replacement was monitored using Rietveld refinements of X-ray powder diffraction patterns of the products. After reaction, all samples preserved their size and external morphology (a pseudomorphic replacement). The grain boundaries of the original marble were preserved although each calcite grain was replaced by multiple fine crystals of fluorite creating inter-crystal porosity. The empirical activation energy E_a (kJ/mol) of the replacement reaction was determined by both model-fitting and model-free methods. The isoconversional method yielded an empirical activation energy of 41 kJ/mol, and a statistical approach applied to the model-fitting method revealed that the replacement of Carrara marble by fluorite is better fitted to a diffusion-controlled process. These results suggest that the replacement reaction depends on the ion diffusion rate in the fluid phase through the newly formed porosity.

Mineral replacement reactions are common phenomena in natural and laboratory environments where solids have re-equilibrated with aqueous solutions and are characterized by the generation or destruction of porosity in the product phase(s). Here, the evolution of porosity during the replacement of calcite by fluorite is used as a model system to characterize the kinetics of volume variations. Non-porous single crystals of calcite were reacted with sodium fluoride solutions for different reaction times. The crystals were pseudomorphically replaced by highly porous fluorite. Complementary use of porosimetry techniques, high resolution imaging, and mass-balance calculations revealed the total, open, and closed porosity in the samples. The infiltration of aqueous fluids in the Earth depends on the evolution of porosity and hence porosity variations are important in various geological processes such as, rock weathering and soil formation, fluid-controlled metamorphism, mineral ore emplacement, or oil and gas reservoir compaction. Such mechanisms can also be used to develop geo-inspired materials designed for industrial and medical applications.