Fluid-mineral interactions are critical components of many natural processes resulting in possible alteration of both the pore structure and mineral composition of earth materials, sometimes leading to complete replacement of one mineral phase by another. Often, these replacement reactions proceed via dissolution of the original phase and subsequent re-precipitation of a different product phase. However, the effects of both the original rock texture and chemical reactivity on the rates, structures and extent of porosity evolution during replacement are poorly understood, although a quantitative understanding of these processes is crucial for predicting contaminant transport in a range of energy applications, e.g. nuclear waste management, oil and gas exploration, geothermal energy. We present a systematic experimental study of limestone dissolution and subsequent reprecipitation in the model system calcite-fluorite (CaCO3-CaF2) in which we varied both texture and chemical reactivity of the parent rock. Limestones and dolostones were reacted with NH4F solution for 2 – 45 days at 200 °C. By combining X-Ray and neutron ultra-small angle scattering ((U)SANS), we quantified changes in the porosity as a function of depth within the sample and time. Porosity characterization was coupled with chemical mapping by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDX) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). The results demonstrate that changes in the replacement mechanism are controlled by changes in chemical reactivity, whereas microstructure primarily controls replacement speed and the size regime of porosity development. Abundance of features that enhance the reactive surface area, such as porosity, grain boundaries or twinning within minerals, leads to a faster replacement via these pathways. Our results demonstrate the importance of the initial microstructure for the replacement reaction speed and mechanism.

Microstructural changes in dissolution/reprecipitation of CaCO3-CaMg(CO3)2 and CaF2

2019 Fall National Meeting

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