Fluid−mineral interactions can alter the pore structure and mineral composition of earth materials via a number of interactions. One important process, sometimes leading to complete replacement of mineral grains and the formation of pseudomorphs, is replacement. Such replacements can either increase or decrease the porosity of the rock, depending on the change in solid molar volume of the reactions. Increases in porosity may either partially or completely armor the rock from further reaction, or cause disaggregation due to crystallization pressures. A quantitative understanding of these processes is needed for the prediction of contaminant transport in nuclear waste management, oil and gas exploration, geothermal energy production, and in many geological processes. We have performed a series of systematic experimental studies replacing limestone, granite and dunite, as well as single crystals of calcite, in order to evaluate this process varying both the texture and chemical reactivity of the parent rock. Limestone experiments have involved replacement in the model systems calcite-fluorite, calcite-dolomite-magnesite, calcite-siderite and calcite-malachite. Silicates were reacted with NH4F, H2O ± NaCl and NaOH solutions. The results depended not only on the solid/fluid reactivity, but on the initial porosity. In the case of positive volume change reactions, for instance, high-permeability rocks tended to be disaggregated by crystallization pressures, while low-permeability rocks tended to react only at the edges, with coreward reactions proceeding along grain boundaries By combining X-ray and neutron (ultra) small-angle scattering we have quantified changes in porosity and mineralogy as a function of depth with time. This spatial resolution was obtained using a set of annular Cd-masks during neutron scattering and a small beam and a rotating sample for X-ray scattering. Microstructural changes were also investigated using scanning electron microscopy (SEM), chemical imaging by time-of-flight secondary ion mass spectrometry (ToF−SIMS) and SEM-energy dispersive X-ray spectroscopy (SEM-EDX).

Volume and texture controls on replacement reactions in carbonates and silicates

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.