Volume expansion reactions significantly influence the rate of fluid-mineral interactions in various geological processes, including silicate weathering, retrograde metamorphism, and mineralization. Previous research has largely focused on fracture generation and propagation resulting from these reactions, suggesting that self-stress within minerals plays a crucial role. However, the impact of volume expansion on the overall rate of mineral replacement reactions remains poorly understood. Compaction of reaction products can inhibit reactant exchange, while the formation of new minerals can create fractures and increase reactive surface area. This study aims to investigate the interplay between these competing effects by examining the replacement of pyrite by chalcopyrite under controlled conditions. Pyrite was chosen due to its ease of collection and preparation, rapid transformation at 200°C (allowing observation on a laboratory timescale), and simple composition, which eliminates complications from chemical changes during hydrothermal experiments.

Existing literature highlights the role of reaction-enhanced permeability in processes such as retrogressive metamorphism and silicate weathering. Studies have shown that self-stress from fluid-mineral interaction drives fracture propagation. However, the effect of volume expansion on mineral replacement remains unclear. While some studies suggest that volume expansion can lead to compaction and inhibition of reactions, others emphasize the positive effect of fracture generation on increasing reactive surface area. There's a lack of research comprehensively addressing the competing influences of these two processes on mineral replacement reaction rates during volume expansion.

Natural pyrite crystals were cut into 2 mm cubes, polished to ensure consistent surface roughness, and reacted with an aqueous CuCl solution at 200°C under saturated vapor pressure. The pH was buffered at 4.5 using an acetate solution. Experiments were run for durations ranging from 5 to 63 days. Post-reaction, the cubes were cleaned, weighed, and their dimensions measured. Scanning electron microscopy (SEM), electron probe microanalysis (EPMA), micro-area and powder X-ray diffraction (µ-XRD), Raman spectroscopy, and transmission electron microscopy (TEM) were employed to characterize the reaction products and textures. A Particles (Pores) and Cracks Analysis System (PCAS) was used to quantify the perimeters of fractured pyrite grains in polished sections. Solubility diagrams were generated using the Geochemist's Workbench to assess the relative solubilities of pyrite and chalcopyrite and to understand the rate-limiting step in the reaction. Crystallization pressure was estimated using standard free energy change calculations.

Microscopic observations revealed that the replacement of pyrite by chalcopyrite resulted in a significant volume increase (up to 67%), consistent with theoretical calculations. The reaction initiated at localized sites, forming chalcopyrite mounds. Abundant fractures were observed, classified as radial fractures, fractures along cleavage planes, and hierarchical fractures. Initially, open fractures separated pyrite grains, enhancing reactive surface area. However, at longer reaction times, these fractures became filled with chalcopyrite, reducing permeability and inhibiting further reaction. Quantitative analysis of volume and weight changes revealed a two-stage process. In the first stage, reaction rates increased due to increased surface area from fracturing. In the second stage, rates decreased due to the reduced permeability caused by compacted reaction products. The analysis using PCAS indicated a fold increase in unreplaced length between days 5 and 34, and decrease in the second stage(between days 45 and 63). The preservation of oriented surface scratches indicated that the reaction proceeds primarily through dissolution of pyrite and precipitation of chalcopyrite near the reaction surface, rather than through an overgrowth mechanism. The calculated crystallization pressure during chalcopyrite formation (up to 700 MPa) is sufficient to induce fracturing and deformation, independent of external stress.

The observed two-stage fluid pathway process, characterized by initial fracture-controlled permeability followed by grain boundary-controlled limitation, highlights the competing effects of volume expansion on the reaction rate. The first stage is dominated by increased reactive surface area due to fracturing, while the second stage is limited by the compacted reaction products blocking access to unreacted pyrite. The findings suggest that the completion of volume expansion reactions in geological settings (like retrograde metamorphism or sulfide transitions) may be difficult due to this two-stage process. The observed compacted products hinder fluid access to unreacted mineral, preserving unbalanced textures that might otherwise be erased by continuous reaction. The two-stage model might also apply to other replacement reactions, like silicate weathering, where initial fracturing is followed by filling with secondary products, impacting the contact between fluids and the reactive surface.

This study demonstrates a two-stage fluid pathway mechanism during volume expansion reactions, driven by the competing effects of fracture generation and product compaction. The initial fracture generation enhances the reaction rate, while the subsequent compaction of the reaction products hinders further reaction, leading to incomplete replacement. These findings have implications for understanding the kinetics of volume expansion reactions in various geological settings and highlight the need to consider fluid pathway evolution when interpreting microtextures in minerals. Future research should explore the applicability of this two-stage model to other mineral replacement reactions and investigate the influence of different fluid compositions and flow rates on the reaction kinetics.

The experiments were conducted at a constant temperature and pressure, which might not fully represent the complex conditions in natural geological systems. The use of a simplified chemical system (pyrite and chalcopyrite) might limit the applicability of the findings to more complex mineral assemblages. The study focuses on a single mineral pair, and further research is needed to assess the generality of the two-stage fluid pathway mechanism for different mineral systems.