Two-stage fluid pathways generated by volume expansion reactions: insights from the replacement of pyrite by chalcopyrite

Volume expansion reactions affect fluid–mineral interaction rates in diverse geological settings (e.g., retrograde metamorphism, silicate weathering, serpentinization, hydrothermal mineralization). Prior work emphasized fracture generation/propagation driven by reaction-induced stress, but the specific influence of volume expansion on mineral replacement remains unresolved. Volume increases may compact products and inhibit reactant transport to parent mineral surfaces, while crystallization/growth of replacements can induce fracturing and expose new reactive surfaces. This study examines how volume expansion influences replacement rates by investigating pyrite-to-chalcopyrite replacement under controlled hydrothermal conditions. The chosen system allows rapid transformation at 200 °C, controlled geometry, and simple compositions, minimizing confounding chemical complexities.

Previous studies linked reaction-induced fracturing to enhanced permeability and reaction progress in metamorphic and weathering contexts, highlighting roles of self-stress and hierarchical fracture networks. Conversely, interface-coupled dissolution–precipitation can produce compact, low-permeability product layers that inhibit further reaction. The balance between exposure of new pristine surfaces (which accelerates reactions) and passivating product layers (which decelerate reactions) is central. For sulfide systems, large volume increases during replacements (e.g., chalcopyrite/bornite formation) have been documented. Conceptual and experimental frameworks (e.g., Putnis’s mineral replacement paradigm) emphasize molar volume and relative solubility contrasts in ICDR reactions, along with possible force-of-crystallization effects driving deformation and fracturing.

Natural pyrite crystals (~1 cm) from Hunan, China, were verified by powder XRD and FE-SEM, then polished (SiC P1000) and cut into 2 mm cubes along (100). Cubes were ultrasonically cleaned and dimensions measured four times (Vernier calipers ±0.02 mm) to compute pre-reaction volumes and weights. Experiments used aqueous CuCl as Cu source with 1 M NaCl to stabilize Cu(I), buffered at pH 4.5 with acetate (0.1 M CH3COOH + 0.1 M CH3COONa + 1 M NaCl) prepared with O2-free water. In an anaerobic glove box, a single pyrite cube, 150 mg CuCl, and 8 mL buffer were loaded into a 12 mL PTFE tube and sealed within a stainless-steel bomb. Hydrothermal runs were conducted at 200 °C (saturated vapor pressure ~1.5 MPa) for 5–63 days, then quenched in cold water (15 min). Post-reaction, cubes were cleaned, dried, re-weighed, and re-measured to assess volume and weight changes; change rates of percentage volume and weight were derived versus time. Characterization included: SEM (Carl Zeiss Supra 55) of surfaces and polished cross-sections; micro-area and powder XRD (Rigaku D/max Rapid II) with Rigaku 2DP analysis; Raman spectroscopy (Horiba LabRAM HR800, 532.11 nm, 600 gr/mm grating); EPMA (JEOL 8100, 15 kV, 20 nA, 10 μm defocused beam, ZAF corrections) for quantitative chemistry; FIB preparation (Zeiss Auriga) and TEM (FEI Tecnai F20) for textures and selected area electron diffraction. Fracture network quantification used the PCAS software: SEM images binarized and eroded to isolate pyrite domains; total perimeters (Lt), unreplaced lengths (Lu), and replaced lengths (Lr = Lt − Lu) were computed. Thermodynamic solubility diagrams at 200 °C were generated in Geochemist’s Workbench (thermo.dat; Fe(II)-Cl association constants from Testemale et al.) for Fe–S–Cu–Cl–H2S systems. Crystallization pressure was estimated using ΔP = ΔG/ΔVs, with ΔG from a balanced reaction and ΔVs approximated by the molar volume change for chalcopyrite formation.

- Mineralogical products: Chalcopyrite is the main replacement product with minor bornite and magnetite on external surfaces. Chalcopyrite shows columnar textures without preferred orientation (SAED); pores are essentially absent in chalcopyrite. - Textures and fractures: Replacement initiates heterogeneously on pyrite surfaces forming chalcopyrite mounds. Three fracture types generated during reaction: radial, along pyrite cleavage planes ((100)/(110)), and hierarchical. Early runs (≤21 days) show abundant open fractures that subdivide large grains; longer durations show fracture infilling by products and deformation within chalcopyrite and near reaction fronts in pyrite (TEM). - Volume change: Cross-section area analysis shows an ~84% local volume increase in replaced zones, consistent with the theoretical molar volume increase for 1 mol pyrite → 1 mol chalcopyrite. Macroscopic post-reaction increases reached ~67% in volume and ~24% in weight by 63 days (Run No. 7). - Kinetics and two-stage behavior: From 5 to ~35–45 days, change rates increased (volume rate up to ~1.6 d−1 around day 35; weight rate up to ~0.6 d−1 around day 45). PCAS revealed a fold increase in unreplaced fracture length between days 5 and 34, indicating expanding reactive surface. From ~45–63 days, both volume and weight change rates decreased, as did unreplaced lengths, marking a transition to a slower stage. - Reaction mechanism: Solubility diagrams show chalcopyrite requires 3–5 orders of magnitude lower FeCl2(aq) or H2S(aq) concentrations than pyrite for precipitation at 200 °C; together with preserved surface scratches, this indicates a dissolution-limited ICDR mechanism with precipitation near reaction fronts. Estimated crystallization pressure for chalcopyrite growth at 200 °C can reach ~700 MPa, sufficient to induce fracturing/deformation independent of external pressure (~1.5 MPa).

The replacement of pyrite by chalcopyrite proceeds as a dissolution-limited, volume-increasing ICDR reaction. In stage 1, crystallization-induced stress generates abundant fractures, exposing new pristine pyrite surfaces and providing fracture-controlled fluid pathways that enhance reactant access and accelerate reaction progress. As chalcopyrite nucleates and grows near reaction fronts, products are pushed outward, increasing solid volume and inducing deformation in both reactant and product minerals. In stage 2, fractures become filled with compacted, low-permeability products; fluid access shifts to grain boundary-controlled pathways, reducing bulk solution contact with pristine pyrite and impeding reaction rates. The lag of the peak weight-change rate behind the peak volume-change rate suggests preferential infilling of fractures by chalcopyrite before creation of additional reactive surface area. These findings reconcile how volume expansion can both augment (via fracture generation) and inhibit (via product compaction) replacement reactions. The results imply that in natural systems (e.g., retrograde metamorphism, sulfide mineralization, silicate weathering), reaction-induced fractures may initially enhance permeability and reaction, but subsequent product sealing can preserve unbalanced replacement textures and limit reaction completion over geologic timescales.

This study demonstrates a two-stage fluid pathway during a volume expansion replacement reaction: pyrite is replaced by chalcopyrite through a dissolution-limited ICDR process that initially accelerates due to reaction-induced fracturing and later decelerates as compact, low-permeability products seal fractures and isolate pristine surfaces. Local volume increases match the theoretical ~84% molar volume change; macroscopic increases reached ~67% (volume) and ~24% (weight) over 63 days. Thermodynamic analyses indicate pyrite dissolution is rate-limiting, while crystallization pressures during chalcopyrite growth can attain ~700 MPa, sufficient to drive fracturing and deformation. These mechanistic insights clarify how volume expansion simultaneously promotes and hinders replacement reactions and highlight the need to consider evolving fluid pathways when extrapolating laboratory reaction rates to natural settings. Future work could explore variable temperatures, fluid chemistries, and different mineral systems to generalize the two-stage framework and quantify permeability evolution and sealing kinetics.

Experiments were conducted under controlled hydrothermal conditions (200 °C, specific Cu–Cl–H+ chemistry, closed PTFE-lined bombs) on small, fabricated pyrite cubes (2 mm) over relatively short durations (5–63 days). The study focuses on a single replacement system (pyrite→chalcopyrite) and does not explicitly quantify in situ permeability or fluid transport properties of compacted products. Extrapolation to diverse natural settings with varying temperatures, pressures, fluid compositions, and rock fabrics should be made with caution.