Mineral Replacement Reactions Research Articles

The Replacement of Celestine (SrSO4) by Strontianite (SrCO3) in Aqueous Solution Studied In Situ and in Real Time Using Fluid-Cell Raman Spectroscopy

The replacement of celestine (SrSO4) by strontianite (SrCO3) has extensively been studied over the past few decades. It also represents an ideal reaction to in situ study the kinetic and fundamental mechanistic details of mineral replacement reactions in aqueous solutions via fluid-cell Raman spectroscopy. This technique allows us to study the reaction process in real time by continuously analysing the solution or by imaging the solid–liquid interface where the reaction takes place and while the replacement process is ongoing. Two sets of experiments were conducted, the first of which was carried out with celestine powder and an equimolar Na2CO3 solution in a heated fluid cell between 30 and 60 °C. The progress of the reaction was followed by Raman spectroscopic in situ measurements of the solution. Another experiment was performed with a polished cuboid cut from a single celestine crystal and a 1 M Na2CO3 solution in a fluid cell at room temperature (21 °C). In this experiment, the reaction was studied in situ, spatially resolved, and in real time. The results of both types of experiments revealed that the replacement occurs via a coupled dissolution–precipitation mechanism and that the evolution of the solution composition cannot fully be explained by a single rate law derived from a shrinking core model. By applying the model-independent time-to-a-given-fraction method, three kinetic regimes could be identified and the associated activation energies quantified.

Nano-mineral assemblages in mercury- and silver-contaminated soils: records of sequestration, transformation, and release of mercury- and silver-bearing nanoparticles.

Mercury-bearing nano-mineral assemblages (Hg-NMAs) are chemically and mineralogically heterogeneous, micrometer-sized aggregates of nanoparticles (NPs) found in contaminated soils and sediments. Although these NMAs control sequestration and release of Hg that is a global contaminant, our understanding is limited with respect to the conditions of different types of Hg-NMAs, the diversity of its minerals, the size distribution of its NPs and whether mineral replacement and alteration reactions in these NMAs result in the release of Hg-bearing NPs. For this purpose, Hg-NMAs in four sediment samples from the Guanajuato Mining District (GMD) in Mexico, a region that was polluted by Hg and silver (Ag) due to historical mining involving Hg amalgamation, are characterized at the micro- and nanoscale. Microscale examinations with SEM show that the majority of Hg-NMAs occurs in mineral surface coatings (MSC) and fillings in fractures within quartz grains and are enriched in Hg and sulfur (S) relative to Ag, and in S and selenium (Se) relative to chloride (Cl). Examinations at the nanoscale show that Hg-NMAs contain (a) residuals of the patio process such as amalgam phases and elemental Ag; (b) associations of Hg- and Ag-sulfide NPs with pyrite and marcasite; (c) associations of Hg- and Ag-sulfide NPs with goethite and clay minerals along the rims of the MSC. The latter minerals replaced the Fe-Si-rich matrix at high-water rock ratios most likely due to an increase in porosity during flooding of the Pastita River. Consequently, the rims are depleted in Hg-Ag-sulfide NPs relative to the unaltered Fe-Si-rich matrices indicating that changes in the physiochemical conditions of soils and sediments in the GMD can result in the release of Hg-Ag-bearing NPs. In this context, this study discusses whether release and dissolution of Hg-Ag-bearing NPs contribute to the recently observed elevated gaseous elemental Hg concentrations in the soil, interstitial air and ambient air, and to the fate and effects of Hg in local aquatic environments.

Morphological and crystallographic controls in the replacement of calcite and aragonite by cerussite and otavite

Mineral replacement reactions are essential solid–fluid interactions in natural and industrial processes including metasomatism, diagenesis, and metamorphism, and sequestration of metal toxins from polluted water. Here, we explore the morphological evolution of aragonite and calcite (polymorphs of CaCO3) during their replacement by cerussite (PbCO3, an isomorph of aragonite) and otavite (CdCO3, an isomorph of calcite) in acidic aqueous solutions (with initial pH 3 to 4). Observations using scanning electron and synchrotron-based transmission X-ray microscopies reveal the formation of pseudomorphic shells (∼5–10 µm thick) of cerussite and otavite when Pb2+ is reacted with either calcite or aragonite and when Cd2+ is reacted with aragonite. The formation of pore space at the substrate-precipitate interface and within the precipitate is found to be key in promoting chemical exchange between the dissolving phase and the solution. In contrast, minimal reactivity is observed when Cd2+ is reacted with calcite. These results demonstrate that the lack of substrate-precipitate epitaxy (either because of inconsistent symmetries or due to large lattice constant mismatch) is a critical factor that enables replacement of primary CaCO3 minerals by Pb- or Cd-bearing secondary minerals while epitaxial relationships can inhibit those reactions. When calcite and aragonite coexist, the replacement of aragonite by cerussite and otavite occurs preferentially over that of calcite. We postulate that the relative reactivity of calcite and aragonite to Pb2+ and Cd2+ can be determined by interplay between morphological and crystallographic properties of CaCO3 materials as well as solution chemistry. Our results reveal how coupled dissolution-precipitation processes of carbonate minerals can control mobility and bioavailability of lead and cadmium in environments where acidic metal-contaminated water interacts with carbonate-rich rocks and soils.

Mechanism and kinetics of the pseudomorphic replacement of anhydrite by calcium phosphate phases at hydrothermal conditions

Abstract Mineral replacement reactions mediated by fluids are common in sedimentary basins, where they influence geochemical cycles. Phosphorous (P) pollution of soils, sediments and water bodies is currently a widespread problem. Some apatite accumulations in sediments may have formed through the interaction of P-bearing aqueous solutions with mineral surfaces that result in mineral replacement reactions. Here, we investigate the pseudomorphic replacement of anhydrite single crystals by aggregates of β-tricalcium phosphate and hydroxyapatite upon interaction with a P-bearing solution at temperatures between 120 and 200 °C. SEM imaging is used to study the texture of the aggregates. Rietveld refinement of the X-ray diffraction patterns and Raman spectra analysis of the reacted samples provide information on the kinetics of the replacement. At all temperatures β-tricalcium phosphate forms alongside hydroxyapatite at early stages of the replacement reaction. At T ≥ 180 °C, hydroxyapatite/β-tricalcium phosphate ratio rapidly increases, and hydroxyapatite is the only phase in fully replaced samples. At T < 180 °C hydroxyapatite/β-tricalcium phosphate ratio increases slowly and fully replaced samples still contain significant amounts of β-tricalcium phosphate. The progress of the replacement is facilitated by the formation of porosity. The evolution of the hydroxyapatite/β-tricalcium phosphate ratio and the crystal habit of both phases strongly influence the arrangement of this porosity. The empirical activation energy Ea (kJ/mol) of the replacement reaction is determined by the Avrami and the iso-conversion methods. Both approaches yield an Ea of ~40 kJ/mol. Anhydrite dissolution appears as the rate-limiting process and the overall kinetics of the replacement reaction is controlled by the rate diffusion of dissolved species through the porosity network. The ripening of the metastable β-tricalcium phosphate into hydroxyapatite affects the characteristics of the porosity network and further modulates the kinetics of the replacement. These results may improve the understanding of the mechanisms of P-sequestration by mineral surfaces through coupled dissolution–precipitation reactions and shed light on the origin of apatite accumulations associated to evaporitic sedimentary rocks.

Targeted Crystallization of Rare Earth Carbonate Polymorphs at Hydrothermal Conditions via Mineral Replacement Reactions.

The interaction between rare earth element (REE)-rich (La, Pr, Nd, Dy) aqueous solutions, dolomite (CaMg(CO3)2), and aragonite (CaCO3) at low temperature hydrothermal conditions (25-220°C) is studied. The experiments result in the solvent-mediated surface precipitation and subsequent pseudomorphic mineral replacement of the dolomite and aragonite seeds by newly formed REE-carbonates. The host grains are replaced from periphery inward. The newly formed REE-bearing carbonates in La-, Pr-, and Nd-doped systems follow the crystallization sequence: lanthanite [REE2(CO3)3·8H2O] → kozoite [orthorhombic REECO3(OH)] → hydroxylbastnasite [hexagonal REECO3(OH)]. The interaction of Dy-bearing solutions with dolomite results only in the crystallization of kozoite [orthorhombic DyCO3(OH)]. However, experiments with aragonite reveal a two-step crystallization pathway: tengerite [Dy2(CO3)3·2-3(H2O)] → kozoite [orthorhombic DyCO3(OH)]. The temperature, the dissolution rate of the host mineral, and the ionic radii of the REE3+ in question are found to control the kinetics of the replacement reaction, the polymorph selection, and the crystallization pathways toward bastnasite. The findings allow to gain a more in-depth understanding of the formation REE-bearing carbonates, particularly the mineral bastnasite, which is the main source of REEs for industry. This knowledge can be used to improve REE separation, exploration, exploitation methods, as well to produce carbonate minerals with tailored structures.

Silver enrichment and trace element deportment in hydrothermal replacement reactions: Perspective from the Nageng Ag-polymetallic deposit, East Kunlun Orogen, NW China

Mineral replacement reactions (dissolution-reprecipitation and solid-state diffusion) have been reported in various types of hydrothermal deposits. However, the metal transfer and partitioning between the parent and daughter minerals and the role of different replacement reactions in silver enrichment remains poorly understood. Here we present the petrographic textures, major and trace elements, and sulfur isotopes of different generations of pyrite and marcasite associated with replacement reactions from the Nageng epithermal Ag-polymetallic deposit, East Kunlun Orogen (EKO, NW China). The mineralization includes the pyrite-quartz (I), quartz-siderite-sulfides (II), quartz-fluorite-rhodochrosite-sulfides-sulfosalts (III), and quartz-calcite (IV) stage. This deposit is characterized by a metal zonation from deep-level Cu-Pb-Zn mineralization to shallow-level Ag-Pb-Zn mineralization, which correspond to main ore stages II and III, respectively. Stage II ores contain the colloform pyrite (Py1) and porous pyrite aggregate (Py2), both of which have higher Co, Ni, and W contents than stage III pyrite (Py3 and Py4) and marcasite (Mc1 and Mc2). In contrast, stage III pyrite/marcasite are enriched in Ag and other ore metals (e.g., Pb, Zn, Sb, Mn, and Sn). The sharp contact between Py3 and Py4/Mc1, together with the porosity in Py4/Mc1, suggests that Py3 and Py4/Mc1 represent the parent and daughter phase of dissolution and reprecipitation. The crystallographically-oriented and metallic inclusion-rich Mc1 has higher Ag concentrations (median 246 ppm) than Py3 (median 180 ppm), and commonly occurs in breccia-type ores related to hydraulic fracturing. This suggests that (i) the dissolution-reprecipitation reaction may have remobilized and concentrated silver and associated ore metals, and (ii) the fluid oxidation related to hydraulic fracturing may contribute to dissolution-reprecipitation reaction. However, the randomly-oriented Mc2 is porous and has homogeneous compositions. This, together with the prevailing occurrence with rhodochrosite in vein-type ores, indicates that Mc2 was transformed from Mc1 via Mn-rich fluid-driven solid-state diffusion under relatively mild physicochemical changes. Mc2 has the most homogeneous and highest Ag concentrations (avg. 1142 ppm) and other ore meals among all the studied Fe disulfides, suggesting that the marcasite has higher trace element compatibility than pyrite, and that the solid-state diffusion reactions promoted Ag enrichment more efficiently than dissolution-reprecipitation reactions. The mostly positive δ34S values (−1.09 to 8.50‰) of the Nageng pyrite and marcasite are consistent with a magmatic origin. The δ34S variations among different pyrite and marcasite generations are attributed to the fluctuations between episodic replenishment of magmatic fluids and subsequent oxidation in respond to hydraulic fracturing and mixing with meteoric water. Our findings reveal higher Ag compatibility of marcasite over pyrite, and that replacement reactions (especially solid-state diffusion) greatly promote ore metal enrichment.

Coupled dissolution-precipitation reactions as a potential method for mitigating contaminant transport in carbonate aquifers

• Coupled dissolution-precipitation reactions can reduce porosity and permeability in porous media. • Injection of oxalic acid into a calcitic 2D flow-through cell caused the formation of a barrier that limited solute transport and fluid flow. • Such a method could be used to contain contaminant transport in calcareous aquifers. Subsurface barriers are often used to mitigate the spread of contaminants in groundwater. Current methods typically involve the injection of slurries into aquifers, a process that clogs porosity and reduces hydraulic conductivity. An alternative way to achieve this effect in calcareous aquifers could be to inject a fluid that dissolves the existing carbonate minerals and induces the formation of phases that have higher molar volumes. While such mineral replacement reactions are known to affect hydraulic conductivity, their ability to impede contaminant transport has not been tested. In this study, we injected oxalic acid into a pseudo-2D flow through cell containing calcitic sand, which simulated a calcareous aquifer. The oxalic acid causes the dissolution of calcite, as well as the precipitation of calcium oxalate minerals, in a reaction zone that spreads out from the point of injection. We found that this process reduces the water discharge in the cell by up to 80%, and caused the flow velocity around the reaction zone to drop from 20 cm h −1 to <0.1 cm h −1 . Mineralogical analyses and electron microscopy indicate that permeability is reduced primarily by partitioning of the intergranular voids by micrometer sized calcium oxalate crystals. Our results suggest that coupled dissolution-precipitation reactions could be a feasible method for deploying subsurface barriers in aquifers.

Anatomy of a complex mineral replacement reaction: Role of aqueous redox, mineral nucleation, and ion transport properties revealed by an in-situ study of the replacement of chalcopyrite by copper sulfides

The fluid-driven transformation of chalcopyrite (CuFeS2) into Cu-rich sulfides (e.g., digenite, Cu1.8S; covellite, CuS; and chalcocite, Cu2S) is a complex mineral replacement reaction where the reaction pathway is controlled by the interplay between evolving mineral make-up, texture/porosity, and solution chemistry. This transformation was investigated in CuCl2 + H2SO4 solutions under mild hydrothermal conditions (180 to 300 °C); the reaction kinetics, nature of minerals formed, and oxidation states of aqueous Fe and Cu were followed in-situ in real-time using synchrotron powder X-ray diffraction (PXRD) and X-ray absorption spectroscopy (XAS). These results are corroborated by an analysis of the textures of reaction products from comparative ex-situ quench experiments. The in-situ and ex-situ experiments revealed that: (i) aqueous Cu2+ quickly reduced to Cu+ during chalcopyrite replacement in all experiments, and Fe dissolved as Fe2+; (ii) covellite was the first mineral to form, followed by digenite-high with delayed nucleation; and (iii) a non-quenchable hydrated Fe sulfate mineral (szomolnokite, FeSO4.H2O) formed at 240 °C at relatively low concentrations of added CuCl2, which supressed the formation of digenite-high. The quantitative mineral phase evolution retrieved using in-situ PXRD was modelled using a novel dual power law (dual Avrami approach). Avrami exponents revealed that chalcopyrite replacement proceeded initially via a 3-dimensional growth mechanism, followed by diffusion-controlled growth. This is consistent with initial formation of a porous covellite rim around chalcopyrite, confirmed by the observation of the ex-situ reaction products, followed by a second reaction stage where the transport properties of aqueous Fe (released from the chalcopyrite) and aqueous Cu (added from the initial solution) to and from the reaction front become the rate-limiting step; and these two kinetic stages exist even where covellite was the only replacement product. The activation energies calculated for these two kinetic stages were 42.9 ± 7.4 kJ mol−1 and 39.3 ± 13.1 kJ mol−1, respectively. We conclude that (i) the replacement of chalcopyrite by covellite and digenite proceeds via an interface coupled dissolution and reprecipitation mechanism; (ii) availabilities of aqueous Cu+ and of Fe2+ play a critical role in covellite nucleation and on the sequence of mineral precipitation during chalcopyrite replacement; the Cu+ to Cu2+ ratio is controlled by the kinetics of Cu2+ to Cu+ reduction, which increases with increasing temperature, and by the transport of Cu2+ through the daughter mineral to the reaction front, while Fe2+ availability is limited at high temperature by the formation of insoluble ferrous sulfate; and (iii) this reaction evolves from a bulk fluid-chemistry controlled reaction (initial formation of covellite) to an interface-controlled reaction (digenite-high or further transformation to covellite). The current findings highlight the complex feedback between Cu2+/Cu+ aqueous redox, mineral nucleation, and ion transport properties during replacement reactions, and the applicability of combined in-situ PXRD and XAS techniques in deciphering complex fluid-driven mineral replacement reactions.

A new mode of mineral replacement reactions involving the synergy between fluid-induced solid-state diffusion and dissolution-reprecipitation: A case study of the replacement of bornite by copper sulfides

Mineral replacement reactions are one of the most important phenomena controlling the geochemical cycle of elements on Earth. In the early years, solid-state diffusion was proposed as the main mechanism for mineral replacement reactions, but over the past 20 years the importance of the coupled dissolution-reprecipitation (CDR) mechanism has been recognized. In the presence of a fluid phase and at low temperatures (e.g., <300 °C), CDR is the predominant mineral replacement process compared to relatively slow solid-state diffusion. However, in the present case study, we show that the rate of solid-state diffusion is comparable to the rate of CDR processes during the replacement of bornite (Cu5FeS4) by copper sulfides at 160–200 °C. The experiments initially produced chalcopyrite lamellae homogeneously distributed in the entire bornite grain, and each lamella was enveloped by digenite. The lamellae were formed by removing Fe3+ from bornite via solid-state diffusion, since there was no evidence for fluid entering the bornite grains during lamellae formation. An interesting discovery is that solid-state exsolution of chalcopyrite lamellae was induced by the bulk hydrothermal fluids surrounding the mineral grains, because in the absence of fluids under otherwise identical conditions, no exsolution occurred, and because the exsolution rate and lamellae size were sensitive to the composition of hydrothermal fluids. We hypothesize that this fluid-induced solid-state diffusion (FI-SSD) mechanism is made possible by the similar topology of the crystal structure of these phases. The solid-state diffusion of Fe3+ within bornite and across the resultant chalcopyrite and digenite phase boundaries is facilitated by the near-identical S framework. Parallel to and after lamellae exsolution, CDR reactions proceeded from the surface to the interior of the grains or along fractures, replacing chalcopyrite by digenite, and digenite by covellite and/or chalcocite, depending on experimental conditions. The synergy between FI-SSD and CDR resulted in complex reaction pathways for reactions in five acidic hydrothermal fluids with or without added Cu2+, Cu+, Cl−, SO42−, and SO32−. The outcomes of these experiments imply that (1) under conditions where cation diffusion rates are of the same order of magnitude as dissolution and precipitation rates, hydrothermal fluids can induce and control solid-state diffusion processes, e.g., exsolution; (2) mineral replacement can be a result of the synergy between FI-SSD and CDR mechanisms; this happens at low temperatures (≤200 °C) in chalcogenide systems, but could affect silicate and oxide systems at amphibolite to granulite to eclogitic metamorphic grade; and (3) the synergy between FI-SSD and CDR mechanisms can lead to complex reaction pathways that cannot be easily predicted empirically.

Characteristic of gold mineralization associated with granites at Hamash old gold mine, South Eastern Desert, Egypt

Well-known controversy over the genesis of orogenic gold mineralization in granites has been a subject of research worldwide. Hamash gold mine is considered one of the pharaohs mines in the South Eastern Desert of Egypt, where gold mineralization is hosted by granites. In this study, we utilized field, ore microscopy, and geochemical data to determine the genesis of granites and the associated gold deposits. We found that the granites, intruded into island-arc metavolcanics and metavolcanoclastic assemblages, are characterized by variable major and trace elements, calc-alkaline, and I-type granitoids that reflect a fractional crystallization. Their trace and rare earth elements reflect magma that is related to subduction tectonic setting. The gold mineralization is confined to quartz veins that cut through granitic rocks, extend in NW-SE and N-S directions, and are formed during shear-extensional tectonics. The gold is found with sulfides in quartz veins and wallrock alteration. Silicification and sericitization are the most hydrothermal alteration zones associated the ore formation, comprise an inner quartz-sericite-carbonate-chlorite-sulfide assemblage. The results of chemical analysis reveal an increase of SiO2, Fe2O3, S, and L.O.I. concentrations and moderate depletion in K2O, Na2O, and LILE suggesting that they were affected by mineral replacement reactions and sulfidation in the altered wallrocks close to Au-quartz veins. The gold is represented by tiny inclusions, with minor chalcopyrite, galena, and arsenopyrite that mainly fill the microfracture in quartz and pyrite grains. Furthermore, a general model of gold mineralization suggests that the gold was contemporaneously formed during syn-orogenic stage, transpressional deformation assigned to D2 deformation, and subduction-related thermal processes followed by gold-bearing fluids that fill the fractures, forming veins.

Trace element catalyses mineral replacement reactions and facilitates ore formation

Reaction-induced porosity is a key factor enabling protracted fluid-rock interactions in the Earth’s crust, promoting large-scale mineralogical changes during diagenesis, metamorphism, and ore formation. Here, we show experimentally that the presence of trace amounts of dissolved cerium increases the porosity of hematite (Fe2O3) formed via fluid-induced, redox-independent replacement of magnetite (Fe3O4), thereby increasing the efficiency of coupled magnetite replacement, fluid flow, and element mass transfer. Cerium acts as a catalyst affecting the nucleation and growth of hematite by modifying the Fe2+(aq)/Fe3+(aq) ratio at the reaction interface. Our results demonstrate that trace elements can enhance fluid-mediated mineral replacement reactions, ultimately controlling the kinetics, texture, and composition of fluid-mineral systems. Applied to some of the world’s most valuable orebodies, these results provide new insights into how early formation of extensive magnetite alteration may have preconditioned these ore systems for later enhanced metal accumulation, contributing to their sizes and metal endowment.

Hydrothermal solubility of TbPO4, HoPO4, TmPO4, and LuPO4 xenotime endmembers at pH of 2 and temperatures between 100 and 250 °C

Rare earth element (REE) phosphates such as xenotime (YPO4) are important hosts to the heavy (H)REE in natural systems. Xenotime is commonly associated with hydrothermal alteration and mineral replacement reactions and its composition may yield important clues about the mineralization processes from aqueous fluids in critical mineral deposits. Robust underlying thermodynamic data for the REE phosphate endmembers and aqueous species are required to simulate the stability of xenotime and the mobility of REE in natural hydrothermal fluids. In this study, the solubility of synthetic TbPO4, HoPO4, TmPO4, and LuPO4 endmembers has been measured in aqueous solutions between 100 and 250 ∘C at saturated water vapor pressure. The solubility products (Ks0) determined in the experiments were compared to values retrieved from a combination of calorimetric data for the REE phosphates and thermodynamic properties of the aqueous REE species at elevated temperatures. The solubility of xenotime is retrograde and generally higher in the experiments than predicted by different sources of thermodynamic data. To resolve these discrepancies, the solubility data were used to optimize the thermodynamic properties of the REE phosphate endmembers and REE aqueous species. These optimizations permit retrieving a set of provisional standard Gibbs energies of formation for REE3+ and REEOH2+ at elevated temperature and were used to derive the following updated Ks0 values (uncertainty of ± 0.2 at the 95% confidence) for the reaction REEPO4 (s) = REE3+ + PO43−:Unlabelled Tablet (∘C)logKs0(TbPO4)logKs0(HoPO4)logKs0(TmPO4)logKs0 (LuPO4)100−27.3−27.7−27.9−28.1150−28.8−29.2−29.5−29.6200−30.6−30.9−31.2−31.4250−32.7−32.9−33.3−33.4The updated thermodynamic data generated from the solubility experiments have a significant impact on simulated xenotime compositions and predicted mobility of REE in crustal fluids. Future efforts are necessary to better constrain the properties of REE hydroxyl species at elevated temperature and possible non-ideal solid solution behavior for REE with ionic sizes significantly different from Y3+.

Formation of Mg-carbonates and Mg-hydroxides via calcite replacement controlled by fluid pressure

Reactive fluid flow can control the mineralogical, mechanical and chemical evolution of the Earth’s crust. When rocks are exposed to differential stresses (i.e., vertical stress ≠ horizontal stress ≠ pore-fluid pressure (Pf)) during reactive fluid flow, effective pressure is usually assumed to control the overall reaction process. Here, we show that fluid pressure can play an important role in mineral replacement reactions. We conducted experiments in which calcite (CaCO3) grains (fraction size 53–150 µm) reacted with a Mg-rich solution at ~ 200 °C both in a closed system and under reactive fluid flow conditions with different fluid flow rates and fluid pore pressures, but with similar confining pressure (σn = 10 or 20 MPa) and effective pressure (Pe). Under closed system, vapor-saturated pressures, the magnesite formed with large pores between the magnesite and the calcite. In the open system flow-through experiments, however, brucite (Mg(OH)2) or magnesite (MgCO3) formed, depending on pore-fluid pressure. The main reaction product was brucite at low pore-fluid pressure (0.2 MPa), but magnesite at higher pore-fluid pressures (≥ 1 MPa). Calcite dissolution and precipitation of the product mineral increased concomitantly with flow rate, but the flow rate did not affect the nature of the products. The permeability of the reacting rock was related to the reaction pathway, i.e. the nature of the products. Magnesite replaced the pristine calcite in a pseudomorphic manner, and mantled the pristine calcite with 10–100 µm wide pores. In contrast, tabular and/or platy brucite blocked the porosity and resulted in a decrease in permeability. Our results show that the pore-fluid pressure can be a significant parameter controlling the reaction products and reaction processes in volatile-rich (e.g., CO2, HCl, H2S and SO2) systems at conditions close to phase separation; these conditions occur for example in epithermal and porphyry hydrothermal systems, and in carbonate-replacement and some metamorphic environments.

Replacement reactions of copper sulphides at moderate temperature in acidic solutions

Understanding mineral transformations of composite copper sulphide ores and the associated reaction mechanism at low to medium temperatures in acidic solutions with oxidant restriction is important for in situ recovery operations because natural ore bodies typically comprise multiple sulphide species and their dissolution is oxidant consuming. Here we present experimental observations on mineral replacement reactions of coarse-grained natural bornite (Cu5FeS4)–chalcocite (Cu2S) samples in acidic solutions at 90 °C and in the absence of oxidant. Transformations were found to follow a three-stage reaction sequence for bornite and one stage for chalcocite. Bornite replacement commenced with the formation of digenite (Cu1.8S) along fractures and surfaces. Bornite–digenite interfaces are sharp and digenite rims contain nanochannels that connect to the original bornite. When copper dissolution exceeded 30%, copper-deficient lamellae exsolved from the bornite by solid-state diffusion; this texture appeared only in stronger acidic solutions with extended times and did not occur in the heating experiments without fluid. Chalcocite that was present together with bornite in the unreacted samples transformed into digenite characterized by laminar replacement textures. Finally, all phases were replaced by covellite (CuS) accompanied by the formation of porosity. These replacements showed the concomitant operation of fluid-induced solid-state diffusion and fluid-mediated mineral replacement by coupled dissolution–precipitation reactions. The textures found in the resulting covellite showed a promising increase in porosity that could be beneficial for hard-rock in situ recovery and suggests a pathway for refractory sulfide processing by acid pre-treatment.

Tracking Metamorphic Dehydration Reactions in Real Time with Transmission Small- and Wide-Angle Synchrotron X-ray Scattering: the Case of Gypsum Dehydration

AbstractWe present a review of a unique non-destructive method for the real-time monitoring of phase transformations and nano-pore evolution in dehydrating rocks: transmission small- and wide-angle synchrotron X-ray scattering (SAXS/WAXS). It is shown how SAXS/WAXS can be applied to investigating rock samples dehydrated in a purpose-built loading cell that allows the coeval application of high temperature, axial confinement, and fluid pressure or flow to the specimen. Because synchrotron sources deliver extremely bright monochromatic X-rays across a wide energy spectrum, they enable the in situ examination of confined rock samples with thicknesses of ≤ 1 mm at a time resolution of order seconds. Hence, fast kinetics with reaction completion times of about hundreds of seconds can be tracked. With beam sizes of order tens to hundreds of micrometres, it is possible to monitor multiple interrogation points in a sample with a lateral extent of a few centimetres, thus resolving potential lateral spatial effects during dehydration and enlarging sample statistics significantly. Therefore, the SAXS/WAXS method offers the opportunity to acquire data on a striking range of length scales: for rock samples with thicknesses of ≤ 10-3 m and widths of 10-2 m, a lateral interrogation-point spacing of ≥ 10-5 m can be achieved. Within each irradiated interrogation-point volume, information concerning pores with sizes between 10-9 and 10-7 m and the crystal lattice on the scale of 10-10 m is acquired in real time. This article presents a summary of the physical principles underpinning transmission X-ray scattering with the aim of providing a guide for the design and interpretation of time-resolved SAXS/WAXS experiments. It is elucidated (1) when and how SAXS data can be used to analyse total porosity, internal surface area, and pore-size distributions in rocks on length scales from ∼1 to 300 nm; (2) how WAXS can be employed to track lattice transformations in situ; and (3) which limitations and complicating factors should be considered during experimental design, data analysis, and interpretation. To illustrate the key capabilities of the SAXS/WAXS method, we present a series of dehydration experiments on a well-studied natural gypsum rock: Volterra alabaster. Our results demonstrate that SAXS/WAXS is excellently suited for the in situ tracking of dehydration kinetics and the associated evolution of nano-pores. The phase transformation from gypsum to bassanite is correlated directly with nano-void growth on length scales between 1 and 11 nm for the first time. A comparison of the SAXS/WAXS kinetic results with literature data emphasises the need for future dehydration experiments on rock specimens because of the impact of rock fabric and the generally heterogeneous and transient nature of dehydration reactions in nature. It is anticipated that the SAXS/WAXS method combined with in situ loading cells will constitute an invaluable tool in the ongoing quest for understanding dehydration and other mineral replacement reactions in rocks quantitatively.

Coupling between mineral replacement reactions and co-precipitation of trace elements: An example from the giant Olympic Dam deposit

The mechanisms of scavenging of minor elements in reactive, fluid-dominated systems control the distribution, nature, concentration, and reactivity of economically valuable or deleterious elements in many environments, ranging from hydrothermal ore deposits to sedimentary basins. Here we show that in a high-grade Cu-U ore from the super-giant Olympic Dam ore deposit, remobilized U was scavenged during the fluid-driven replacement of chalcopyrite (CuFeS2) by bornite (Cu5FeS4). Hydrothermal experiments show that the pseudomorphic replacement of chalcopyrite by bornite proceeded via an interfaced coupled dissolution reprecipitation (ICDR) reaction mechanism, with further bornite growing via overgrowth. Addition of U(VI) to the experimental fluid resulted in scavenging of U in the form of nano- to micro-particles of uraninite. Uraninite precipitation occurred concurrently with replacement, resulting in abundant uraninite decorating the initial surface of the parent chalcopyrite grain, as well as the coarse porosity that developed within overgrowth bornite.Our results highlight the importance of disequilibrium or local equilibrium in controlling the endowment of U in complex ores. Under far from equilibrium, fluid-dominated conditions, e.g., during seismic or eruptive events, fast fluid movements associated with brecciation and/or faulting can lead to fast mineral reactions. The resulting ultra-local conditions (e.g., surface of reacting mineral; reaction-induced porosity) play an important role in controlling the precipitation of minor elements, U in this case, and interpretation of precipitation mechanisms as reflecting the evolution of bulk fluid may be erroneous.

The impact of hydrothermal mineral replacement reactions on the formation and alteration of carbonate-hosted polymetallic sulfide deposits: A case study of the Artemis prospect, Queensland, Australia

Hydrothermal sulfidation of carbonate rocks and the reverse replacement of sulfides by carbonates are important processes responsible for the formation and subsequent alteration of ore deposits. These processes have been recognized in the case study of the Artemis Zn-Cu-Au prospect, which is located in the Cloncurry district within the Eastern Succession of the Mount Isa Inlier, NW Queensland, Australia. The high-grade polymetallic mineralization displays a complex association of massive sulfides and carbonates hosted in a vertical marble lens adjacent to garnet-bearing schist, staurolite-muscovite schist, and quartzo-feldspathic psammite. Petrographic analyses of drill core specimen by optical and electron microscopy, micro-X-ray florescence elemental maps, electron probe microanalyses, cathodoluminescence, and bulk geochemistry suggested three major stages of mineralization: (i) pre-ore stage marble (calcite-0); (ii) ore stage involving sulfidation of the marble and precipitation of calcite-1 and spherical sulfide inclusions in the first episode and the formation of calcite-2 and massive sulfide in the second episode; and (iii) post-ore stage involving replacement of sulfides by calcite-3 and garnet.Mineral replacement reactions are dominant processes in rocks of ore stage and post-ore stage. The ore stage involved the dissolution of coarse and pristine calcite-0 (Ca0.95Mn0.3Fe0.2CO3) and the precipitation of sulfides and calcite-1 (Ca0.96Mn0.3Fe0.1CO3) and calcite-2 (Ca0.96Mn0.3Fe0.1CO3). The sulfide assemblage consists of pyrrhotite, sphalerite, chalcopyrite, galena, arsenopyrite, cobaltite, costibite, Bi, cubanite ± Ag-sulfides and Au, in decreasing order of abundance. In episode 1, calcite-1 contains spherical sulfide inclusions while in episode 2, calcite-2 envelopes calcite-1. The replacement of Co-rich arsenopyrite (Fe0.75Co0.26As1.13S0.83) by relatively Co-poor arsenopyrite (Fe0.86Co0.15As0.99S0.99) via a coupled dissolution-precipitation reaction occurred in episode 2. Based on arsenopyrite geothermometry, the temperatures for the first and second episode are 650 ± 50 and 450 ± 70 °C, respectively. Micro-XRF elemental maps of the garnet-bearing schist, adjacent to the sulfidized marble, revealed the sulfidation front where pre-ore garnet and amphibole were replaced by pyrrhotite, chalcopyrite, Au, biotite, and muscovite. In the post-ore stage, calcite-2 was partially replaced by siderite and calcite-3 (Ca0.94Mn0.4Fe0.2Mg0.1CO3), which shows sectoral and oscillatory zoning. Galena was replaced by calcite-3 as documented by embayed lobate grain boundaries between these minerals, and pyrrhotite Fe(1-x)S, (x = 0.091–0.119) was pervasively replaced by garnet (Mn1.2Fe1.2Ca0.6)Al2Si3O12. The microscale investigations of the mineralization processes in the Artemis orebody document the impact of hydrothermal mineral replacement reactions on metal sequestration. Mineral replacement reactions facilitate the formation of ore-deposits and subsequent alteration processes, which may lead to the loss of economic value of the deposit.

Dynamic Permeability Related to Greisenization Reactions in Sn-W Ore Deposits: Quantitative Petrophysical and Experimental Evidence

Massive greisens are commonly associated with Sn-W mineralization and constitute low-grade high-tonnage deposits. The formation of this type of deposit results from an intense pervasive metasomatic alteration involving a major fluid and mass transfer through a nominally impermeable parental granite. A decrease in the volume of the solid phases associated with the mineral replacement reactions may be a potential process for creating pathways to enhance fluid flow. Here, we explore the effects of the replacement reactions related to greisenization on the granite’s mineralogy and petrophysical properties (density, porosity, and permeability), as well as their potential implications for fluid flow in the case of the world-class Panasqueira W-Sn-(Cu) deposit, Portugal. Mineralogical and microtextural analyses of greisenized facies show that the total replacement of feldspars by muscovite is associated with a volume decrease of the solid phases that induces a significant porosity generation in greisen (~8.5%). Greisenization experiments coupled with permeability measurements show that the replacement of feldspars by muscovite permits new pathways at the crystal scale that significantly enhance the transient permeability. Moreover, permeability measurements performed on representative samples with different degrees of greisenization show that permeability increases progressively with the level of alteration from 10-20 m2 in least granite to 10-17 m2 in greisen. The correlation between the permeability and porosity evolutions demonstrates that the porous texture developed during replacement reactions creates new pathways that enhance significantly the permeability in greisen systems. The occurrences of mineral-bearing metals such as cassiterite in the newly formed porosity of greisen provide evidence that greisenization can be a decisive process for enhancing fluid flow and promoting transport of metals in Sn-W deposits. Finally, we present a model involving a positive feedback between greisenization and permeability, in which mineralizing fluids are able to generate their own pathways in initially impermeable granite via replacement reactions, which in turn promote further hydrothermal alteration and mass transport.

Development of Phosphatized Calcium Carbonate Biominerals as Bioactive Bone Graft Substitute Materials, Part I: Incorporation of Magnesium and Strontium Ions.

Synthetic materials based on calcium phosphate (CaP) are frequently used as bone graft substitutes when natural bone grafts are not available or not suitable. Chemical similarity to bone guarantees the biocompatibility of synthetic CaP materials, whereas macroporosity enables their integration into the natural bone tissue. To restore optimum mechanical performance after the grafting procedure, gradual resorption of CaP implants and simultaneous replacement by natural bone is desirable. Mg and Sr ions released from implants support osteointegration by stimulating bone formation. Furthermore, Sr ions counteract osteoporotic bone loss and reduce the probability of related fractures. The present study aimed at developing porous Ca carbonate biominerals into novel CaP-based, bioactive bone implant materials. Macroporous Ca carbonate biominerals, specifically skeletons of corals (aragonite) and sea urchins (Mg-substituted calcite), were hydrothermally converted into pseudomorphic CaP materials with their natural porosity preserved. Sr ions were introduced to the mineral replacement reactions by temporarily stabilizing them in the hydrothermal phosphate solutions as Sr-EDTA complexes. In this reaction system, Na, Mg, and Sr ions favored the formation of correspondingly substituted β-tricalcium phosphate over hydroxyapatite. Upon dissolution, the incorporated functional ions became released, endowing these CaP materials with bioactive and potentially osteoporotic properties.

The role of fluorine in hydrothermal mobilization and transportation of Fe, U and REE and the formation of IOCG deposits

The fluorine(F)-rich nature in iron oxide-copper-gold (IOCG) deposits has received much attention since it was recognized that the giant Olympic Dam (OD) deposit contains >2 wt% fluorite. Yet, the significance of the F-U-REE-Fe association remains poorly understood, with four existing hypotheses regarding the role of F: (i) fluoride increases the solubility of metals by forming stable aqueous complexes; (ii) fluoride acts as a precipitating rather than a transporting agent, due to the low solubility of some fluoride minerals; (iii) high F contents may simply reflect the source of the ore fluids; and (iv) the presence of HF(aq) in acidic aqueous solutions may improve leaching of metals and create fluid pathways. In this study, we investigated these hypotheses using thermodynamic modelling, and performed complementary experiments to evaluate the nature and stability of Fe(II/III)-F complexes at elevated temperature.Our in-situ X-ray absorption spectroscopy (XAS) data show that at room temperature, the Fe(II/III) fluoride complexes are more stable than corresponding Fe(II/III) chloride complexes, and the Fe(III) fluoride complexes are important in F-only solutions at low temperature (≤100 °C). Increasing temperature causes precipitation of Fe from F-only solutions, so that above 200 °C there was little detectable Fe left in the solution. In mixed F-Cl solutions, the experimental data and the thermodynamic calculations show that Fe(III)-F complexes are important at low temperature (~≤150 °C) while Fe(II)-Cl complexes predominate at temperatures higher than 200 °C, causing an increase in Fe solubility.We further investigated the potential of granitic rocks as a source for F and metals (Fe, REE and U) using thermodynamic calculations (hypotheses (i), (ii) and (iii)). Our results show that U and La solubilities are mainly controlled by temperature: U solubility is relatively high at T < 200 °C; La solubility is relatively high at T > 250 °C. Fluoride significantly enhances the solubility of U and La compared to F-free system by affecting fluid chemistry or forming stable complexes (La).Our simulations also show that Si solubility is enhanced in F-bearing solutions (hypothesis (iv)), mainly attributed to the increased solubility of H3SiO4− and NaHSiO3(aq). Moreover, the solubility of Si-F increases dramatically. These indicate that F may help breaking the SiO bond in silicates and releasing Si into fluids. Therefore, fluorine may play a key role in enhancing the porosity of the wall rock and breccia, and in mineral replacement reactions. Collectively, these would enhance fluid-rock reaction, and thus may help to drive metal precipitation.Overall, the thermodynamic considerations suggest that the common F + Fe + REE ± U association in IOCGs may not only reflect the source of the metals, but also that F contributes to the metal endowment of some IOCG deposits via a combination of processes, including increasing the metal (U and REE, but not Fe) carrying capacity of the fluids; and dissolving silicates and enhancing the porosity of the breccia and the wall rocks, thus contributing to increased fluid pathways and ore-forming reactions.