Yung Ngothai

Dissolution-reprecipitation vs. solid-state diffusion: Mechanism of mineral transformations in sylvanite, (AuAg)2Te4, under hydrothermal conditions

Abstract Under hydrothermal conditions, diffusion-driven solid-state reactions can compete with fluid-mediated reaction mechanisms. We have obtained an insight into the complex textures resulting from this competition by studying experimentally the transformation of Au-Ag-telluride sylvanite to Au-Ag alloy under hydrothermal conditions, and exploring the effects of temperature (160-220 °C), pH (2-10), and redox conditions on the sample textures and the reaction kinetics. Sylvanite transformed to Au-Ag alloy over all hydrothermal conditions investigated, but not under dry conditions. The replacement was pseudomorphic, as the Au-Ag alloy preserved the external dimensions of the sylvanite grains. The resulting Au-Ag alloy was porous, consisting of worm-like aggregates with diameters ranging from 200 nm to 1 μm. In addition to Au-Ag alloy, a range of other phases were observed as intermediate products, including petzite (Ag3AuTe2), hessite (Ag2Te), and two compositions of calaverite: an Ag-rich-Te-depleted composition, (Au0.78Ag0.22)Te1.74, and a normal calaverite, (Au0.93Ag0.07)Te2. The transformation of sylvanite to Au-Ag alloy follows a complex reaction path, with competing reactions proceeding either via interface-coupled dissolution and reprecipitation (ICDR) mechanism or via solid-state exsolution. Initially, sylvanite was replaced by an Au-Ag alloy following an ICDR mechanism, with sylvanite dissolution being the rate-limiting step relative to Au-Ag alloy precipitation. Tellurium was lost to the bulk solution as tellurite or telluride complexes, depending on the redox conditions. Once the concentration of Te in solution reached a critical state, the reaction switched and sylvanite dissolution was coupled to the precipitation of an Ag-rich-Te-depleted calaverite. This Agrich- Te-depleted calaverite decomposes via exsolution to calaverite and phase X (Ag3+xAu1-xTe2 with 0.1 < x < 0.55), which in turn breaks down to a mixture of low petzite and low hessite below 120 °C via exsolution. As the reaction continues, the calaverite and phase X are all transformed to Au-Ag alloy via ICDR. In the ICDR reactions the Au-Ag alloy precipitated locally near the telluride dissolution site. Such local Au-Ag alloy precipitation is facilitated by fast heterogeneous nucleation onto the sylvanite, calaverite, and petzite surfaces. The dissolution of sylvanite and of the intermediate telluride species, and the overall reaction, are oxidation reactions. The diffusion of oxygen through the porous Au-Ag alloy layer plays an important role in sustaining the reaction. A similar combination of dissolution-reprecipitation and solid-state processes may be responsible for the formation of some of the Au and Au-Ag telluride assemblages observed in Nature. These processes may also play a role in the formation of mineral assemblages in Cu-Fe sulfide systems, where the solid-state mobility of Cu+ ions is relatively high at moderate temperatures. The interplay of different reaction mechanisms results in complex textures, which could easily be misinterpreted in terms of complex geological evolution. At 220 °C, solid-state replacement of sylvanite by Au-Ag alloy is slow (months), but under hydrothermal conditions sylvanite grains ~100 μm in size can be fully replaced in as little as 96 h, providing a possible alternative to roasting as a pre-treatment of telluride-rich gold ores.

The replacement of chalcopyrite by bornite under hydrothermal conditions

Abstract We report the replacement of chalcopyrite by bornite under hydrothermal conditions in solutions containing Cu(I) and hydrosulfide over the temperature range 200-320 °C at autogenous pressures. Chalcopyrite was replaced by bornite under all studied conditions. The reaction proceeds via an interface coupled dissolution-reprecipitation (ICDR) mechanism and via additional overgrowth of bornite from the bulk solution. Initially, the reaction is fast and results in a bornite rim of homogeneous thickness. Reaction rates then slow down, probably reflecting healing of the porosity, and the reaction proceeds predominantly along twin boundaries of the chalcopyrite. The composition of the bornite product is generally Cu-rich, corresponding to the bornite-digenite (Cu5FeS4-Cu9S5; Bn-Dg) solid solution (bdss). The Cu and Fe contents were controlled principally by temperature, with solution pH having only a small effect. The percentage of Cu in bdss decreased and the percentage of Fe increased with increasing reaction temperature: at 200 °C a composition of Bn47Dg53 was obtained; at 300 °C the composition was Bn90Dg10 and at 320 °C it was near-stoichiometric bornite. The influence of temperature rather than solution chemistry on the composition of bdss, as well as the homogeneity of the bornite product grown both via replacement of chalcopyrite and from the bulk solution as overgrowth, are interpreted to reflect buffering of the bornite activity in bdss via solids (e.g., reaction chalcopyrite + 2 chalcocite = bornite). Only the end-member compositions of the bdss are found in nature, indicating that the products obtained are metastable, and illustrating the importance of reaction mechanism for controlling the chemistry of the mineral product. The unique features of the chalcopyrite to bornite reaction investigated here are related to interaction between a solution controlled ICDR reaction with solid-state diffusion processes driving porosity healing.

Experimental study of the formation of chalcopyrite and bornite via the sulfidation of hematite: Mineral replacements with a large volume increase

Abstract Chalcopyrite (CuFeS2) and bornite (Cu5FeS4) are the most abundant Cu-bearing minerals in hydrothermal Cu deposits, forming under a wide range of conditions from moderate-temperature sedimentary exhalative deposits to high-temperature porphyry Cu and skarn deposits. We report the hydrothermal synthesis of both chalcopyrite and bornite at 200-300 °C under hydrothermal conditions. Both minerals formed via the sulfidation of hematite in solutions containing Cu(I) (as a chloride complex) and hydrosulfide, at pH near the pKa of H2S(aq) over the whole temperature range. Polycrystalline chalcopyrite formed first, followed by bornite. Assuming that Fe behaves conservatively, the transformation of hematite to chalcopyrite involves a large increase in volume (~290%). The reaction proceeds both via direct replacement of the existing hematite and via overgrowth around the grain. Chemical exchanges between bulk solution and hematite are enabled by a network of micrometer-size pores. However, in some cases the chalcopyrite overgrowth develops large grain sizes with few apparent pores and in these cases fluid transport may have been via a network of fractures. Similarly to the replacement of hematite by chalcopyrite, bornite forms via the replacement of chalcopyrite. The reaction has a large positive volume (~230%), and proceeds both via chalcopyrite replacement and via overgrowth. This study shows that replacement reactions can proceed via coupled dissolution-reprecipitation even where there is a large volume increase between parent and product mineral. This study also provides further evidence about the controls of reaction pathways onto the final mineral assemblage. In this case, the host initial fluid was undersaturated with respect to Fe-bearing minerals. Upon slow release of Fe at the surface of hematite, a mineral assemblage of chalcocite, bornite, and finally chalcopyrite is expected. However, in practice chalcocite did not nucleate on the surface of hematite. Rather relatively slow nucleation of bornite enabled high concentrations of Fe to build up near the dissolving hematite, so that chalcopyrite (high-sulfidation experiments) or chalcopyrite+pyrite (low sulfidation) crystallized first.

The kinetics of the α → β transition in synthetic nickel monosulfide

Abstract The kinetic behavior of the α-Ni1-xS → β-NiS transition was investigated via a series of annealquench experiments using Rietveld quantitative phase analysis of powder X-ray diffraction data. Initial compositions of α-Ni1-xS were found to play an important role in the kinetics of the transition. The activation energy (Ea) for this α- to β-phase transition is 16.0 (±0.5) kJ/mol for NiS in the temperature range 343 to 423 K, and 13.0 (±0.5) kJ/mol in the temperature range 523 to 623 K. For Ni0.97S, however, Ea decreases from 73.0 (±0.5) to 17.0 (±0.5) kJ/mol over the course of the reaction in the temperature range 573 to 593 K. The relationship between Ea and extent of transition (y) for the initial bulk Ni0.97S was derived using the Refined Avrami method. For Ni-deficient compositions, α-Ni1-xS, the transformation to β-NiS is accompanied by the exsolution of a progressively more Ni-deficient α-Ni1-xS and Ni3S4, and the reactions become more sluggish for more metal-deficient compositions.

Premixed, injectable PLA-modified calcium deficient apatite biocement(cd-AB)with washout resistance

By using a non-aqueous solution as the mixing liquid, the washout resistance of the calcium deficient apatite biocement (cd-AB) was significantly improved, over that of the conventional method of using cd-AB with water as the liquid phase. In this study, premixed and injectable cd-AB was prepared, which had the advantage of being stable in the syringe and hardens only after being delivered to the defect area. The cd-AB powder with a Ca/P ratio of 1.5 consists of a mixture of tetracalcium phosphate (TTCP) and dicalcium phosphate anhydrous (DCPA). A solution of polylactide (PLA) in N-methyl-2-pyrrolidone (NMP) was used as the liquid phase of the premixed cd-AB. The premixed cd-AB paste injected into an aqueous environment exhibited excellent washout resistance. The premixed cd-AB had longer setting time and lower compressive strength than conventional cd-AB. The hydration products of premixed cd-AB were a mixture of calcium deficient hydroxyapatite (cd-HA) and PLA. In vitro Tris-HCl immersion tests demonstrated that the premixed cd-AB could be degradable. The results revealed that the premixed cd-AB was cytocompatible and had no adverse effects on the attachment and proliferation of MG-63 osteoblast-like cells in vitro. The most distinct advantages of premixed and injectable PLA-modified cd-AB were its excellent washout resistance and in vitro degradability, suggesting that it may be a promising candidate for bone repair.

A thermosyphon-driven hydrothermal flow-through cell for in situ and time-resolved neutron diffraction studies

A flow-through cell for hydrothermal phase transformation studies by in situ and time-resolved neutron diffraction has been designed and constructed. The cell has a large internal volume of 320 ml and can operate at temperatures up to 573 K under autogenous vapor pressures (ca 8.5 × 106 Pa). The fluid flow is driven by a thermosyphon, which is achieved by the proper design of temperature difference around the closed loop. The main body of the cell is made of stainless steel (316 type), but the sample compartment is constructed from non-scattering Ti–Zr alloy. The cell has been successfully commissioned on Australias new high-intensity powder diffractometer WOMBAT at the Australian Nuclear Science and Technology Organization, using two simple phase transformation reactions from KAlSi2O6 (leucite) to NaAlSi2O6·H2O (analcime) and then back from NaAlSi2O6·H2O to KAlSi2O6 as examples. The demonstration proved that the cell is an excellent tool for probing hydrothermal crystallization. By collecting diffraction data every 5 min, it was clearly seen that KAlSi2O6 was progressively transformed to NaAlSi2O6·H2O in a sodium chloride solution, and the produced NaAlSi2O6·H2O was progressively transformed back to KAlSi2O6 in a potassium carbonate solution.

Uranium scavenging during mineral replacement reactions

Abstract Interface coupled dissolution-reprecipitation reactions (ICDR) are a common feature of fluid-rock interaction during crustal fluid flow. We tested the hypothesis that ICDR reactions can play a key role in scavenging minor elements by exploring the fate of U during the experimental sulfidation of hematite to chalcopyrite under hydrothermal conditions (220-300 °C). The experiments where U was added, either as solid UO2+x(s) or as a soluble uranyl complex, differed from the U-free experiments in that pyrite precipitated initially, before the onset of chalcopyrite precipitation. In addition, in UO2+x(s)- bearing experiments, enhanced hematite dissolution led to increased porosity and precipitation of pyrite+magnetite within the hematite core, whereas in uranyl nitrate-bearing experiments, abundant pyrite formed initially, before being replaced by chalcopyrite. Uranium scavenging was mainly associated with the early reaction stage (pyrite precipitation), resulting in a thin U-rich line marking the original hematite grain surface. This “line” consists of nanocrystals of UO2+x(s), based on chemical mapping and XANES spectroscopy. This study shows that the presence of minor components can affect the pathway of ICDR reactions. Reactions between U- and Cu-bearing fluids and hematite can explain the Cu-U association prominent in some iron oxide-copper-gold (IOCG) deposits.

The mechanism and kinetics of α-NiS oxidation in the temperature range 670–700 °C

Abstract The oxidation behavior of synthetic α-NiS in air has been investigated over the temperature range 670-700 °C. The α-NiS was ground and sieved to give a particle size ranging from 53 to 90 μm. Three oxidation paths were observed: (i) α-NiS + 3/2 O2 → NiO + SO2 (ii) 3α-NiS +O2 → Ni3S2 + SO2 (iii) Ni3S2 + 7/2 O2 → 3NiO + 2SO2 No Ni3S2 (heazlewoodite) was observed over the course of α-NiS oxidation at 670 and 680 °C. The dominant oxidation path at this temperature is path i. At 700 °C, however, all three oxidation paths were observed. As an intermediate oxidation product, Ni3S2 steadily exsolved from α-NiS, reaching a maximum quantity after about 80 min of oxidation, declining afterward, and approaching annihilation at 160 min of oxidation. Experimental results show that the exsolution of Ni3S2 is likely triggered by the loss of one third of S in the α-NiS structure with the release of SO2 rather than by an intrinsic thermal decomposition of α-NiS to α-Ni1-xS + Ni3S2. The eventual annihilation of Ni3S2 was caused by a further oxidation of Ni3S2 to NiO. Oxidation paths 2 and 3 form a typical single chain reaction: The approximate values of k1 are k2 are 3 × 10-4s-1 and 5 × 10-4s-1 respectively. Oxidation temperature was found to play important roles both in the oxidation kinetics and the oxidation mechanism. By decreasing 10 °C from 680 to 670 °C, the average reaction rate (dy/dt, where y is the reaction extent) over the experiment time scale almost decreased to one third of its original rate (from 3.3 × 10-5s-1 to 1.2 × 10-5s-1). The reaction mechanism in the temperature range 670 to 680 °C is constant with Ea = 868.2 kJ/mol.

Thermodynamic Modeling of Poorly Complexing Metals in Concentrated Electrolyte Solutions: An X-Ray Absorption and UV-Vis Spectroscopic Study of Ni(II) in the NiCl2-MgCl2-H2O System

Knowledge of the structure and speciation of aqueous Ni(II)-chloride complexes is important for understanding Ni behavior in hydrometallurgical extraction. The effect of concentration on the first-shell structure of Ni(II) in aqueous NiCl2 and NiCl2-MgCl2 solutions was investigated by Ni K edge X-ray absorption (XAS) and UV-Vis spectroscopy at ambient conditions. Both techniques show that no large structural change (e.g., transition from octahedral to tetrahedral-like configuration) occurs. Both methods confirm that the Ni(II) aqua ion (with six coordinated water molecules at R Ni-O = 2.07(2) Å) is the dominant species over the whole NiCl2 concentration range. However, XANES, EXAFS and UV-Vis data show subtle changes at high salinity (> 2 mol∙kg-1 NiCl2), which are consistent with the formation of small amounts of the NiCl+ complex (up to 0.44(23) Cl at a Ni-Cl distance of 2.35(2) Å in 5.05 mol∙kg-1 NiCl2) in the pure NiCl2 solutions. At high Cl:Ni ratio in the NiCl2-MgCl2-H2O solutions, small amounts of [NiCl2]0 are also present. We developed a speciation-based mixed-solvent electrolyte (MSE) model to describe activity-composition relationships in NiCl2-MgCl2-H2O solutions, and at the same time predict Ni(II) speciation that is consistent with our XAS and UV-Vis data and with existing literature data up to the solubility limit, resolving a long-standing uncertainty about the role of chloride complexing in this system.

Effect of cation vacancy and crystal superstructure on thermodynamics of iron monosulfides

Iron monosulfides, Fe1 − xS (0 < x < 0.125), are extremely complex in their chemical and physical behaviours, which are largely attributed to their nonstoichiometric nature and myriad superstructures. The chemical composition of Fe1 − xS affects the polymorph formation for iron monosulfides, their mineral reactivity, surface sulfur fugacity, and thermal expansion. In this paper, the effects of cation vacancy and crystal superstructure on the thermodynamics of iron monosulfides are reviewed and discussed.