Interface-coupled Dissolution-precipitation Research Articles

Interface-coupled PbSO4 dissolution and PbS precipitation and its effect on sulfidization flotation of anglesite

Anglesite sulfidization mechanism was systematically investigated using flotation tests, X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM), Raman spectroscopy, and ultraviolet− visible diffuse reflectance spectroscopy (UV−Vis DRS). The activation effect of sodium sulfide on anglesite flotation was demonstrated by the flotation tests; however, sodium sulfide concentration must be properly controlled to avoid excessive sulfide ions causing flotation depression. The results of XPS, Raman, and UV−Vis DRS revealed that PbSO4 was replaced by PbS during anglesite treatment with a sodium sulfide aqueous solution. FESEM imaging shows the dissolution of PbSO4 and the precipitation of PbS nanoparticles during sulfidization. Thus, it can be suggested that the reaction of anglesite with a sodium sulfide aqueous solutionproceeds via an interface-coupled dissolution–precipitation mechanism: upon contact with a sodium sulfide aqueous solution, anglesite dissolution releases Pb2+ and SO42– into the fluid boundary layer, which becomes oversaturated with respect to PbS phase; then the PbS nanoparticles nucleate and grow on the surface of anglesite. The PbS nanoparticles grown on anglesite can improve the floatability of anglesite.

Aqueous dissolution of Li-Na borosilicates: Insights from machine learning and experiments

Previously acquired data could be utilised in predicting glass dissolution kinetics at long times, but the application of machine learning methods needs to be assessed. Here, the dissolution processes of two Li-Na borosilicate ‘base glasses’ at 40 and 90 °C were investigated by SEM-EDS, NMR and Raman spectroscopy. Boron and sodium machine learning predictions were excellent when considering other normalised releases as features. However, extrapolating the training feature space yielded poorer performance and the absence of incorporated waste elements resulted in underestimated predicted long-term lithium and silicon releases. Faster dissolution kinetics were observed for MW than MW-½Li but the MW-½Li gel layer at 40 °C trapped more water. Whilst BO3 rings leached preferentially at 90 °C, surface enrichment of BO3 at 40 °C suggested [BO4]− transformed prior to dissolution. Results were consistent with interdiffusion being significant at 40 °C and interface-coupled dissolution precipitation beyond 7 days at 90 °C.

Corrosion of glaze in the marine environment: study on the green-glazed pottery from the Southern Song “Nanhai I” shipwreck (1127–1279 A.D.)

The “Nanhai I” shipwreck is an important discovery in the underwater archaeology of China, and many ceramics have been unearthed. These ceramics are important material artifacts of China’s “Maritime Silk Road” and have considerable significance for the study of foreign trade in the Southern Song Dynasty (1127–1279 A.D.). However, these ceramics have been buried in a marine environment for approximately 800 years and have all been corroded to varying degrees, with green-glazed pottery being the most severely corroded. In this study, the chemical compositions of five samples of green-glazed pottery and the corrosion morphology and mechanism of a representative sample were analyzed by optical microscopy, scanning electron microscopy-energy dispersive X‐ray spectrometry (SEM–EDS), Raman spectroscopy, and X-ray diffraction (XRD). Results: The green glaze is a low-temperature silica-aluminum oxide-lead oxide (SiO2-Al2O3-PbO) glaze with copper ions (Cu2+) as the main colorant. The corrosion morphology is characterized by alternating silicon (Si)- and lead (Pb)-rich layers, a sharp reaction interface between the Si-rich layer and the pristine glaze, and a relatively high porosity of the Si-rich gel layer, which is formed by the accumulation of spherical hydrated silica colloidal particles. These features suggest that the glaze was corroded through an interface-coupled dissolution–precipitation mechanism and that the properties of the gel pores controlled the reaction kinetics. Fluctuations in the solution properties at the reaction interface produced the complex morphology of the gel layer, whereas changes in the dryness and humidity of the environment are not essential factors. The samples have been corroding in the marine environment for nearly a thousand years, and explorations of the corrosion morphology and mechanism could provide reference information on the corrosion of various ancient ceramics and glasses and a basis for scientific conservation of these objects.

Natural glass alteration under a hyperalkaline condition for about 4000years.

Silicate glasses are durable materials in our daily life, but corrosion rate accelerates under alkaline aqueous environment. Such situation has raised concerns, for example, in nuclear waste disposal where vitrified wastes encounter to alkaline leachate from surrounding concrete materials. Here we report volcanic glass example surviving with a hyperalkaline groundwater (pH > 11) and high flow rate for about 4000 years. The tiny glass fragments were extracted from the volcanic ash layer sandwiched between ultramafic sediments using microanalytical techniques. Sharp elemental distributions at the glass surface, where amorphous-like smectite precursors and crystalline smectites coexist, suggest the corrosion by an interface-coupled dissolution–precipitation mechanism rather than inter-diffusion. The corrosion rate was maintained at, the minimum, 2.5 orders of magnitude less than the rate observed for fresh glass, even in the presence of Fe and Mg that might have consumed Si through the silicate precipitation.

A mechanism of ion exchange by interface-coupled dissolution-precipitation in the presence of an aqueous fluid

The equilibration of minerals in the presence of an aqueous fluid phase, with which it is out of equilibrium, has been described in terms of a number of potential mechanisms, specifically mainly either by solid state exchange of elements within the solid phase and the aqueous solution, or by interface-coupled dissolution-precipitation where equilibration is approached by the incremental dissolution of the parent solid and the coupled precipitation of a new solid. The conditions determining the equilibration mechanism can be defined by the specific chemical potential differences at the mineral interface, the kinetics of potential reactions, the solubility of the solid phase in the specific fluid and physical properties such as the mineral: fluid ratio as well as the surface area: fluid ratio, temperature and pressure. This review focusses on the mechanism of ion exchange in a range of solids by interface-coupled dissolution-precipitation in the presence of an aqueous solution.

Pseudomorphic Replacement in the Transformation between Metal–Organic Frameworks toward Three-Dimensional Hierarchical Nanostructures

The construction of three-dimensional hierarchical materials from nano building blocks has received tremendous attention because of the intriguing architecture and promising application potential of these materials. However, the use of traditional assembly methods to construct hierarchical nanostructures with complicated 3D shapes across multiple length scales remains challenging. Herein, we developed a new strategy to fabricate hierarchical nanostructures by borrowing the concept of pseudomorphic replacement from mineralogy; this concept refers to a specific conversion between minerals with the preservation of the pristine shape through an interface-coupled dissolution–precipitation mechanism. By employing pseudomorphic replacement to the transformation between metal–organic frameworks (MOFs), a series of hierarchical MOF materials with similar nanostructures but various shapes and dimensions were successfully fabricated. The experiments verified that these transformations not only preserve the shape of the pristine MOF crystals but also proceed via an interface-coupled dissolution–recrystallization process. To our knowledge, our investigation is the first explicit demonstration of pseudomorphic replacement in the transformation between MOFs, offering a new route to hierarchical materials with enhanced control ability and extended control scope.

Mobilization and fractionation of HFSE and REE by high fluorine fluid of magmatic origin during the alteration of amphibolite

High field strength elements (HFSE) and rare earth elements (REE) are normally immobile in hydrothermal fluids whereas experimental studies indicate some anionic ligands can significantly enhance their solubility in hydrothermal fluids. However, the role of fluorine (F) in hydrothermal fluids revealed by experiments and natural samples is different or even contradictory, especially in the mechanism of elements mobilization and fractionation. Here, we report a field-based geochemical study of amphibolites with extremely high Nb/Ta ratios (up to 98.7) and enriched HREE concentrations. The Paleoproterozoic amphibolites are intruded by a granitic dike with zircon UPb age of 132 ± 3 Ma. Felsic veinlets are observed in thin-sections of the amphibolite. Biotites with high F contents occur as aggregates along the boundary between the felsic veinlets and amphiboles, which indicates they are formed by reaction between amphibole and the granitic melt. The amphiboles have core-mantle-rim structure with high F actinolitic mantle surrounding low F magnesio-hastingsitic core. Some actinolites intergrow with biotites, titanites and magnetites. These evidences indicate that the actinolites are produced by hydrothermal metasomatism or interface-coupled dissolution–precipitation. The metasomatism occurred at 0.3–0.9 kbar and 500–650 °C within the hornfels facies. Hydrothermal titanites from amphibolites yield a lower intercept age of 131 ± 4.3 Ma, which is consistent with the crystallization age of the granitic dike. These observations indicate that the fluids are derived from fractional crystallization of a felsic magma for diking, and the released fluids are channeled along the interface between dike and amphibolite. The metasomatic minerals show high F and no Cl contents, indicating the hydrothermal fluids are high in fluorine (0.04–0.12 wt%) and low in chlorine. The F contents of minerals negatively correlate with HFSE, REE contents and (La/Yb)N ratios, but positively with Nb/Ta ratios. These reveal the progressive leaching and fractionation of HFSE and REE by the hydrothermal fluids. Mass balance estimates that most trace elements lose 20–60% during the hydrothermal alternation of amphibole. The calculation results also reveal that more Ta and LREE are lost than Nb and HREE, indicating Ta and LREE are more soluble than Nb and HREE, respectively, in the hydrothermal fluids. All these observations suggest that HFSE and REE are not only mobilized but also fractionated because of different degrees of mobilization by fluoride-bearing fluids.

From schwertmannite to natrojarosite: Long-term stability and kinetic approach

Abstract This work examines the transformation of iron-bearing precursors to jarosite-like minerals in the absence of bacteria or other organic compounds. The composition of the aqueous solution determines the transformation, through which crystallinity and long-term stability of jarosite increase, whereas the temperature of the environment affects the kinetics of the process. Spectroscopic techniques (FTIR and XPS) were used to characterize the chemical species present on the transformed mineral surfaces. Schwertmannite is the first phase to precipitate as a result of homogeneous nucleation and growth in the bulk of the supersaturated solution. This metastable phase transforms into a crystalline Na-rich member of the (Na,H3O)Fe3(SO4)(OH)6 solid-solution family after aging for either 3 h at 70 °C or 1 day at 20 °C. XRD analyses show that the crystallinity of natrojarosite increases progressively with reaction time, although its cell parameters and crystallite size remain nearly constant during aging, which reveals the stability of the crystal structure of this secondary phase. Interestingly, the mechanisms governing the transformation from aggregates of schwertmannite into natrojarosite crystals consist of interface-coupled dissolution–precipitation reactions that involve an internal structural reorganization within the individual nanoparticles of the secondary phase, in which Fe3+ is transferred from the solid to the solution while SO42−, OH–, and Na+ move in the opposite direction. The spectroscopic study confirms the mineralogical results and suggests that the crystal structure of jarosite-like minerals may offer interesting geochemical information about the aqueous solutions where they were formed. The transformation kinetics and the apparent activation energy (Ea = 52.1 kJ/mol) of the transformation were estimated using the so-called “time to a given fraction” method, and a temperature-transformation-time (TTT) diagram was established in the range 20–70 °C to define the reaction pathways during the process.

A reactive transport modeling perspective on the dynamics of interface-coupled dissolution-precipitation

In interface coupled dissolution-precipitation systems, the dynamics of the mineral-fluid interface depends on two intertwining processes: the dissolution of the primary mineral that is needed for subsequent precipitation and the passivation of the dissolution reaction as a result of secondary mineral precipitation. The resulting thickness and texture of the precipitating coating layer will affect the progression of geochemical reactions, flow and transport processes at the macroscopic scale. Understanding the interplay between macroscopic flow regimes and microscopic reaction mechanisms (e.g., nucleation and crystal growth pathways) in controlling the dynamics of the mineral-fluid interface has important implications for predicting natural weathering processes, scaling in the subsurface energy production systems, etc. In this study, we use a micro-continuum pore-scale reactive transport model to investigate the feedback loop between reaction rate and solute transport with explicit consideration of the surface passivation and the diffusion process through the coating layer, as well as the impacts of saturation-dependent nucleation rate on the textures of precipitates that will largely dictate the diffusion properties of the coating layer. Our model results highlight that the drastically different coating behaviors at the macroscopic scale and their dependence on solution supersaturation observed in previous column experiments are primarily controlled by the interplay between mineral reaction rates, advective flow, and diffusion through the dynamically forming coating layer. The diffusion properties of the coating layer also play a secondary but non-negligible role in shaping the evolution of the co-dissolution and precipitation system. The probabilistic nucleation model building on the framework of classical nucleation theory highlights the complex dependence of precipitates’ texture on solution chemistry and substrate properties, which can affect the diffusion process within the precipitates. The modeling observations also underscore the necessity of further investigations to better characterize the properties of the coating layer and to improve modeling descriptions of the nucleation processes.

Interactions between deformation and dissolution-precipitation reactions in plagioclase feldspar at greenschist facies

Rocks often undergo deformation and metamorphism simultaneously. However, relatively little research has been carried out on the interactions between deformation, fluid influx and a recently identified type of metamorphic reaction, termed interface-coupled dissolution-precipitation. In this study, optical microscopy and electron backscatter diffraction (EBSD) were used to investigate the interactions between deformation and dissolution-precipitation reactions in feldspar from metagabbros deformed at mid-crustal conditions. Fracturing and fluid influx promoted two types of dissolution-precipitation reactions that worked in tandem to convert Ca-bearing plagioclase to pure albite. Conventional dissolution and precipitation, here defined as dissolution precipitation reactions involving a transport step, worked to heal fractures and produce fine-grained albite. In parts of original grains that were not fractured, interface-coupled dissolution-precipitation occurred to albitize mm-cm scale grain fragments. Because interface-coupled replacement displays fast kinetics, reaction fronts were not preserved, so coupled dissolution-precipitation was identified using the following microstructural criteria: a lack of preserved zoning, indicating a fast reaction mechanism; orientation inheritance during reaction indicating epitaxial nucleation/topotactic replacement; intracrystalline strain containing Burgers vectors that indicate distortion was not derived from crystal plasticity; intragranular microporosity and second phase inclusions which share orientations with twin and cleavage planes of their parent grains. These criteria could be applied to any system to identify interface-coupled replacement in the absence of preserved reaction fronts. Brittle fracturing and dissolution-precipitation resulted in an overall grain size reduction, and a transition from dominantly brittle to dominantly viscous deformation, producing mylonites at greenschist facies, and contributing to the development of a regional-scale shear zone.

Corona formation around monazite and xenotime during greenschist-facies metamorphism and deformation

Abstract. Epidote/allanite–fluorapatite coronae around monazite and xenotime are investigated in Permian pegmatites deformed under greenschist-facies conditions during Alpine tectonometamorphism in the Austroalpine basement, Eastern Alps. The aim was to evaluate the replacement reactions involved in the formation of a corona microstructure, its age and relation to deformation. In the corona core, monazite and xenotime single crystals show domains with different composition and age. Monazite (Mnz1) and xenotime (Xen1) dating by electron microprobe (EPM) reveals an age of 250–287 Ma, consistent with the Permian magmatic age of the pegmatites. These are partly replaced by secondary monazite (Mnz2) and xenotime (Xen2) compositions yielding younger Mesozoic (170–210 Ma) and Alpine (30–120 Ma) ages. The same crystallographic orientation of the primary and secondary monazite and xenotime indicates interface-coupled dissolution–precipitation reactions. Allanite U–Th–Pb dating by laser ablation inductively coupled mass spectrometry in the corona revealed an age of 60±6 Ma, interpreted as the age of corona formation. The coronae around monazite consist of an inner zone of equant fluorapatite grains surrounded by prismatic allanite, which are surrounded by epidote enriched in heavy rare earth elements (HREEs) and REE-poor epidote grains. Compared to coronae around monazite, fluorapatite has higher REE contents and no allanite occurs in the coronae surrounding the xenotime. General reactions for monazite and xenotime breakdown can be written as follows: Mnz1+(Si,Ca,Al,Fe,F)fluid→Mnz2+LREE-Ap+Aln+HREE-Ep+Ep+(Th,U)O2+(Th,U)SiO4,Xen1+(Si,Ca,Al,Fe,F)fluid→Xen2+HREE-Ap+HREE-Ep+Ep+(Th,U)O2. The amount of replacement (judged by the relative proportions of monazite and fluorapatite) is low for monazite included in tourmaline but high within the mylonitic foliation. This dependence on the degree of replacement on the local surrounding microfabric indicates that fluid availability along grain boundaries in the matrix and cracks controlled reaction advancement, allowing the elementary mass transfer required for corona formation (e.g. input of Ca, Al, Si, Fe, F). The oblate shape of the coronae aligned within the foliation of the pegmatites and the deflected foliation around the coronae, without an outer rim of prismatic epidote showing signs of deformation, indicate that the main stage of corona formation took place during deformation and reactions were still ongoing after the main stage of deformation. The corona microstructure documents replacement reactions of a single reactant into multiple distinct mineral growth zones by dissolution and precipitation processes at non-isostatic, greenschist-facies conditions, which prevailed in the area to the north of the Defereggen–Antholz–Vals shear zone between the middle Cretaceous and the Oligocene. These reactions ceased before being completed, and REE gradients within single grains within the corona and on the thin-section scale are preserved, which suggests restricted and/or episodic transport of REE in the fluid phase and/or availability of fluid.

Model of interface-coupled dissolution-precipitation mechanism of pseudomorphic replacement reaction in aqueous solutions based on the system of cerussite PbCO3 – pyromorphite Pb5(PO4)3Cl

The results obtained from carefully designed experiments with the lead carbonate–lead apatite system presented in the study are used to propose a general model for the development of microstructures upon polycrystalline pseudomorphic transformation of minerals that are formed from the reaction with an aqueous solution. In several experiments, cerussite PbCO3 powder or crystals were reacted with a phosphate solution, in the presence or absence of Cl− ions, in an autoclave at 140 °C for up to 5 days at a near-neutral pH. In the presence of Cl− ions, cerussite powder is replaced by pyromorphite Pb5(PO4)3Cl, whereas in the absence of these ions, Pb3(PO4)2 precipitates first and recrystallizes into hydroxylpyromorphite Pb5(PO4)3OH within 3 days of transformation. The reaction of single cerussite crystals with phosphate solutions proceeds in stages via an interface-coupled dissolution–precipitation mechanism through which PbCO3 is replaced by lead phosphates. A structural control is imposed on the reaction progress when the dimensions of the dissolving crystal are much larger than the thickness of the outermost layer of the secondary phase formed by the reaction with unaltered bulk solution. Since various components of the solution exhibit different diffusivity and their migration through the porous layer is retarded to various degrees (a chromatography effect), the conditions at the reaction front differ from those at the contact with the bulk solution, resulting in the crystallization of transitional phases. With time, all the components of bulk solution diffuse through the porous layers and the transitional phases transform into thermodynamically favored reaction products. At the reaction front, spatially separated dissolution of the host mineral (associated with the formation of the porous layer) and heterogeneous precipitation of the secondary phase are observed. Several similar features of the lead carbonate–lead apatite system have been reported before, which suggests that the proposed model is universal and the interpretation can be extrapolated to other systems.

Partitioning of chromium between garnet and clinopyroxene: first-principle modelling versus metamorphic assemblages

Abstract. Understanding the geochemical behaviour of trace and minor elements in mineral assemblages is of primary importance to study small- and large-scale geological processes. Partition coefficients are frequently used to model the chemical evolution of minerals and fluids during melting and in metamorphic rocks of all grades. However, kinetic effects hampering equilibrium partitioning may invalidate the modelling. This study aims at calculating partition coefficients and testing their applicability in natural mineral assemblages, choosing Cr in garnet and clinopyroxene via exchange with Al as a case study. First-principle modelling has been combined with measurements and element mapping to estimate partition coefficients for Cr and the deviation from equilibrium. Results highlight the role of crystal chemistry over the strain field around point defects, controlling the dynamics of the Cr3+ = Al3+ exchange between clinopyroxene and garnet. Ab initio calculations allowed estimation of Cr partition coefficients between garnet and clinopyroxene, using a thermodynamic approach based on endmembers and mixing models simplified for trace element behaviour. The Cr3+ = Al3+ exchange reaction between garnet and the jadeite component of clinopyroxene depends on the grossular and pyrope content, with Cr preferentially incorporated into grossular over jadeite but preferentially incorporated into jadeite over pyrope. Comparison of predicted partition coefficients to measured concentrations in natural samples, together with element mapping, shows large disequilibrium. Cr-rich and Cr-poor sectors exhibit disequilibrium attributed to slow diffusivity of Cr during crystal growth and interface-coupled dissolution–precipitation, even for garnet–clinopyroxene assemblages crystallized around 850 ∘C.

The contribution of aqueous catechol-silica complexes to silicification during carbonate diagenesis

Pore-filling and carbonate-replacing silica is exceedingly common in carbonates, but the fundamental geochemical mechanisms that drive these silicification reactions during diagenesis remain poorly understood. An existing model has proposed that carbonate silicification proceeds through an interface-coupled dissolution-precipitation reaction, but it lacks a mechanism that enables pore fluids to reach the requisite level of supersaturation with respect to silica to allow nucleation and growth. Here, we present a sequence of batch experiments ranging in duration from 7 to 49 days designed to test the hypothesis that these reactions are facilitated by the formation and destruction of organo-silica complexes during diagenesis. Our results illustrate that the stability of organo-silica complexes is dependent upon the concentration of organic molecules in solution, as well as pH, salinity, and solution redox state. Together, these results allow us to present the following scheme for organo-silica complex mediation of silicification reactions: Firstly, the breakdown of organic matter in the presence of siliceous material creates organo-silica complexes, leading to silica-enriched pore fluids, a process which is enhanced by the anoxic conditions accompanying sediment burial. Then, as environmental conditions evolve (fO2, salinity, light, fCO2, pH…), the stability of the organo-silica complexes diminishes, and the organo-silica complexes break down. Simultaneously, the pore fluids become intensely silica-supersaturated, in direct proportion to the amount of organic material remaining in solution. The resulting supersaturation drives carbonate silicification via the precipitation of silica minerals, a process which is aided by the presence of silica “nuclei” (such as sponge spicules). This study contributes new data and a conceptual model that will aid in the ongoing quest to understand carbonate silicification reactions and their potential applications in hydrocarbon exploitation and geologic CO2 storage. Moreover, it helps to explain the common association between silica precipitates and organic mineral in the sedimentary rock record.

Baryte cohesive layers formed on a (010) gypsum surface by a pseudomorphic replacement

The mineral replacement of gypsum (CaSO4·2H2O) by baryte (BaSO4) is relevant to technological and industrial applications, including its use as a plaster or stone consolidant in cultural heritage conservation. In the present study, we provide experimental evidence suggesting that, during the interaction of gypsum cleavage surfaces with barium-bearing solutions, a pseudomorphic replacement takes place and results in the formation of a crystallographically oriented baryte layer. This mineral replacement process is favoured by the porosity generated, due to the differences in molar volume and solubility between parent and product sulfate phases, allowing the progress of the reaction. The homogeneous micrometre-sized layer of baryte occurs most likely via a fluid-mediated interface-coupled dissolution–precipitation mechanism. A certain degree of crystallographic control on the polycrystalline BaSO4 product layer by the structure of the parent substrate (gypsum) is confirmed by electron microscopy observations and X-ray diffraction analyses. The structural control exerted by the cleavage gypsum surface on the baryte layer can be defined by the epitactic relationship: Gyp (010) || Bar (010). The formation of baryte increases with reaction time until passivation occurs at the replacement interface, probably due to a decreased porosity and loss of connectivity that thereby prevents further reaction. The investigation of these processes occurring on freshly cleaved single crystals of gypsum were complemented by studying the replacement of polycrystalline gypsum cubes, showing a homogeneous baryte surface layer on the sample. The results of this study thus offer interesting insights into the application of the replacement of gypsum by baryte as a conservation method for gypsum sculptures and plasterwork, increasing their resistance against water and humidity while preserving the surface features of the original mineral substrate.

Role of myrmekite and associated deformation fabrics in controlling development of granitic mylonites in the Pofadder Shear Zone of southern Namibia

Myrmekites developed within metagranites of the Pofadder Shear Zone (PSZ) in southern Namibia were investigated to understand how alkali feldspar deforms under fluid-poor mid-crustal conditions. Myrmekites showed two different morphologies: (1) ‘Wormy’ myrmekite with a fine-grained vermicular structure that occurred closest to the alkali feldspar host; and (2) ‘recrystallised’ myrmekite composed of regularly sized quartz grains that formed 120° boundaries with the plagioclase and occurred along the exterior of alkali feldspar and in contact with the mylonitic matrix. These textures were interpreted to represent two generations of myrmekite growth. Recrystallised myrmekite masses were dismembered and incorporated into the fine-grained matrix. This suggests that myrmekite formation during PSZ deformation contributed to the grain size reduction of alkali feldspar and aided in the generation of the fine-grained polymineralic matrix which accommodated most of the strain. Syn-kinematic reactions, such as myrmekite and muscovite production, were internally balanced and T–MH2O pseudosections indicated that progressive deformation could have occurred under closed conditions. The biotite–muscovite–plagioclase–alkali feldspar–quartz equilibrium assemblage was stable with decreasing fluid contents until fluid-saturated or slightly fluid-undersaturated conditions were reached around 2 mol% H2O. The variability of the quartz rod spacing and the wide range of results from two-feldspar thermometry and phengite barometry on associated muscovite growth indicates that peak metamorphic conditions were around 500 °C, 0.5 GPa and that myrmekite growth continued as the shear zone was exhumed. The myrmekite-forming reaction is a deformation-induced interface-coupled dissolution–precipitation reaction with relict magmatic plagioclase supplying the needed cationic inputs. These results indicate that deformation within granitic units of the PSZ was aided by the formation and deformation of myrmekite and could have taken place under closed relatively fluid-poor conditions.

Fluid-driven resetting of titanite following ultrahigh-temperature metamorphism in southern Madagascar

LA-ICP-MS U-Pb dates and EPMA trace-element maps of titanite were collected from calc-silicate gneisses of the Ikalamavony (peak T: 750–800 °C) and Anosyen (peak T: 900–950 °C) domains in southern Madagascar to evaluate how titanite responds to high-temperature metamorphism, cooling, and retrogression. Fluid-mediated replacement of precursor titanite by titanite domains of different composition (interface-coupled dissolution-precipitation, ICDR) was the primary mechanism by which titanite was reset following high-grade metamorphism. Comparison of titanite U-Pb dates (530–490 Ma) with independent petrology and thermochronology indicates that the alteration occurred at temperatures as low as 300–500 °C. Apparent Zr temperatures (temperatures calculated assuming titanite–quartz–zircon equilibrium) in altered titanite are less than or equal to the metamorphic peak, but higher than the inferred alteration temperature, implying that Zr was removed, but that titanite–quartz–zircon equilibrium was not achieved during alteration. Although evidence for ICDR was observed in titanite samples separated by 10's to ~100 km, differences in U-Pb dates among samples, and even among titanite grains in the same thin section suggest that alteration at any given time was localized.

The Carbonation of Wollastonite: A Model Reaction to Test Natural and Biomimetic Catalysts for Enhanced CO2 Sequestration

One of the most promising strategies for the safe and permanent disposal of anthropogenic CO2 is its conversion into carbonate minerals via the carbonation of calcium and magnesium silicates. However, the mechanism of such a reaction is not well constrained, and its slow kinetics is a handicap for the implementation of silicate mineral carbonation as an effective method for CO2 capture and storage (CCS). Here, we studied the different steps of wollastonite (CaSiO3) carbonation (silicate dissolution → carbonate precipitation) as a model CCS system for the screening of natural and biomimetic catalysts for this reaction. Tested catalysts included carbonic anhydrase (CA), a natural enzyme that catalyzes the reversible hydration of CO2(aq), and biomimetic metal-organic frameworks (MOFs). Our results show that dissolution is the rate-limiting step for wollastonite carbonation. The overall reaction progresses anisotropically along different [hkl] directions via a pseudomorphic interface-coupled dissolution–precipitation mechanism, leading to partial passivation via secondary surface precipitation of amorphous silica and calcite, which in both cases is anisotropic (i.e., (hkl)-specific). CA accelerates the final carbonate precipitation step but hinders the overall carbonation of wollastonite. Remarkably, one of the tested Zr-based MOFs accelerates the dissolution of the silicate. The use of MOFs for enhanced silicate dissolution alone or in combination with other natural or biomimetic catalysts for accelerated carbonation could represent a potentially effective strategy for enhanced mineral CCS.

Pb2+–Calcite Interactions under Far-from-Equilibrium Conditions: Formation of Micropyramids and Pseudomorphic Growth of Cerussite

The presence of impurity ions is known to significantly influence mineral surface morphology during crystal growth from aqueous solution, but knowledge on impurity ion–mineral interactions during dissolution under far-from-equilibrium conditions remains limited. Here we show that calcite (CaCO3) exhibits a rich array of dissolution features in acidic Pb-bearing solutions. During the initial stage, calcite exhibits nonclassical surface features characterized as micropyramids that developed spontaneously. Subsequent pseudomorphic growth of cerussite (PbCO3) was observed, where nucleation occurred entirely within a pore space created by dissolution at the calcite/substrate interface. Uneven growth rates yielded a cerussite shell made of lath- or dendritic-shaped crystals. The cerussite phase was separated from the calcite by pores of less than 200 nm under transmission X-ray microscopy, consistent with the interface-coupled dissolution–precipitation mechanism. These results show that impurity metal ions exer...

Silica nanoparticle aggregation in calcite replacement reactions

Natural nanoparticles are fundamental building blocks of Earth’s bio- and geosphere. Amorphous silica nanoparticles are ubiquitous in nature, but fundamental knowledge of their interaction mechanisms and role in mineral replacement reactions is limited. Here we show how silica nanoparticles replace Cretaceous calcite bivalve shells in a volume- and texture-preserving process. Electron tomography reveals that mineral replacement transfers calcite crystallographic orientations to twinned photonic crystals composed of face-centered cubic silica sphere stacks. During the face-specific replacement process, silica nanoparticles continuously nucleate, aggregate, and form a lattice of uniform spheres parallel to calcite low-energy facets. We explain the replacement process with a new model that unifies recently proposed, probably universal mechanisms of interface-coupled dissolution-precipitation and aggregation-based crystallization; both key mechanisms in geological processes and nanomaterials design and synthesis.