Interface-coupled Dissolution-precipitation Mechanism Research Articles

Utilization of Eggshell Waste Calcite as a Sorbent for Rare Earth Element Recovery.

The green energy transition requires rare earth elements (REE) for the permanent magnets used in electric cars and wind turbines. REE extraction and beneficiation are chemically intensive and highly damaging to the environment. We investigated the use of eggshell waste as a sustainable alternative sorbent for the capture and separation of REE from aqueous solutions. Hen eggshell calcite was placed in multi-REE (La, Nd, Dy) solutions at 25 to 205 °C for up to 3 months. A pervasive diffusion of the REE inside the eggshell calcite was observed along pathways formed by the intracrystalline organic matrix and calcite crystal boundaries. At 90 °C, kozoite (REECO3OH, orthorhombic) spherulites precipitate on the surface of the dissolving calcite. At 165 and 205 °C, an interface-coupled dissolution-precipitation mechanism is observed, resulting in the complete dissolution of the calcite shell and its pseudomorphic replacement by polycrystalline kozoite. At 205 °C, kozoite is slowly replaced by hydroxylbastnäsite (REECO3OH, hexagonal), the stable form of the rare earth hydroxycarbonate polymorphs. Our results demonstrate two potential applications of eggshell waste for the uptake of rare earth elements in solution: at low temperatures, as a mixed organic-inorganic adsorbent and absorbent, given sufficient sorption time; and at higher temperatures, as an efficient sacrificial template for the precipitation of rare earth hydroxycarbonates.

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.

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.

Coupled dissolution-precipitation and growth processes on calcite, aragonite, and Carrara marble exposed to cadmium-rich aqueous solutions

Calcium carbonate and cadmium-rich fluid interactions have been studied at the nano and microscale with fluid flow and static fluid conditions for three forms of CaCO3: calcite in single crystals of Iceland Spar, calcite in a polycrystalline Carrara marble, and aragonite single crystals. Atomic Force Microscopy (AFM) showed the nanoscale effect of cadmium on CaCO3 dissolution and growth under flow-through conditions at ambient temperature, with the modification of calcite dissolution behaviour and simultaneous precipitation of a Cd-rich phase on all the different samples. Hydrothermal experiments at 200 °C revealed that the reactivity of single calcite crystals is passivated by epitaxial growth of the less soluble Cd-rich endmember of the (Ca,Cd)CO3 solid-solution on the sample surface due to the similar crystallographic structures of calcite and otavite (CdCO3). Conversely, the presence of grain boundaries in Carrara marble or the change of crystallographic structure and reaction-induced fracturing in aragonite allowed, to some extent, the pseudomorphic replacement of Carrara marble and aragonite samples by a porous (Ca,Cd)CO3 solid-solution phase of variable composition. These phenomena have been observed in solutions undersaturated with respect to all solid phases and are the result of an interface-coupled dissolution-precipitation mechanism where the dissolving CaCO3 provides ions to supersaturate the mineral-fluid interfacial layer, leading to the precipitation of a Cd-containing phase on the samples' surfaces. This coupled dissolution-precipitation mechanism could potentially be used as a remediation process to sequester cadmium from contaminated effluents.

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.

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.

Carbonation of calcium-magnesium pyroxenes: Physical-chemical controls and effects of reaction-driven fracturing

The weathering of primary silicates, and their carbonation in particular, is key for the geochemical cycling of elements, strongly affecting the C cycle and the long-term regulation of the Earth's climate. The knowledge on the controlling factors and mechanisms of aqueous carbonation of primary silicates is however still far from complete. This precludes a better understanding of their chemical weathering in nature and is a strong handicap to implement effective Carbon Capture and Storage (CCS) strategies. Here, dissolution-carbonation reactions of two abundant Ca-Mg pyroxenes, augite and diopside, have been investigated in experiments conducted at hydrothermal conditions, in the presence/absence of different carbonate sources (NaHCO3 and Na2CO3). We show that the main reaction products are low-magnesium calcite and amorphous silica. A higher conversion of augite (∼38 wt%) than diopside (∼15 wt%) was achieved. The presence of abundant Fe and Al (and minor Na) in the former pyroxene strongly enhances the release of cations to the solution, and contributes to the formation of abundant secondary crystalline and amorphous silicates as well as carbonates (Na-phillipsite and magnesium silicate hydrate, MSH, as well as calcite). In particular, Na-phillipsite nucleates in etch pits exerting a crystallization pressure ∼100 MPa exceeding the tensile strength of augite (i.e, failure under tension due to breaking of inter-atomic bonds as well as opening and propagation of subcritical fractures or flaws) and causing extensive fracturing. This takes place via an interface-coupled dissolution-precipitation mechanism, despite the bulk system being undersaturated with respect to this phase. Limited reaction-induced fracturing was also observed following MSH precipitation within diopside crystals. Reaction-induced fracturing increases exposed reactive surface area and creates channels for solution flow, thereby contributing to the progress of the reaction via a positive feedback loop. Ultimately, our results help to understand differences in the kinetics and mechanisms of chemical weathering of these two abundant rock forming inosilicates relevant for CCS strategies, showing that secondary phase formation (other than carbonates) are fostered by moderately alkaline pH and the presence of alkali metals, resulting in reaction-driven fracturing that enables the progress of silicate carbonation for an effective, safe, and permanent CO2 mineral storage. We also show that under our experimental conditions the precipitation of amorphous silica and calcite cannot generate sufficient pressure as to create fracturing, an effect that limits carbonation of Mg-Ca-Fe pyroxenes.

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.

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.

Direct Observations of the Coupling between Quartz Dissolution and Mg-Silicate Formation

Although quartz is a stable mineral at Earth surface conditions, field samples have shown its rapid dissolution in combination with the precipitation of Mg-silicate phases. Atomic force microscopy (AFM) experiments were performed to investigate the dissolution of quartz and the precipitation of secondary phases in high-pH, Mg-rich solutions both in situ and ex situ. Experiments were conducted at room temperature with varying MgCl 2 or MgSO 4 concentrations (0.1-100 mM), pH (8.9-12) and ionic strength (<1-530 mM). The results suggest that quartz dissolves by the removal of nanoparticles on the time scale of minutes, and that a nanometer-scale gel-like layer of amorphous silica forms on the quartz surface and is thicker at higher pH. During the in situ experiments, soft and poorly attached precipitates form on the surface when the Mg concentration is high (100 mM). After 20 h in a high-pH, Mg-rich solution, solid Mg-rich precipitates can be observed at places on the surface where the gel-like silica layer is present, predominantly near surface edges where dissolution is enhanced. This suggests a coupling between the dissolution of quartz, that resulted in the gel-like layer, and the formation of secondary phases, indicating an interface-coupled dissolution-precipitation mechanism. The precipitates could not be precisely identified but evidence suggests they are likely to be amorphous Mg-silicate phases. Such a coupled reaction may provide a pathway for Mg-Si phase formation suitable as a new environmentally friendly cement.

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...

Unusual replacement of Fe-Ti oxides by rutile during retrogression in amphibolite-hosted veins (Dabie UHP terrane): A mineralogical record of fluid-induced oxidation processes in exhumed UHP slabs

The replacement of rutile by Fe-Ti oxides is a common phenomenon during the retrogression of eclogites. Here, we report an unusual case regarding the replacement of Fe-Ti oxides by rutile during greenschist-facies metamorphic overprinting of veins in amphibolites (retrograded eclogites) from the Dabie ultrahigh-pressure (UHP) terrane, eastern China. The veins mainly consist of plagioclase, Fe-Ti oxides, and quartz, which crystallized from a Ti-rich amphibolite-facies fluid that formed during exhumation of the eclogites. Two types of textures involving the replacement of Fe-Ti oxides by rutile are recognized in the veins: (1) the first type is characterized by the development of rutile coronas (Rt-C) and other silicates (high-Fe epidote, muscovite, and chlorite) around the external boundaries of the Fe-Ti oxide grains, and (2) the second type is characterized by the formation of symplectitic intergrowths of rutile (Rt-S) and magnetite after exsolved hemo-ilmenite (H-Ilm) lamellae in the Fe-Ti oxides. The micro-textures, mineral assemblages, and Zr-in-rutile thermometry indicate that both replacement reactions involved mineral re-equilibration processes in the presence of an infiltrating fluid phase at ~476–515 °C, taking place by an interface-coupled dissolution-precipitation mechanism. Thermodynamic modeling reveals that both replacement reactions occurred during oxidation processes under relatively high-oxygen fugacity ( f O 2 ) conditions, approximately 2.5–4.5 log f O 2 units higher than the fayalite-magnetite-quartz (FMQ) reference buffer. In situ Sr isotopic analyses of epidote (Ep-C) coexisting with the Rt-C suggest that the infiltrating fluid involved in the greenschist-facies replacement reactions was externally derived from the surrounding granitic gneisses (the wall rocks of the amphibolites). Compared with the rutile in the UHP eclogites (Rt-E) and amphibolites (Rt-A), the Rt-C is characterized by distinctly lower contents of Nb ( 340 ppm) and V (>1580 ppm). These results provide a geochemical fingerprint for distinguishing the low-pressure (LP) rutile from relic high-grade phases in retrograded HP-UHP rocks. Our results reveal that rutile can form during LP retrograde stage in UHP rocks by high- f O 2 fluid-induced replacement reactions. The unusual replacements of Fe-Ti oxides by rutile-bearing assemblages during retrogression provide important constraints on fluid-mineral reactions and f O 2 variations in exhumed UHP slabs.

Subduction channel fluid–rock interaction and mass transfer: Constraints from a retrograde vein in blueschist (SW Tianshan, China)

One widely neglected part of the subduction zone fluid cycle is the retrograde fluid flow along the plate interface. This fluid flow may facilitate the exhumation of slab-derived rocks within the subduction channel. However, our understanding on the nature and behavior of these retrograde fluids is still insufficient. Retrograde veins preserved in HP–UHP rocks can provide valuable information about fluid activities and mass transfer at the plate interface and exhumation processes. In this study, we present a petrological and geochemical investigation of a typical retrograde vein and its blueschist host rock, which occurs throughout the whole Akeyazi (U)HP terrane in the Chinese southwestern Tianshan. The magnetite-bearing albite–calcite vein, representing an external fluid pathway, crosscuts the host blueschist that mainly consists of glaucophane, garnet, epidote and titanite. Along the fluid conduit the immediate wall-rock experienced greenschist-facies overprinting as displayed by an albite-rich selvage that grades into a chlorite–biotite selvage towards the host rock. The replacements of garnet by chlorite and glaucophane by albite are thought to have occurred due to an interface-coupled dissolution-precipitation mechanism. Mass balance calculations show that during fluid–rock interaction large amounts of LILE (K–Rb–Ba–Cs and Pb), Cu and CO2 were added to the selvages (mass gain >200%), whereas the altered wall-rock released significant amounts of Ca, Li, Sr, REE and transition metal elements (V–Mn–Co–Ni–Zn) (mass loss mainly between 40 and 100%) due to the dissolution of glaucophane, garnet and epidote. The chlorite–biotite selvage experienced additional SiO2 and Na2O mass losses as well as FeO, MgO and H2O mass gains, whereas the albite-rich selvage underwent exactly the opposite mass transfer with respect to these components. The Al, Ti, Nb, Ta and Cr contents were constant during the alteration. Therefore, the retrograde fluid causing greenschist-facies overprinting of the blueschist host in the subduction channel (at ca. 30km depth) is proposed to be oxidized and enriched in LILE, Cu and CO2 components, whereas Ca, Li, Sr, REE, Y and transition metal elements (V–Mn–Co–Ni–Zn) are thought to have been mobilized during fluid-rock interaction. In addition, the main fluid sources (sediments, oceanic crust or serpentinites) diversely contribute to retrograde fluids at different depths in the subduction channel.

The role of reacting solution and temperature on compositional evolution during harzburgite alteration: Constraints from the Mesoarchean Nuasahi Massif (eastern India)

We investigate the microtextural–chemical features of partially serpentinized harzburgites from the lower ultramafic unit of the Mesoarchean Nuasahi Massif, eastern India, in order to understand the role of reacting fluid composition and temperature on the phase evolution across replacement interfaces during progressive alteration. Two distinct types of pseudomorphic replacement textures are identified. Type-1 replacement texture was developed after primary orthopyroxene and is composed of talc + olivine + lizardite + tremolite + magnetite. Primary olivine was replaced by mesh-textured Mg-rich lizardite + magnetite at the center of the olivine grains and successive layers of relatively Fe-rich lizardite, magnesite, and calcite toward olivine rims, defining type-2 replacement texture. The Nuasahi harzburgite was initially out-of-equilibrium with respect to H2O–CO2-bearing reacting solution and the secondary compositions have mainly evolved in the CaO–MgO–FeO–SiO2–Al2O3–H2O–CO2 system as a result of fluid–rock interaction. The alteration process across orthopyroxene interfaces has started at relatively higher temperature conditions (400<T<675°C) than that at primary olivine interface (T≤330°C). Each replacement process across reaction interfaces was controlled via an interface-coupled dissolution–precipitation mechanism. The sequential development of different secondary compositions in these replacement rims indicates a micrometer-scale variation in silica activity and H2OCO2aq ratio in the solution across replacement interfaces. The Fe2+Mg−1 chemical exchange potential of the equilibrating system plays an important role in dictating the Fe/Mg ratio of the secondary compositions and the molar proportions of magnetite. The precipitation of tremolite and calcite in some isolated areas reflects a variation in Ca activity in the reacting fluid. The precipitation of carbonates may also be associated with an increase in pH in the interfacial solution. The pattern in phase evolution across type-1 interfaces suggests a greater extent of diffusion of solutes toward inner interfaces relative to the rim center during progressive alteration of the Nuasahi harzburgite, possibly due to an increase in permeability in replaced phases. With increasing magnitude of diffusion toward type-1 inner reacting interfaces, the widespread presence of arrested replacement features suggests an alteration event under low fluid/rock ratio and/or sluggish reaction kinetics with decreasing temperature.

Mineralogical controls on metamorphic fluid flow in metabasaltic sills from Islay, Scotland

In this study we show that mineralogy was the primary control of metamorphic fluid flow in the well-studied metabasaltic sills in the SW Scottish Highlands. Here, basaltic sills have been partially carbonated by H2O–CO2 fluids at greenschist facies conditions. This has led to mineral zonation with carbonate-poor sill interiors separated from carbonate-rich sill margins by reaction fronts. Although deformation set the stage for metamorphic fluid flow in the SW Scottish Highlands by causing the preferred alignment of mineral grains, metamorphic fluid flow was not coupled with active deformation but occurred later utilizing the pre-existing mineral alignment as a means of accessing the sill interiors. The sills which were studied were partially carbonated with well-preserved reaction fronts. They were selected because (atypically for the SW Scottish Highlands) they are mineralogically heterogeneous making them ideal for a study of mineralogical controls of metamorphic fluid flow. Their mineralogical heterogeneity reflects chemical heterogeneity arising from magmatic flow differentiation and spilitization, which occurred before greenschist facies metamorphism. Magmatic flow differentiation resulted in parts of the sill containing large crystals with no preferred alignment. Large (up to 3cm) plagioclase phenocrysts were concentrated in the sill interior whereas large (up to 1cm) amphibole (after pyroxene) grains formed cumulate layers close to the sill margins. These large randomly oriented crystals were replaced by an interface-coupled dissolution–precipitation mechanism. Replacement is constant volume and with hydration and carbonation affecting the cores of these minerals while the rims are remained intact and unaltered. This finding points to intra-granular metamorphic fluid flow. In contrast inter-granular metamorphic fluid flow was facilitated by mineral alignment on different scales. Pre-metamorphic spilitization, produced layers of epidote called segregations, whereas regional deformation caused preferred alignment mainly of amphibole and chlorite. Epidote undergoes a series of volume changes during greenschist facies metamorphism. This created porosity which produced preferred pathways for metamorphic fluids affecting the advancement of fluid-driven reaction fronts. Preferred alignment of amphibole and chlorite also affected the advancement of reaction fronts. In this case, fluid flow was preferentially parallel to the foliation. In both cases, inter-granular metamorphic fluid flow utilized a pre-existing fabric albeit on different scales. These results show intra-granular metamorphic fluid flow in unfoliated rock and inter-granular metamorphic fluid flow in foliated rock. In both cases metamorphic fluid flow occurred after deformation controlled by pre-existing mineralogical heterogeneities, such as grain composition and shape anisotropy as well as preferred alignment of mineral grains.

The pseudomorphic replacement of marble by apatite: The role of fluid composition

The replacement of a natural carbonate rock (Carrara marble) by apatite was used as a model to study the role of fluid chemistry in replacement reactions, focusing on the mineralogy, chemical composition, and porosity of the replacement product. Carrara marble was reacted with diammonium phosphate solutions ((NH4)2HPO4), in the presence and absence of four salt solutions (NH4Cl, NaCl, NH4F, and NaF) at different ionic strengths, at 200°C and autogenous pressure. The replacement products were analyzed using powder X-ray diffraction, Scanning electron microscopy (SEM), electron microprobe analysis (EMPA), and Raman spectroscopy. The reaction in all samples resulted in pseudomorphic replacements and shared the characteristics of an interface-coupled dissolution–precipitation mechanism. Increasing the ionic strength of the phosphate fluid increased the replacement rates. With a fixed concentration of phosphate, replacement rates were reduced with the addition of NH4Cl and NaCl and increased significantly with the addition of NaF and NH4F. The addition of different salts resulted in specific porosity structures resulting from the formation of different phosphate phases. Chloride-containing fluids showed a higher degree of fluid percolation through grain boundaries. This study illustrates the significant impact that small differences in solvent composition can have in the progress of replacement reactions, the nature of the products and the resultant porosity.

Formation of Mg-rich Olivine Pseudomorphs in Serpentinized Dunite from the Mesoarchean Nuasahi Massif, Eastern India: Insights into the Evolution of Fluid Composition at the Mineral–Fluid Interface

We present a detailed investigation of the micrometer- to nanometer-scale textural–chemical features in partially serpentinized dunites from the lower ultramafic unit of the Mesoarchean Nuasahi Massif, eastern India; these data are used to interpret the evolution in fluid chemistry at the reaction interface during progressive hydration of olivine. In the first stage of serpentinization, a mixed layer of Mg-rich lizardite + brucite + magnetite formed within fractures producing a typical mesh texture, followed by homogeneous brucite-free lizardite at the inner interface of the mixed layer. A unique feature of the serpentinization in the Nuasahi dunites, which to our knowledge has not been previously described, is the formation of a pseudomorphic rim of Mg-rich olivine (Fo 98 ) directly in contact with the primary olivine (Fo 92 ). The textural–chemical relations, such as (1) sharp but irregular textural–chemical reaction interfaces, (2) propagation of replacement fronts along fractured pathways, allowing transfer of aqueous species towards the center of olivine crystals, and (3) the presence of micro- to nano-porosity within product phases, indicate that each replacement process was governed by an interface-coupled dissolution–precipitation mechanism. Additionally, Mg-rich chromite, distributed within the samples, was partially pseudomorphically replaced by an Fe-rich chromite rim and/or isolated magnetite. Mass-balance calculations for different replacement reactions suggest that lizardite formation reactions may be coupled with chromite replacement reactions, allowing a micrometer-scale transfer of aqueous species (e.g. Al 3+ , Cr 3+ ) between chromite and olivine reaction interfaces. The lizardite formation reactions have produced hierarchical patterns of reaction-induced fractures within primary olivine crystals and the replacement front has progressed from the outer rim towards the center of the olivine grain. The sequential formation of different pseudomorph phases during progressive hydration of primary olivine reflects a micrometer-scale variation in silica and/or water activity in the precipitating solution across the reaction interface, and may also be associated with a decrease in temperature. The Fe 2+ Mg –1 chemical exchange potential of the equilibrating system at the reaction interface also plays an important role, controlling the Fe/Mg ratio of the secondary minerals and the molar proportions of magnetite. The pattern of fluid-mobile element mobilization (e.g. Li, B) during replacement suggests that serpentinization of the Nuasahi dunite occurred during interaction with a B-rich solution, possibly in an ocean-floor setting.

The mechanism of infiltration of metamorphic fluids recorded by hydration and carbonation of epidote-amphibolite facies metabasaltic sills in the SW Scottish Highlands

In this study we investigate a group of metabasaltic sills from the SW Scottish Highlands metamorphosed at epidote-amphibolite facies conditions that provide useful insight into the mechanisms and characteristics of fluid infiltration during metamorphism. The sills are amphibole and garnet bearing and exhibit a strong foliation in the sill margins that developed pre- to syn-peak metamorphism. Fluid infiltration caused hydration and carbonation in the sills, expressed as (1) replacement of garnet and amphibole by chlorite and calcite and (2) replacement of amphibole and epidote to form chlorite and calcite. Using garnet-amphibole and garnet-chlorite geothermometers we show that these reactions occurred after peak metamorphism at T = 290 to 400 °C. Reaction textures show that the fluid infiltration into the sill that caused hydration and carbonation occurred in the absence of deformation. The fluid infiltration was mineralogically controlled with greater fluid access in areas of abundant fine-grained elongate minerals such as amphibole and chlorite. The replacement of garnet by chlorite most likely occurred by an interface-coupled dissolution-precipitation mechanism as evidenced by perfect pseudomorphic textures of garnet, porosity generation behind the reactive interface and fracturing ahead of this interface. Porosity generated in the product chlorite enhanced fluid access to the replacement front. The study shows that deformation was not required for extensive fluid infiltration and alteration during metamorphism. Fluid flow uses a pre-existing foliation to gain access to the rock, taking advantage of the anisotropic shape of the aligned minerals.

Coupled fluctuations in element release during dolomite dissolution

Atomic force microscopy has been used to determine more precisely the mechanism of the initial stages of dolomite dissolution. Analysis of outflow solutions initially shows fluctuations of both Ca and Mg release with concentrations of Ca &gt;&gt; Mg. The dolomite surface dissolves congruently in the presence of slightly acidified water as confirmed by the regular spreading of characteristic rhombohedral etch pits. Direct in situ observations show that a new phase precipitates on the dissolving surface simultaneously. As the Ca and Mg release decreases with time, the precipitated phase can be seen to spread across the dolomite surface. These observations indicate that the apparent incongruent dissolution of dolomite is a two-step process involving stoichiometric dissolution with the release of Ca, Mg and CO3 ions to solution at the mineral–fluid interface coupled with precipitation of a new Mg-carbonate phase. The coupled element release confirms the interface-coupled dissolutionprecipitation mechanism.