Solid Solution Of Co Research Articles

SYNTHESIS AND INVESTIGATION OF SOME PHYSICOCHEMICAL PROPERTIES OF NANOSTRUCTURED CO-PT SYSTEM

The phase compositions and particle morphology of nanostructured Co-Pt system, obtained by the reduction of aqueous solutions of precursors with hydrazine hydrate, have been studied (for the first time in the Pt-rich region) by X-ray diffraction analysis (XRD), transmission electron microscopy (coupled with electron diffraction (ED)), thermogravimetry, small-angle X-ray scattering analysis (SAXS) and elemental analysis of the samples. As determined by XRD, with Co content less than approximately 60 at%, the only phase is determined: the face-centred cubic (fcc) phase of the solid solution of Co in Pt, with the upper limit of Co solubility in Pt equal to 18±1 at%. For Co content in the solid solution higher than its solubility limit during the synthesis, diffraction-undetectable phase (phases) is also formed in addition to the fcc solid solution phase. The presence of such phases was confirmed by the results of SAXS, and their metal nature, rather than oxide-hydroxide one, was confirmed by XRD, ED, and thermal analysis.

Synthesis and study of some physicochemical features of Co-Pt nanostructured system

In the work by methods of XRD, TEM, SAED, XPS combined with mass-spectrometry of gaseous products, SAXS, and elemental analysis of samples the phase compositions and morphology of particles of nanostructured Co-Pt system synthesized by the method of reduction of hydrazine-hydrate aqueous solutions of precursors were studied (in the region rich in Pt for the first time). It was found that in the composition range up to ≈ 60 at. % Co. XRD-analysis of synthesized samples fixes as the only FCC phase of solid solution of Co in Pt, with the established upper limit of solubility %18±1 at. At the content of Co above the limit, along with it, the X-ray diffraction unregistered phases with the content of Co above the limit in solid solution are also formed. Their presence was confirmed by the SAXS method, and their metallic nature (not oxide-hydroxide) nature follows from the results of XRD, SAED. When FCC phase formed metal reduction process proceeded directly without cobalt hydroxides forming. According to results of XPS with the Ar-etching of samples there were internal platinum-rich sub-regions in Co-Pt nanoalloys.

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.

Electrical conductivity of siderite and its implication for high conductivity anomaly in the slab-mantle wedge interface

Carbonate minerals as a dominant carbon host can be transported to the Earth’s deep interior via subduction of the oceanic lithosphere, and their physicochemical behavior potentially has a significant influence on the compositional heterogeneity and physical properties in the deep mantle. In this study, we measured the electrical conductivity of natural siderite at 1–3 GPa and 100–700°C using a complex impedance analyzer in a large volume multi-anvil high-pressure apparatus. A sharp increase in conductivity was observed at ∼400°C under various pressures, and subsequently, the electrical conductivity keeps anomalously high values in the whole temperature range owing to a small quantity of interconnected highly conductive phases (graphite and magnetite) produced from the low degree decarbonation of siderite. The change in electrical conductivity and activation enthalpy suggest that the conduction mechanisms before and after low degree decarbonation of siderite are the small polaron (electron hopping in Fe2+–Fe3+) and highly conductive phases, respectively. Our results indicate the incipient decarbonation temperatures at 1–3 GPa are considerably lower than the decomposition boundary of siderite determined by phase equilibrium experiments, implying the initial decarbonation reaction of Fe-bearing carbonates in the subducting oceanic crust occurs at a shallower depth. The 30 vol.% of siderite is required to enhance the electrical conductivity of (Mg, Fe)CO3 solid solutions. Magnetite and graphite generated from the decarbonation reaction of the siderite component of Fe-bearing carbonate make a significant contribution to the high conductivity anomaly observed in the slab-mantle wedge interface.

Dissolution and Solubility of the Calcite–Otavite Solid Solutions [(Ca1−xCdx)CO3] at 25 °C

A complete series of the calcite–otavite solid solutions [(Ca1−xCdx)CO3] were prepared, and their dissolution processes lasting nine months were experimentally investigated. For the dissolution in the N2-degassed water, the Ca concentrations of the aqueous phases increased up to the steady states after 5040 h of dissolution, and the Cd concentrations of the aqueous phases increased up to the highest values and then decreased gradually to the steady states of 0.017–6.476 μmol/L after 5040 h of dissolution. For the dissolution in the CO2-saturated water, the Ca and Cd concentrations of the aqueous phases increased up to the peak values and then decreased gradually to the steady states of 0.94–0.46 mmol/L and 0.046–9.643 μmol/L after 5040 h of dissolution, respectively. For the dissolution in the N2-degassed water at 25 °C, the mean solubility products (log Ksp) and the Gibbs free energies of formation (ΔGfθ) were estimated to be −8.45–−8.42 and −1129.65–−1129.48 kJ/mol for calcite [CaCO3] and −11.62–−11.79 and −671.81–−672.78 kJ/mol for otavite [CdCO3], respectively. Generally, the log Ksp values decreased non-linearly, and the ΔGfθ values increased linearly with the increasing Cd/(Ca+Cd) mole ratio (XCd) of the (Ca1−xCdx)CO3 solid solutions. In the Lippmann diagrams constructed for the sub-regular (Ca1−xCdx)CO3 solid solutions with the estimated Guggenheim coefficients a0 = −0.84 and a1 = −3.80 for the dissolution in the N2-degassed water or a0 = −1.12 and a1 = −3.83 for the dissolution in the CO2-saturated water, the (Ca1−xCdx)CO3 solid solutions dissolved incongruently, moved progressively up to the quasi-equilibrium curves for otavite and then along the quasi-equilibrium curve from right to left, approached the solutus curve and finally reached the minimum stoichiometric saturation curve for calcite. The considerably Cd-poor aqueous phases were finally in equilibrium with the CdCO3-rich solid phases.

Dissolution of the smithsonite–rhodochrosite (ZnCO3-MnCO3) solid solutions in aqueous solution at 25 °C

A series of smithsonite-rhodochrosite solid solutions [(Zn1-xMnx)CO3] (XMn = 0.00–1.00) were prepared in lab to study their dissolution process in aqueous solution. The molar ratios Mn/(Zn + Mn) of the synthetic solids (XMn) were very close to that of the starting mixtures. The X-ray diffraction (XRD) patterns of the synthetic solids showed continuously peak shifting to lower 2θ value with the increasing of XMn. For the dissolution in the nitrogen-degassed and the air-saturated water, the Zn and Mn contents in aqueous phase went up quickly to a constant value; after dissolution, the weak (200) peak of Zn5(CO3)2(OH)6 (hydrozincite) emerged in the XRD patterns of the solids with XMn = 0.00–0.30. For the dissolution in the carbon-dioxide-saturated water, the Zn and Mn contents in water reached high values quickly after 48 h ~ 120 h, and then declined to a stable value. After 270 d of dissolution, the aqueous molar ratios Mn/(Zn + Mn) were greater than or close to XMn of the corresponding solids. For the dissolution in the nitrogen-degassed, the air-saturated and the carbon-dioxide-saturated waters at 25 °C, the mean log IAP values (ion activity product) at the equilibrium state (≈ solubility product log Ksp) were determined to be −10.56, −10.54 and − 10.71 for smithsonite, and − 10.29, −10.26 and − 10.18 for rhodochrosite, respectively. With the increasing of XMn, the log IAP values increased non-linearly from −10.54 ~ −10.71 to −10.18 ~ −10.29, while the ΔGf° decreased linearly from −735.08 to −736.04 kJ/mol and − 813.99 to −814.63 kJ/mol. The constructed Lippmann diagrams showed that the (Zn1-xMnx)CO3 solid solutions dissolved incongruently, and the dissolution data points moved progressively up and exceeded the solutus curve, and then move along the (Zn1-xMnx)CO3 saturation curves from left to right or right to left for the solids with lower XMn (0.10–0.30) and higher XMn (0.40–0.90), respectively.

Incorporation of Europium into (Ba,Ca)2[formula omitted

Synchrotron-based powder diffraction measurements in combination with inductively coupled plasma optical emission spectrometry, Raman and fluorescence spectroscopy show that (Ba,Ca)2(CO3)2 can incorporate significant amounts (up to 6 ​mol%) of europium. This solid solution is therefore of potential interest for the solidification of nuclear waste streams involving aqueous nitrate solutions of lanthanides. Europium replaces Ba/Ca on lattice sites and is not incorporated as an interstitial defect. Charge compensation is likely due to the presence of OH−-groups as we could exclude a coupled substitution involving Na+. The Eu-containing compound is stable to at least 723 ​K. We show that the one-phase-field of (Bax,Ca(1−x))CO3 solid solutions at ambient conditions is larger (0.36 <x< 0.51) than previously thought. The synthesis routes employed here lead to compounds which have similar molar volumes than those of the naturally occurring (Ba,Ca)-double carbonates, in noted contrast to another synthetic phase, “balcite”.

Re-examination of the heterotype solid solution between calcite and strontianite and Ca-Sr fluid-carbonate distribution: An experimental study of the CaCO3-SrCO3-H2O system at 0.5–5 kbar and 600 °C

Abstract Carbonates are excellent carriers for divalent cations such as Ca, Mg, and Sr, and knowledge about their stability and solid solutions is important to understanding the global strontium cycle. To shed light on the topology of the two-phase field between calcite-type and aragonite-type (Ca,Sr)CO3 solid solutions as a function of temperature and pressure, and to learn more about the distribution of Sr and Ca between carbonates and fluid, we studied the system CaCO3-SrCO3-H2O at 600 °C in the pressure range 0.5–5 kbar. Conventional and rapid-quench hydrothermal syntheses were performed using a range of different starting materials. All bulk compositions were within the assumed/postulated two-phase field of calcite-type and aragonite-type (Ca,Sr)CO3 solid solutions. The run products were analyzed by scanning electron microscopy, electron microprobe analysis, and powder X-ray diffraction with Rietveld refinement. The results show that the heterotype solid solution is more extensive than previously assumed, with calcite incorporating up to 20 mol% SrCO3, which is twice as much as previously predicted. The compositional range of the aragonite-type solid solution was identical to that found in the literature. Using the data from this study, an updated version of the phase diagram P-X (Sr) at 600 °C for the CaCO3-SrCO3 system was calculated. The phase diagram does not support a phase transition within the trigonal (Ca,Sr)CO3 solid solution associated with rotational disorder of the CO3-groups. This order-disorder phase transition was previously postulated to explain some observed compositional trends in this system. Our new data are in line with other more recent studies. The distribution of Sr and Ca between the fluid and solid phases D = XSrsolid/XSrfluid is near to 1.0 for calcite-type and on average around 2.0 for aragonite-type solid solutions. This contrasts with silicate-fluid systems in which Sr typically shows a strong preference for the fluid phase compared with Ca.

Dissolution and solubility of calcite-rhodochrosite solid solutions [(Ca1-xMnx)CO3] at 25\xa0\xb0C

A complete series of calcite-rhodochrosite solid solutions [(Ca1-xMnx)CO3] are prepared, and their dissolution processes in various water samples are experimentally investigated. The crystal morphologies of the solid solutions vary from blocky spherical crystal aggregates to smaller spheres with an increasing incorporation of Mn in the solids. Regarding dissolution in N2-degassed water, air-saturated water and CO2-saturated water at 25 °C, the aqueous Ca and Mn concentrations reach their highest values after 1240–2400 h, 6–12 h and < 1 h, respectively, and then decrease gradually to a steady state; additionally, the ion activity products (log_IAP) at the final steady state (≈ solubility products in log_Ksp) are estimated to be − 8.46 ± 0.06, − 8.44 ± 0.10 and − 8.59 ± 0.10 for calcite [CaCO3], respectively, and − 10.25 ± 0.08, − 10.26 ± 0.10 and − 10.28 ± 0.03, for rhodochrosite [MnCO3], respectively. As XMn increases, the log_IAP values decrease from − 8.44 ~ − 8.59 for calcite to − 10.25 ~ − 10.28 for rhodochrosite. The aqueous Mn concentrations increase with an increasing Mn/(Ca + Mn) molar ratio (XMn) of the (Ca1-xMnx)CO3 solid solutions, while the aqueous Ca concentrations show the highest values at XMn = 0.53–0.63. In the constructed Lippmann diagram of subregular (Ca1-xMnx)CO3 solid solutions, the solids dissolve incongruently, and the data points of the aqueous solutions move progressively up to the Lippmann solutus curve and then along the solutus curve or saturation curve of pure MnCO3 to the Mn-poor side. The microcrystalline cores of the spherical crystal aggregates are preferentially dissolved to form core hollows while simultaneously precipitating Mn-rich hexagonal prisms.

Structure and emission properties of barium calcium aluminates synthesized by room-temperature solid-state reaction approach

The two conventional aluminate precursor preparation methods are liquid-state co-precipitation and mechanical mixture, needing high temperature to fabricate thermionic cathodes for vacuum devices. Herein, we introduce the room temperature solid states (RTSS) approach to fabricate the precursors of barium calcium aluminates (BCA). For this, crystalline hydrates are used to prepare the precursor through mechanical grinding. The resulting product comprises of crystallized α-(Ba,Ca)CO3 solid solution and Al3+-contained amorphous precipitants. After sintering at 1200 °C, the active phases of Ba3CaAl2O7 and Ba3Al2O6 were obtained. The direct current (dc) emission measurements in a close-space diode configuration revealed that the divergent current density (Jdiv) of the test cathode reaches 7.2 A/cm2 at 1130 °CB, which is comparable to that of the co-precipitation liquid phase method. Considering the virtue of an environmentally-friendly and low-temperature preparation approach, the preparation of aluminates using the RTSS technique has great potential for the fabrication of dispenser cathodes for vacuum devices.

Effect of cationic substitution on the pressure-induced phase transitions in calcium carbonate

Abstract The high-pressure CaCO3 phase diagram has been the most extensively studied within the carbonates group. However, both the diverse mineralogy of carbonates and the abundance of solid solutions in natural samples require the investigation of multi-component systems at high pressures (P) and temperatures (T). Here we studied a member of the CaCO3–SrCO3 solid-solution series and revealed the effect of cationic substitution on the pressure-induced phase transitions in calcium carbonate. A synthetic solid solution Ca0.82Sr0.18CO3 was studied in situ by Raman spectroscopy in a diamond-anvil cell (DAC) up to 55 GPa and 800 K. The results of this work show significant differences in the high-pressure structural and vibrational behavior of the (Ca,Sr)CO3 solid solution compared to that of pure CaCO3. The monoclinic CaCO3-II-type structure (Sr-calcite-II) was observed already at ambient conditions instead of the “expected” rhombohedral calcite. The stress-induced phase transition to a new high-pressure modification, termed here as Sr-calcite-IIIc, was detected at 7 GPa. Sr-calcite-VII formed already at 16 GPa and room T, which is 14 GPa lower compared to CaCO3-VII. Finally, crystallization of Sr-aragonite was detected at 540 K and 9 GPa, at 200 K lower T than pure aragonite. Our results indicate that substitution of Ca2+ by bigger cations, such as Sr2+, in CaCO3 structures can stabilize phases with larger cation coordination sites (e.g., aragonite, CaCO3-VII, and post-aragonite) at lower P-T conditions compared to pure CaCO3. The present study shows that the role of cationic composition in the phase behavior of carbonates at high pressures should be carefully considered when modeling the deep carbon cycle and mantle processes involving carbonates, such as metasomatism, deep mantle melting, and diamond formation.

Microstructure Evolution, Thermal and Mechanical Property of Co Alloyed Sn-0.7Cu Lead-Free Solder

The effect of Co addition (0.5-2.0 wt.%) on the microstructure evolution, thermal and mechanical property of Sn-0.7Cu (wt.%) solder was investigated. The microstructure of Sn-Cu alloy was refined with the addition of Co, and CoSn2 phase formed during solidification. The volume fraction of CoSn2 phase increases with the increase of Co content. The melting temperature of Sn-0.7Cu-xCo alloys increases with increasing Co content. The melting range of these alloys is small (< 4°C), and decreased with the increase of Co content. The ultimate tensile strength and microhardness of Sn-Cu-xCo (x = 0.5, 1.0, 1.5, 2.0) alloys increase with the increase of the Co content. The relationships between tensile strength/microhardness and Co content of Sn-Cu-xCo alloy are established. Meanwhile, Sn-0.7Cu-1.5Co (wt.%) and Sn-0.7Cu-2.0Co (wt.%) alloys revealed an indentation size effect. Different ISE behaviors of Sn-0.7Cu-xCo alloys are mainly related to the presence of CoSn2 phase and the volume fraction of it. The indentation creep property of Sn-Cu-xCo alloys is modified with the addition of Co. Enhanced creep property of the alloys is depended on the solid solution of Co and precipitate out of CoSn2 phase.

The System K2CO3–CaCO3–MgCO3 at 3 GPa: Implications for Carbonatite Melt Compositions in the Shallow Continental Lithosphere

Potassic dolomitic melts are believed to be responsible for the metasomatic alteration of the shallow continental lithosphere. However, the temperature stability and range of compositions of these melts are poorly understood. In this regard, we performed experiments on phase relationships in the system K2CO3–CaCO3–MgCO3 at 3 GPa and at 750–1100 °C. At 750 and 800 °C, the system has five intermediate compounds: Dolomite, Ca0.8Mg0.2CO3 Ca-dolomite, K2(Ca≥0.84Mg≤0.16)2(CO3)3, K2(Ca≥0.70Mg≤0.30)(CO3)2 bütschliite, and K2(Mg≥0.78Ca≤0.22)(CO3)2. At 850 °C, an additional intermediate compound, K2(Ca≥0.96Mg≤0.04)3CO3)4, appears. The K2Mg(CO3)2 compound disappears near 900 °C via incongruent melting, to produce magnesite and a liquid. K2Ca(CO3)2 bütschliite melts incongruently at 1000 °C to produce K2Ca2(CO3)3 and a liquid. K2Ca2(CO3)3 and K2Ca3(CO3)4 remain stable in the whole studied temperature range. The liquidus projection of the studied ternary system is divided into nine regions representing equilibrium between the liquid and one of the primary solid phases, including magnesite, dolomite, Ca-dolomite, calcite-dolomite solid solutions, K2Ca3(CO3)4, K2Ca2(CO3)3, K2Ca(CO3)2 bütschliite, K2Mg(CO3)2, and K2CO3 solid solutions containing up to 24 mol % CaCO3 and less than 2 mol % MgCO3. The system has six ternary peritectic reaction points and one minimum on the liquidus at 825 ± 25 °C and 53K2CO3∙47Ca0.4Mg0.6CO3. The minimum point resembles a eutectic controlled by a four-phase reaction, by which, on cooling, the liquid transforms into three solid phases: K2(Mg0.78Ca0.22)(CO3)2, K2(Ca0.70Mg0.30)(CO3)2 bütschliite, and a K1.70Ca0.23Mg0.07CO3 solid solution. Since, at 3 GPa, the system has a single eutectic, there is no thermal barrier for liquid fractionation from alkali-poor toward K-rich dolomitic compositions, more alkaline than bütschliite. Based on the present results we suggest that the K–Ca–Mg carbonate melt containing ~45 mol % K2CO3 with a ratio Ca/(Ca + Mg) = 0.3–0.4 is thermodynamically stable at thermal conditions of the continental lithosphere (~850 °C), and at a depth of 100 km.

Interaction of Co, Mn, and Fe Atoms with Calcite: An X-Ray Photoelectron Spectroscopy Study

Adsorption of Co, Mn, and Fe atoms on the surface of calcite in ultrahigh vacuum and the interaction of the generated adsorption systems with water were studied by X-ray photoelectron spectroscopy. It is demonstrated that Mn and Fe atoms form CaCO3/Mn(Fe)CO3 solid solutions on calcite surface, whereas the dominant compounds of adsorbed Co atoms are CoО and Co3О4. When interacting with water, the Mn and Fe surface compounds are not appreciably modified, whereas the Co compounds are partly transformed into soluble hydroxylated complexes.

Primary microstructure, microsegregation and precipitates characterization of an as-cast new type γ-γ′ Co-Al-Mo-Nb cobalt-based superalloy

Present paper concerns primary structure of new type of tungsten-free cobalt-based superalloys with formula Co-Al-Mo-Nb. Due to replacement of W by addition of Mo and Nb, the desirable dual phase structure of γ/γʹ should be formed after heat treatment of this alloy. In analyzed case, the alloy in as cast state was described. The Co-10Al-5Mo-2Nb (at%) alloy was casted via induction melting process. The specimens were cut from “foot“ area of the rod and analyzed from phases and chemical point of view by XRD and EDS analysis. Detailed analysis of primary microstructure by light and scanning microscopy observations were performed. Obtained data revealed that in as cast state, the main phases detected in the alloy were solid solution of Co(Al,Mo,Nb) and Mo with Nb reach precipitations, probably with formula of Co3(Mo,Nb). The morphology of observed interdendritic precipitations is typical for eutectic reaction.

Amorphous alloy strengthened stainless steel manufactured by selective laser melting: Enhanced strength and improved corrosion resistance

In order to enhance the strength and wear characteristics of stainless steel, Fe-based amorphous alloy with ultra-high strength was introduced to reinforce stainless steel through selective laser melting. The intrigued finding was that the amorphous alloy strengthened stainless steel exhibited an increased tensile strength from 819 MPa to 1090 MPa, a lower coefficient of friction (from 0.62 to 0.49) and a higher corrosion resistance. The enhancements of properties are contributed to the oxygen purification of Y element and solid solution of Co and Mo elements, as well as the grain refinement strengthen with the introduction of the amorphous alloy.

Research into recycling of nickel­cobalt­containing metallurgical wastes by the ecologically­safe technique of hydrogen reduction

We studied kinetic patterns of hydrogen reduction of the scale of a nickel-cobalt containing precision alloy at a temperature of 673‒1573 K over a period from 0 to 180 minutes. The highest degree of reduction was achieved after thermal treatment at 1273 K – 99 %. This is predetermined by the intensification of reduction processes and a sufficient level of porosity, which ensures satisfactory gas exchange. It was discovered that the starting scale consists mainly of Fe 3 O 4 , Fe 2 O 3 and FeO with atoms substituting their alloying elements. The target product of metallization had a sponge-like microstructure and consisted mainly of the solid solution of Co and Ni atoms in γ-Fe and the residual non-reduced Fe 3 O 4 and FeO. The resulting phases had no noticeable susceptibility to sublimation. This has ensured a reduction in the losses of alloying elements while receiving and using the highly-alloyed metallized scale, which was confirmed by experimental- industrial tests. At the same time, recycling of industrial wastes contributes to a reduction in the technogenic intensity of industrial regions and improves ecological safety of the environment

Manganese-calcium intermixing facilitates heteroepitaxial growth at the [formula omitted] calcite-water interface

Abstract In situ atomic force microscopy (AFM) measurements were performed to probe surface precipitates that formed on the 10 1 ¯ 4 surface of calcite (CaCO3) single crystals following reaction with Mn2+-bearing aqueous solutions. Three-dimensional epitaxial islands were observed to precipitate and grow on the surfaces. In situ time-sequenced measurements demonstrated that the growth rates were commensurate with those obtained for epitaxial islands formed on calcite crystals reacted with Cd2+-bearing aqueous solutions of the same range in supersaturation with respect to the pure metal carbonate phase. This finding was unexpected as rhodochrosite (MnCO3) and calcite display a 10% lattice mismatch, based on the area of their 10 1 ¯ 4 surface unit cells, whereas the lattice mismatch is only 4% for otavite (CdCO3) and calcite. Coatings of varying thicknesses were therefore synthesized by reacting calcite single crystals in calcite-equilibrated aqueous solutions with up to 250 μM MnCl2. Ex situ X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), X-ray reflectivity (XRR), and AFM measurements of the reacted crystals demonstrated the formation of an epitaxial (Mn,Ca)CO3 solid solution. The epitaxial solid solution had a spatially complex composition, whereby the first few nanometers were rich in Ca and the Mn content increased with distance from the original calcite surface, culminating in a topmost region of almost pure MnCO3 for the thickest coatings. The effective lattice mismatch was therefore much smaller than the nominal mismatch thus explaining the measured growth rates. These findings highlight the strong influence played by the substrate on the composition of surface precipitates in aqueous conditions.

Dissolution–Recrystallization of (Mg,Fe)CO3 during Hydrothermal Cycles: FeII/FeIII Conundrums in the Carbonation of Ferromagnesian Minerals

Crystallization experiments were performed at high supersaturation to obtain metastable (Mg,Fe)CO3 precipitates and study their evolution during hydrothermal heating in a nitrogen atmosphere. The primary (Mg,Fe)CO3 solids undergo a dissolution–recrystallization process driven by two forces: the preferential partition of Fe2+ toward the (Mg,Fe)CO3 solid solution and the oxidation of aqueous Fe2+ species to form Fe(III)-bearing solids. Under limited oxygen supply, the oxidation of Fe2+ is a slow process, particularly in the presence of chlorine and inorganic carbon. In absence of oxidation, the initial solid solution should become increasingly Fe-rich as the supersaturation decreases and the system approaches equilibrium. Here, the secondary (Mg,Fe)CO3 solids become the wrong way round, Mg-rich. The gradual oxidation of the aqueous Fe2+ species eventually leads to precipitation of magnetite, which is also metastable because the aqueous solution is maximally supersaturated with respect to hematite. In most c...

Heteroepitaxial growth of cadmium carbonate at dolomite and calcite surfaces: Mechanisms and rates

The systematic variation of rates and the mechanism of cadmium uptake on the (104) surface of dolomite (CaMg(CO3)2) were investigated using in situ and ex situ atomic force microscopy (AFM), ex situ specular X-ray reflectivity (XR), and ex situ X-ray fluorescence (XRF). Selected experiments were performed on the calcite (CaCO3) (104) surface for comparison. Aqueous solutions of CdCl2, CaCl2, and NaHCO3, undersaturated with respect to calcite and supersaturated with respect to otavite (CdCO3) and the (CdxCa1−x)CO3 solid solution, were reacted with dolomite surfaces for minutes to days. Calcite substrates were reacted with solutions containing 1–50μM CdCl2, and with no added Ca or CO3. Thin carbonate films following the Stranski–Krastanov growth mode were observed on both substrates. Specular XR and XRF revealed the formation of nm-thick Cd-rich carbonate films that were structurally ordered with respect to the dolomite (104) plane. Epitaxial films adopted the calcite crystal structure with a d104-spacing (3.00Å) larger than those of pure dolomite (2.88Å) and otavite (2.95Å) indicating either a solid solution with x≈0.5, or a strained Cd-rich carbonate with a composition near that of otavite. The growth rate r of this phase increases with the initial supersaturation of the solution with respect to the solid solution, βmax, and follows the empirical relationship, as determined from XRF measurements, given by: r=10-4.88±0.42(βmax2.29±0.24-1),(in units of atoms of Cd/Å2/h).The morphology of the overgrowth also varied with βmax, as exemplified by AFM observations. Growth at step edges occurred over the entire βmax range considered, and additional growth features including 3Å high monolayer islands and ∼25Å high tall islands were observed when logβmax>1. On calcite, in situ XR indicated that this phase is similar to the Cd-rich overgrowth formed on dolomite and images obtained from X-ray reflection interface microscopy (XRIM) reveal the existence of laterally variable Cd-rich domains.