Magnesite Crystals Research Articles

Mechanistic insights into the co-recovery of nickel and iron via integrated carbon mineralization of serpentinized peridotite by harnessing organic ligands.

The rising need to produce a decarbonized supply chain of energy critical metals with inherent carbon mineralization motivates advances in accelerating novel chemical pathways in a mechanistically-informed manner. In this study, the mechanisms underlying co-recovery of energy critical metals and carbon mineralization by harnessing organic ligands are uncovered by investigating the influence of chemical and mineral heterogeneity, along with the morphological transformations of minerals during carbon mineralization. Serpentinized peridotite is selected as the feedstock, and disodium EDTA dihydrate (Na2H2EDTA·2H2O) is used as the organic ligand for metal recovery. Nickel extraction efficiency of ∼80% and carbon mineralization efficiency of ∼73% is achieved at a partial pressure of CO2 of 50 bars, reaction temperature of 185 °C, and 10 hours of reaction time in 2 M NaHCO3 and 0.1 M Na2H2EDTA·2H2O. Extensive magnesite formation is evidence of the carbon mineralization of serpentine and olivine. An in-depth investigation of the chemo-morphological evolution of the CO2-fluid-mineral system during carbon mineralization reveals several critical stages. These stages encompass the initial incongruent dissolution of serpentine resulting in a Si-rich amorphous layer acting as a diffusion barrier for Mg2+ ions, subsequent exfoliation of the silica layer to expose unreacted olivine, and the concurrent formation of magnesite. Organic ligands such as Na2H2EDTA·2H2O aid the dissolution and formation of magnesite crystals. The organic ligand exhibits higher stability for Ni-complex ions than the corresponding divalent metal carbonate. The buffered environment also facilitates concurrent mineral dissolution and carbonate formation. These two factors contribute to the efficient co-recovery of nickel with inherent carbon mineralization to produce magnesium carbonate. These studies provide fundamental insights into the mechanisms underlying the co-recovery of energy critical metals with inherent carbon mineralization which unlocks the value of earth abundant silicate resources for the sustainable recovery of energy critical metals and carbon management.

Characterization of mineral precipitation in artificially fractured chalk during flooding experiments for IOR research

Natural fractures in carbonate reservoirs are essential to effectively transport hydrocarbon in an otherwise low permeable matrix. However, they can reduce the wanted effect of water injection and Improved Oil Recovery (IOR), as the injected water mainly runs through the fractures. We have investigated chemical and textural alteration in and adjacent to an artificial fracture with an aperture of 2.25 (±0.05) mm in Upper Cretaceous chalk from the St. Vaast Formation (Mons Basin, Belgium). The sample has been injected mainly with 0.219 M MgCl₂ under reservoir conditions over two months. The fracture aperture reduced averagely by 76%. The analysis performed by Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD) show that, magnesite is the dominating precipitate in the fracture with sizes ranging from 0.5 to 5 μm. The abundance of magnesium phases in the fracture fill was also confirmed by Scanning Electron Microscopy (SEM) coupled with Energy-dispersive X-ray spectroscopy (EDS) and bulk geochemistry (Inductively Coupled Plasma Mass Spectrometry ICP-MS). Besides magnesite, phyllosilicates were observed by TEM and Helium Ion Microscopy combined with Secondary Ion Mass Spectrometry (HIM-SIMS). The newly formed material, precipitates, and possible reworked minerals are characterized by magnesite crystals being covered occasionally by phyllosilicates. This material has formed a distinct area bounded to the surrounding matrix. The final diameter of the precipitate measures about 2.75 (±0.15) mm, which indicates an incremental widening of the original fracture aperture. Calcite along fracture wall dissolved. Effluent concentration supports dissolution by an increase in Ca2⁺ and precipitation of magnesite by the loss of Mg2+. As either silica or aluminium is a component of the fluids, the origin of these elements is necessarily from the matrix of the chalk. It is yet unknown if phyllosilicates precipitated or was reworked, or both. Formation and concentration of magnesite and phyllosilicates in reservoir fractures can reduce fracture permeability over time and lead to subsequent propagation through the matrix of the reservoir rock. This in-depth mineralogical study of artificial fracture changes has the utmost importance for the implementation of techniques in fractured reservoirs.

Green soil improvement: using carbon dioxide to enhance the behaviour of clay

This research study was conducted to improve the properties of highly plastic clay using carbonate minerals produced using carbon dioxide. The carbonates such as magnesite, siderate and calcite were produced using laboratory-scale carbonation experiments and were used to treat clay samples. The index properties, shear strength and consolidation behaviour of the treated clay samples were then investigated. The results showed that the plasticity index and activity of the treated clay decreased, and shear strength and consolidation behaviour improved. The study confirmed that magnesite had improved clay properties due to the uniform and homogeneous distribution of small crystals of magnesite in the clay compared with those of siderite- and calcite-treated soil. As the process sequesters carbon dioxide, it could be developed as an environmentally friendly soil improvement method subject to further research studies.

A Database of Solution Additives Promoting Mg2+ Dehydration and the Onset of MgCO3 Nucleation.

Formed via aqueous carbonation of Mg2+ ions, the crystallization of magnesite (MgCO3) is a promising route to carbon capture and reuse, albeit limited by the slow precipitation of MgCO3. Although magnesite is naturally abundant, forming at low temperature conditions, its industrial production is an energy-intensive process due to the temperatures required to prevent the formation of hydrated phases. The principal difficulty in aqueous conditions arises from the very strong Mg2+···H2O interaction, with high barriers to Mg2+ dehydration. Using atomistic simulations, we have investigated the influence of 30 additive anions (Xn–, n = 1–3), ranging from simple halides to more complex molecules, on the first two steps of MgCO3 aggregation from solution, as follows: Mg2+ dehydration and subsequent prenucleative Mg2+···CO32– pairing. We have computed the thermodynamic stabilities of solvent shared ion pairs (Mg2+···H2O···Xn–) and contact ion pairs (Mg2+···Xn–) to reveal the propensity of solution additives to inhibit or promote Mg2+···CO32– formation. We have determined the stabilization of undercoordinated hydrated Mg2+ states with a vacant coordination site to which CO32– can bind, subsequently initiating MgCO3 nucleation or Mg2+ incorporation into the crystal lattice. Extensive molecular dynamics simulations of electrolyte solutions containing Na2CO3 with different sources of Mg2+ (i.e., MgCl2, MgSO4, and Mg(CH3COO)2) further show that the degree of dehydration of Mg2+ and the structure of prenucleation MgCO3 clusters change depending on the counterion identity. Through a fundamental understanding of the role of solution additives in the mechanism of Mg2+ dehydration, our results help to rationalize previously reported experimental observation of the effect of solvation environments on the growth of magnesite. This understanding may contribute to identifying the solution composition and conditions that could promote the low-temperature CO2 conversion into MgCO3 at industrially relevant scales.

Heritage Stone 8. Formation of Pinolitic Magnesite at Quartz Creek, British Columbia, Canada: Inferences from Preliminary Petrographic, Geochemical and Geochronological Studies

Rocks in the Late Proterozoic Horsethief Creek Group at Quartz Creek in British Columbia display rare ‘pinolitic’ textures resembling those described in some sparry magnesite deposits elsewhere in the world. Elongated white magnesite crystals up to 30 cm long occur in a contrasting, dark, fine-grained matrix of dolomite, chlorite, organic material, clay minerals and pyrite. The rocks are aesthetically appealing for use in sculpture and as dimension stone. The term ‘pinolite’ is derived from the superficial similarities between these unusual textures and pinecones. Petrographic examination indicates that these textures formed when metasomatic fluids replaced primary sedimentary dolomite with magnesite. Fluids moved along fractures and bedding planes with repeated fracturing yielding magnesite crystals oriented in opposite directions on either side of annealed fractures, and broken magnesite crystals adjacent to later fractures. Magnesite contains dolomite microinclusions and has elevated Ca contents that are consistent with its formation by replacement of dolomite. Low concentrations of Cr, Ni, Co, Ti, Sr, and Ba in magnesite also imply formation in a metasomatic rather than a sedimentary environment. The rare earth element (REE) concentrations in the Quartz Creek magnesite are higher than those in most evaporitic magnesite and REE patterns lack the Ce and Eu anomalies that characterize carbonate rocks from sedimentary environments. Enrichment in light REE relative to heavy REE, and the similarities between dolomite, chlorite, and magnesite REE profiles, imply that metasomatic fluids modified the original sedimentary geochemical signature of the dolostones during formation of the pinolite rocks. A Late Ordovician to Early Silurian U–Pb age (433 ± 12 Ma), for titanite in the black matrix surrounding the sparry magnesite is younger than the local host rocks, and also younger than the Mesoproterozoic to Middle Cambrian stratigraphic ages of the host rocks for nearby magnesite deposits. The ca. 433 Ma titanite overlaps the ages for numerous fault-associated diatremes and volcaniclastic deposits in the area. Possibly the igneous activity furnished heat for, and/or was the source for, metasomatic fluids that produced the pinolite deposits.

Metasomatic interaction of the eutectic Na- and K-bearing carbonate melts with natural garnet lherzolite at 6 GPa and 1100–1200°C: Toward carbonatite melt composition in SCLM

The range of carbonatite melts in equilibrium with the subcontinental lithospheric mantle (SCLM) under geothermal conditions is limited by alkali-rich near-eutectic compositions. Therefore, here we employed eutectic Na/K-Ca-Mg-Fe carbonate mixtures to model the interaction of a metasomatic carbonatite melt with natural garnet lherzolite. The experiments were performed at 1100 and 1200 °C and 6 GPa in graphite capsules using a multianvil press. The run duration was 111 and 86 h, respectively. To verify achieving an equilibrium, a synthetic mixture identical to natural lherzolite was also employed.We have found that both Na- and K-bearing carbonatite melts cause wehrlitization accompanying by the elimination of orthopyroxene and an increase of CaO in garnet at a constant Cr2O3. Interaction with the K‑carbonatite melt alters clinopyroxene composition toward lower Na2O (0.2–0.3 wt%), and higher K2O (0.5–1.0 wt%), whereas the Na‑carbonatite melt revealed the opposite effect. The resulting melts have a following approximate composition [40(Na, K)2CO3∙60Ca0.5Mg0.4Fe0.1CO2 + 0.6–1.4 wt% SiO2] displaying a decrease in Ca# at a nearly constant alkali content relative to the initial composition, where Ca# = 100∙Ca/(Ca + Mg + Fe). We have also found that alkali-poor (≤ 20 mol% (Na, K)2CO3) carbonate mixtures do not melt completely but yield magnesite and alkali- and Ca-rich melts like those in the systems with eutectic mixtures.Under SCLM P-T conditions the range of carbonatite melt compositions is restricted by the full melting field of alkali-rich carbonates in the corresponding Na/K-Ca-Mg carbonate systems. Infiltration of less alkaline higher-temperature carbonatite melt in SCLM and its subsequent cooling to the ambient mantle temperature, 1100–1200 °C at 6 GPa, should cause crystallization of magnesite and shift the melt composition to [30(Na, K)2CO3∙70Ca0.6Mg0.3Fe0.1CO3]. Owing to its high Ca#, this melt is not stable in equilibrium with orthopyroxene yielding its disappearance by CaMg exchange reaction producing clinopyroxene, magnesite, and shifting the melt composition toward higher alkali content. The melts containing 40–45 mol% of alkaline carbonates have no limitation in Ca# because the corresponding binary NaMg and KMg carbonate eutectics are located near 1200 °C. Therefore, these melts can achieve Ca# ≤ 30–40 and, be in equilibrium with garnet lherzolites and harzburgites under the geothermal condition of SCLM.Considering the present results and previous experimental data the following ranges of carbonatite melt compositions can be expected in equilibrium with garnet peridotites at the base of SCLM: Ca# < 30 and > 30 mol% (K, Na)2CO3 in equilibrium with harzburgite; Ca# 30–40, >25 mol% (K, Na)2CO3 in equilibrium with lherzolite; and Ca# 40–60 and >20 mol% (K, Na)2CO3 in equilibrium with wehrlite.

In-situ High-Temperature XRD and FTIR for Calcite, Dolomite and Magnesite: Anharmonic Contribution to the Thermodynamic Properties

In-situ powder X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectra were measured on the natural crystals of calcite (Ca0.996Mg0.004CO3), dolomite (Ca0.497Mg0.454Fe0.046Mn0.003CO3) and magnesite (Mg0.988Ca0.010Fe0.002CO3), with a temperature up to 796 K. The thermal expansion coefficients were evaluated for these carbonate minerals, resulting in the values of 2.7×10-5, 3.3×10-5 and 3.5×10-5 K-1 for calcite, dolomite and magnesite, respectively. The magnitude of these coefficients is in the same order as those for the isothermal and elastic moduli of these carbonates (e.g., calcite dolomite>magnesite. The Debye model (harmonic approximation) would be valid for magnesite to simulating the thermodynamic properties and isotope fractionation β-factor at high P-T condition.

Mantle exhumation at magma-poor passive continental margins. Part II: Tectonic and metasomatic evolution of large-displacement detachment faults preserved in a fossil distal margin domain (Saraillé lherzolites, northwestern Pyrenees, France)

In two companion papers we report the detailed geological and mineralogical study of two emblematic serpentinized ultramafic bodies of the western North Pyrenean Zone (NPZ), the Urdach massif (paper 1) and the Saraillé massif (this paper). The peridotites have been uplifted to lower crustal levels during the Cretaceous rifting period in the future NPZ. They are associated with Mesozoic pre-rift metamorphic sediments and small units of thinned Paleozoic basement that were deformed during the mantle exhumation event. In the Saraillé massif, both the pre-rift cover and the thin Paleozoic crustal lenses are involved in a Pyrenean recumbent fold having the serpentinized peridotites in its core. Based on detailed geological cross-sections microscopic observations and microprobe mineralogical analyses, we describe the lithology of the two major extensional fault zones that accommodated: (i) the progressive uplift of the lherzolites upward the Cretaceous basin axis, (ii) the lateral extraction of the continental crust beneath the rift margins and, (iii) the decoupling of the pre-rift cover along the Upper Triassic (Keuper) evaporites and clays, allowing its gliding and conservation in the basin center. These two fault zones are the (lower) crust-mantle detachment and the (upper) cover décollement located respectively at the crust-mantle boundary and at the base to the detached pre-rift cover. The Saraillé peridotites were never exposed to the seafloor of the Cretaceous NPZ basins and always remained under a thin layer of crustal mylonites. Field constraints allow to reconstruct the strain pattern of the mantle rocks in the crust-mantle detachment. A 20–50 m thick layer of serpentinized lherzolites tectonic lenses separated by anastomosed shear zones is capped by a thin upper damage zone made up of strongly sheared talc-chlorite schists invaded by pyrite crystallization. The cover décollement is a few decameter-thick fault zone resulting from the brecciation of Upper Triassic layers. It underwent strong metasomatic alteration in the greenschist facies, by multi-component fluids leading to the crystallization of quartz, dolomite, talc, Cr-rich chlorite, amphiboles, magnesite and pyrite. These data collectively allow to propose a reconstruction of the architecture and fluid-rock interaction history of the distal domain of the upper Cretaceous northern Iberia margin now inverted in the NPZ.

Phase Relations in the Harzburgite–Hydrous Carbonate Melt at 5.5–7.5 GPa and 1200–1350°С

Phase relations are studied experimentally in the harzburgite–hydrous carbonate melt system, the bulk composition of which represents primary kimberlite. Experiments were carried out at 5.5 and 7.5 GPa, 1200–1350°С, and $${{X}_{{{\text{C}}{{{\text{O}}}_{2}}}}}$$ = 0.39–0.57, and lasted 60 hours. It is established that olivine–orthopyroxene–garnet–magnesite–melt assemblage is stable within the entire range of the studied parameters. With increase of temperature and $${{X}_{{{\text{C}}{{{\text{O}}}_{2}}}}}$$ in the system, Ca# in the melt and the olivine fraction in the peridotite matrix significantly decrease. The composition of silicate phases in run products is close to those of high-temperature mantle peridotite. Analysis of obtained data suggest that magnesite at the base of subcontinental lithosphere could be derived by metasomatic alteration of peridotite by asthenospheric hydrous carbonate melts. The process is possible in the temperature range typical of heat flux of 40–45 mW/m2, which corresponds to the conditions of formation of the deepest peridotite xenoliths. Crystallization of magnesite during interaction with peridotite matrix can be considered as experimentally substantiated mechanism of CO2 accumulation in subcratonic lithosphere.

Generating porosity during olivine carbonation via dissolution channels and expansion cracks

Abstract. The olivine carbonation reaction, in which carbon dioxide is chemically incorporated to form carbonate, is central to the emerging carbon sequestration method using ultramafic rocks. The rate of this retrograde metamorphic reaction is controlled, in part, by the available reactive surface area: as the solid volume increases during carbonation, the feasibility of this method ultimately depends on the maintenance of porosity and the creation of new reactive surfaces. We conducted in situ dynamic X-ray microtomography and nanotomography experiments to image and quantify the porosity generation during olivine carbonation. We designed a sample setup that included a thick-walled cup (made of porous olivine aggregates with a mean grain size of either ∼ 5 or ∼ 80 µm) filled with loose olivine sands with grain sizes of 100–500 µm. The whole sample assembly was reacted with a NaHCO3 aqueous solution at 200 °C, under a constant confining pressure of 13 MPa and a pore pressure of 10 MPa. Using synchrotron-based X-ray microtomography, the three-dimensional (3-D) pore structure evolution of the carbonating olivine cup was documented until the olivine aggregates became disintegrated. The dynamic microtomography data show a volume reduction in olivine at the beginning of the reaction, indicating a vigorous dissolution process consistent with the disequilibrium reaction kinetics. In the olivine cup with a grain size of ∼ 80 µm (coarse-grained cup), dissolution planes developed within 30 h, before any precipitation was observed. In the experiment with the olivine cup of ∼ 5 µm mean grain size (fine-grained cup), idiomorphic magnesite crystals were observed on the surface of the olivine sands. The magnesite shows a near-constant growth throughout the experiment, suggesting that the reaction is self-sustained. Large fractures were generated as the reaction proceeded and eventually disintegrated the aggregate after 140 h. Detailed analysis show that these are expansion cracks caused by the volume mismatch in the cup walls, between the expanding interior and the near-surface which keeps a nearly constant volume. Nanotomography images of the reacted olivine cup reveal pervasive etch pits and wormholes in the olivine grains. We interpret this perforation of the solids to provide continuous fluid access, which is likely key to the complete carbonation observed in nature. Reactions proceeding through the formation of nano- to micron-scale dissolution channels provide a viable microscale mechanism in carbon sequestration practices. For the natural peridotite carbonation, a coupled mechanism of dissolution and reaction-induced fracturing should account for the observed self-sustainability of the reaction.

Temperature effects on rock engineering properties and rock-fluid chemistry in opal-CT-bearing chalk

In this study, eight tri-axial tests on Cretaceous age outcrop chalk from Aalborg have been performed systematically by injecting MgCl2 for the first time at different temperatures (25, 60, 92, 110 and 130 °C) and for comparison, NaCl at 130 °C. Whole-rock geochemistry, stable isotope measurements, pycnometry, Field Emission Gun Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy, X-Ray Diffraction (XRD) and measurements of Specific Surface Area (Brunauer-Emmett-Teller theory (N2)) were applied to analyse unflooded and flooded cores. Based on analyses of changes in brine composition, mineralogy, specific surface area, solid density, porosity and permeability some conclusions can be drawn on temperature effects on rock engineering properties and rock-fluid chemistry.The MgCl2 flooded cores show systematically higher creep rates at higher temperature and the cores tested at 25 and 60 °C show similar creep rates as the two NaCl flooded cores at 130 °C. All fluid-rock interactions were more pronounced at higher temperature. After flooding with MgCl2 at 110 and 130 °C newly formed magnesite is observed. In the cores tested at 25, 60 and 92 °C magnesite crystals have not been positively identified, but minute increases in MgO in whole-rock geochemistry analyses are seen. Si4+ originating from the dissolution of silica bearing phases (mainly diagenetic opal-CT), has taken part in the re-precipitation of Si-Mg-bearing minerals during MgCl2 injection from 25 to 130 °C, leading to an increase of the specific surface area. This is partly balanced by opal-CT dissolution, which should lower the specific surface area. The flaky Si-Mg-bearing minerals, covering significant portions of the pore surfaces and throats are the main drivers to reduce permeability in cores flooded at high temperatures. Additionally, in NaCl flooded cores where mineralogical changes are minute, the dissolution of parts of the existing opal-CT has lowered the SSA. At high temperatures, the following chemical changes must be carefully acknowledged when porosity reduction is calculated: calcite and opal-CT dissolution and precipitation of new minerals, particularly Mg-bearing minerals. The presence of opal-CT strongly influences the mineralogical alterations in flooded cores, hence the geo-mechanical evolution.

Thermogravimetric analysis–mass spectrometry (TGA–MS) of hydromagnesite from Dujiali Lake in Tibet, China

Thermogravimetric analysis–mass spectrometry, in situ X-ray diffraction, scanning electron microscopy and Fourier transform infrared spectroscopy were used to characterize hydromagnesite [Mg5(CO3)4(OH)2·4H2O] from Dujiali Lake in Tibet, China. This study describes the variations in the thermal decomposition mechanisms of hydromagnesite at varying heating rates and under either helium (He) or carbon dioxide (CO2) atmospheres. In a He atmosphere, only two decomposition stages were observed; the loss of the crystalline water followed by the combined dehydroxylation and decarbonation. However, under a CO2 atmosphere, the dehydroxylation and decarbonation occur separately as the inert CO2 gas prevents the decomposition of the MgCO3 component of hydromagnesite. Overall, the thermal decomposition is an endothermic process. A distinctly exothermic process occurs at about 540 °C under conditions of high partial pressure of CO2 or high heating rate and implies the crystallization of magnesite (MgCO3). We propose that the release of H2O and CO2 at different stages likely results from the complicated hydrogen bonds and different carbonate groups in the crystal structure of hydromagnesite.

Indicator Isotope–Geochemical Characteristics of Sulfides from the Satka Magnesite Ore Field (South Urals Province)

The stable enrichment of pyrite from magnesite ores in δ34S isotope (from 5.4 to 6.9‰) compared with pyrite from the host (sedimentary and igneous) rocks was established in the classical Satka sparry magnesite ore field. Concretionary segregations of fine-grained pyrite in dolomite are depleted in the heavy sulfur isotope (δ34S, from–9.1 to–5.8‰). Pyrite from dolerite is characterized by δ34S values (–1.1 and 1.7‰) close to the meteorite sulfur. The δ34S values in barite from the underlying dolomite horizon vary in the range of 32.3–41.4‰. The high degree of homogeneity of the sulfur isotope composition in pyrite from magnesite is a result of thermochemical sulfate reduction during the syngenetic crystallization of pyrite and magnesite from epigenetic brines, formed during dissolution of evaporite sulfate minerals at the stage of early catagenesis of the Riphean deposits.

Room Temperature Magnesite Precipitation

Magnesite (MgCO3) is one of the most stable sinks for carbon dioxide (CO2) and is therefore of great interest for long-term carbon storage. Although magnesite is the thermodynamically stable form of magnesium carbonate, the kinetic inhibition of low-temperature precipitation has hindered the development of carbon sequestration strategies that can be economically conducted under ambient temperature. Here, we document the precipitation of magnesite from waters (magnesite saturation index = 1.45) in batch reactors at room temperature with the aid of carboxylated polystyrene microspheres over the course of 70 days. Microspheres provide surfaces with a high density of carboxyl groups that act to bind and dehydrate Mg2+ ions in solution, thereby minimizing the kinetic barrier and facilitating magnesite formation. Magnesite crystals are observed on sphere surfaces and their organic matrixes. Mineral identification was confirmed by X-ray diffraction and selected area electron diffraction of a thin section obtaine...

Changes in mineral assemblages during serpentine carbonation

The carbonation of serpentine (lizardite [Mg3Si2O5(OH)4]) was investigated at a fixed temperature of 290°C under a 30-bar CO2 atmosphere in aqueous solutions of different alkalinities to understand the pH-dependence of the process. To activate the carbonation, the serpentine was heat-treated at 650°C beforehand for 2h. At pH6, the main reaction products were magnesite (MgCO3) and well-crystallized saponite [NaxMg3(Si4Al4−x)O10(OH)2]. The carbonation rate of the Mg ions was limited at acidic pH, and thermogravimetry (TG) analysis of the solid product showed that only 13% carbonation rate of serpentine to magnesite was achieved. The partial dissolution of SiOSi and the resultant Mg-rich solution provided a favorable environment for the formation of saponite. In weakly alkaline and neutral solutions, the rate of carbonation was dramatically enhanced well-faceted rhombohedral magnesite crystals of approximately 1μm in size with a round shape appeared together with a small amount of “curled-flaky type” saponite. The conversion of silicate to carbonate was found to be approximately 45%. As the pH of the solution increased further, the dissociation of carbonic acid accelerated and the free Mg2+ cations then reacted with the bicarbonate ions to form magnesium carbonate. Thus, active conversion to magnesite resulted in poor formation of saponite because of the low concentration of Mg ions in the solution. At pH2, well-crystallized saponite appeared, but magnesite was not observed. Under strongly alkaline conditions, saponite may form by a mechanism wherein the undissolved Mg layer of serpentine is available to act as a substrate for saponite precipitation. The transformation of serpentine to saponite and magnesite was influenced mostly by temperature, alkalinity, and mineral composition.

Behavior of the Chemical Potential in Calcite and Magnesite Crystals: A Damped Harmonic Oscillation

The behavior of the chemical potential of the species CaCO3 and MgCO3 in proximity of the (10.4) crystal face of calcite and magnesite is determined at the quantum–mechanical level by adopting a recent calculation strategy conceived in our research laboratory. A very peculiar trend of this quantity from the (10.4) surface to the center of the calcite and magnesite crystals was observed. The highest value of the chemical potential is related to the surface layer, while the lowest one is in correspondence to the underlying layer. Interestingly, the chemical potential converges to the bulk value via a damped oscillatory behavior (DOB). We observed that such DOB is strongly related to the structural modification of the slab caused by the presence of the surface, which produces a damped oscillating behavior of the interplanar distances, d10.4, very similar to the one described for the chemical potential. The implications of this finding are discussed.

Uranium Immobilization and Nanofilm Formation on Magnesium-Rich Minerals.

Polarization-dependent grazing incidence X-ray absorption spectroscopy (XAS) measurements were completed on oriented single crystals of magnesite [MgCO3] and brucite [Mg(OH)2] reacted with aqueous uranyl chloride above and below the solubility boundaries of schoepite (500, 50, and 5 ppm) at pH 8.3 and at ambient (PCO2 = 10(-3.5)) or reduced partial pressures of carbon dioxide (PCO2 = 10(-4.5)). X-ray absorption near edge structure (XANES) spectra show a striking polarization dependence (χ = 0° and 90° relative to the polarization plane of the incident beam) and consistently demonstrated that the uranyl molecule was preferentially oriented with its Oaxial═U(VI)═Oaxial linkage at high angles (60-80°) to both magnesite (101̅4) and brucite (0001). Extended X-ray absorption fine structure (EXAFS) analysis shows that the "effective" number of U(VI) axial oxygens is the most strongly affected fitting parameter as a function of polarization. Furthermore, axial tilt in the surface thin films (thickness ∼ 21 Å) is correlated with surface roughness [σ]. Our results show that hydrated uranyl(-carbonate) complexes polymerize on all of our experimental surfaces and that this process is controlled by surface hydroxylation. These results provide new insights into the bonding configuration expected for uranyl complexes on the environmentally significant carbonate and hydroxide mineral surfaces.

Carbonatite melt–peridotite interaction at 5.5–7.0 GPa: Implications for metasomatism in lithospheric mantle

Interaction between carbonatite melt and peridotite is studied experimentally by melting samples of interlayered peridotite–carbonatite–peridotite in graphite containers at 1200–1350°C and 5.5–7.0GPa in a split-sphere multianvil apparatus. Starting compositions are lherzolite and harzburgite, as well as carbonatite which may form in the upper part of a slab or in a plume-related source. Most experimental runs were of 150h duration in order for equilibrium to be achieved.The interaction produced carbonatitic melts with low SiO2 (≤7wt.%) and high alkalis. At 1200°C, melt–peridotite interaction occurs through Mg–Ca exchange, resulting in elimination of orthopyroxene and crystallization of magnesite and clinopyroxene. At 1350°C hybridization of the carbonatite and magnesite-bearing peridotite melts occurred with consumption of clinopyroxene and magnesite, and crystallization of orthopyroxene at MgO/CaO≥4.3. The resulting peridotite-saturated melt has Ca# (37–50) depending on primary carbonatite composition. Compositions of silicate phases are similar to those of high-temperature peridotite but are different from megacrysts in kimberlites. CaO and Cr2O3 changes in garnet produced from the melt–harzburgite interaction at 1200 and 1350°C perfectly match the observed trend in garnet from metasomatized peridotite of the Siberian subcontinental lithospheric mantle. K-rich carbonatite melts equilibrated with peridotite at 5.5–7.0GPa and 1200–1350°C correspond to high-Mg inclusions in fibrous diamond. Carbonatite melt is a weak solvent of entrained xenoliths and therefore cannot produce kimberlitic magma if temperatures are ~1350°C on separation from the lithospheric peridotite source and ~1000°C on eruption.

Conditions of carbonation and wehrlitization of lithospheric peridotite upon interaction with carbonatitic melts

Study of the mechanism of carbonation and wehrlitization of harzburgite upon metasomatism by carbonatitic melts of various genesis was carried out. Experiments with durations of 60–150 h were performed at 6.3 GPa and 1200°C. The data showed that carbonatite with MgO/CaO > 0.3 percolating into the peridotitic lithosphere may provide crystallization of magnesite in it. The influence of all studied carbonatites results in wehrlitization of peridotite. The compositions of melts formed by interaction with harzburgite (∼2 wt % SiO2, Ca# = 36–47) practically do not depend on the composition of the initial carbonatite. Based on the data obtained, we conclude that the formation of magnesite-bearing and magnesite-free metasomatized peridotites may have a significant influence on the CO2 regime in the further generation of kimberlitic magmas of groups I and II.

Melilite-derived mineral inclusions in chromite from the Gaositai complex: Implications for an extensional tectonic setting in Early Permian at the north North China Craton

The Early Permian mafic-ultramafic concentrically zoned Gaositai intrusion at Chengde, on the northern margin of the North China Craton (NCC), is a cumulative complex emplaced along a giant fracture that penetrates deeply into the continental lithosphere. Melt inclusions are present in chromite crystals from the inner dunite and chromitite zones of the Gaositai complex. The melt inclusions have experienced post-trap crystallization and resulted in multiple mineral phases, including melilite, garnet, phlogopite, magnesite and apatite, which can indicate the liquidus minerals of the primitive magma. The characteristics of the melilite+melanite+clinopyxene assemblage indicate that the primary parental magma was highly undersaturated and derived from an alkali-rich mantle source. The crystallization of phlogopite, magnesite and apatite suggests a primary magma rich in K, H2O and CO2. When compared with experimental data, the primary magma of the Gaositai intrusion is concordant with a kamafugite magma originating from partial melting of enriched mantle with H2O and CO2 at pressures greater than 2.7 GPa. This magmatic process would have been related to extensional thinning of the continental lithosphere. The Gaositai primary magmas have high Nb/La ratios, which are similar to those of ocean island basalts, but different from arc-related magmas. This suggests that the northern margin of the NCC was not an active continental margin of the Paleo-Asian Ocean subduction zone during the Early Permian: an extensional tectonic setting during the emplacement of the Gaositai intrusion is more likely.