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Lake Salda, a terrestrial analog for the paleolake in Jezero Crater on Mars, hosts active, subfossil, and fossil hydromagnesite microbialites, making it an ideal location to study microbialite formation and subsequent processes. Our understanding of this record is still limited by an incomplete knowledge of the macro‐ and mesoscale morphotypes of microbialites, along with their spatial distribution and correlation with microbial and geochemical processes that influence microbialite formation. In this study, we investigated the spatial distribution, morphotypes, mineralogy, geochemistry, and microbial diversity of the microbialites and identified six distinct zones (Zone I to Zone VI) with major microbialite build‐ups in Lake Salda. Newly identified microbialites were classified based on the macro‐ and mesostructures. Our work shows that the lake contains stromatolites, thrombolites, stromatolitic thrombolites, dendrolites, and microbially induced sedimentary structures. At macroscale, Lake Salda microbialites exhibit hemispheres, stacked domes, and laterally linked columnar structures while minicolumns, knobs, mesoclots, laminae, and botryoidal structures are common at mesoscale. The macro‐ and mesoscale distribution of different microbialite types spatially correlates with microbial community composition and water depth. Deep‐growing microbialites with a low abundance of Cyanobacteria (∼1%–4%) and high abundance of Firmicutes (28%–93%) exhibit steeply convex lamination, producing finger‐like minicolumnar mesostructures. In contrast, shallow‐growing microbialites with a low abundance of Firmicutes (0%–5%) and high abundance of Cyanobacteria (11%–37%) have well‐preserved gently convex millimeter‐scale lamination, resulting in cauliflower mesostructures. Palygorskite ((Mg, Al)2Si4O10(OH)) is identified in the diatom‐rich microbial layer of the deep‐growing microbialites. Regardless of the microbialite types, hydromagnesite and aragonite are present in the extracellular polymeric substance (EPS)‐rich zone of the shallow and deep‐growing microbialites. Overall, environmental changes (e.g., water depth and, accommodation space) play a major role in the formation and spatial distribution of different microbialite morphologies at the macro‐ and mesoscale. Differences in the relative abundance of dominant microorganisms between mesostructured types suggest that mesomorphology may be influenced by changes in microbial diversity. Spatial variations in the microbialite morphotypes, along with the abundant presence of entombed biomass (e.g., mineralized filaments), may indicate areas that have a high potential for the preservation of biosignatures. [ABSTRACT FROM AUTHOR]

Hydromagnesite (HM for short) is a natural carbonate mineral that is widely distributed. It is a high-quality mineral raw material for preparing flame retardants, magnesium oxides, heavy/light basic magnesium carbonates, magnesium hydroxides, and other Mg products. The evaluation of HM resources is of great significance to the development and utilization of salt lake resources. Using remote sensing technology to observe HM resources in salt lake can overcome the shortcomings of traditional prospecting methods such as discontinuous spatial data, time and effort. In addition, spectral analysis is the basis of hyperspectral remote sensing, and more detailed analysis of the spectral characteristics of HM is still lacking; therefore, we measured the reflection spectral curve of HM samples in the area of Jiezechaka by ASD FieldSpec4 short-wave infrared spectrometer and determined the mineral composition and content of HM samples by X-ray diffraction. The analysis indicated three and seven absorption valleys with high and low absorption intensities, respectively, in the reflectance spectral curves of the HM samples in the Jiezechaka area. Then, on this basis, the Landsat8 OLI multispectral data and ZY1-02D AHSI hyperspectral data were used as the basic data of remote sensing inversion. As the ZY1-02D AHSI data have 166 bands, which is much more than Landsat8 OLI data, it has a stronger ability to characterize the spectral characteristics of HM and can better meet the requirements of remote sensing inversion. The end member spectra were selected based on PPI and SMACC methods, respectively. The HM information around Jiezechaka Salt Lake in Tibet was extracted by the mixture tuned matched filtering method, and the regional distribution map of HM was made. A confusion matrix operation was used to compare the determination results of the two types of data. Among them, based on Landsat8 data, PPI method was used to obtain end members, and the overall accuracy of HM extraction results was > 69%, and the kappa coefficient was 0.688. Based on Landsat8 data, SMACC method was used to obtain end members, and the overall accuracy of HM extraction results was > 67%, and the kappa coefficient was 0.667. Based on ZY1-02D AHSI data, PPI method was used to obtain end members, and the overall accuracy of HM extraction results was > 76%, and the kappa coefficient was 0.743. Based on ZY1-02D AHSI data, SMACC method was used to obtain end members, and the overall accuracy of HM extraction results was > 73%, and the kappa coefficient was 0.728. It shows that the end members selected by PPI method can better express HM information in the image. Finally, through the overlay analysis of the four results, we concluded that HM outcrops in the Jiezechaka area are mainly distributed in the northwestern and southeastern regions of the lake. This study provides a rapid assessment technique for measuring HM information from salt lakes. [ABSTRACT FROM AUTHOR]

A painted wooden shield from the 16th century, commonly traced back to the famous Lepanto Battle (1571), was investigated by a multi‐analytical approach to identify its manufacturing process and shed light on its cultural provenance. The typology is coherent with both the Christian and Ottoman armies and does not allow for an attribution. Radiocarbon dated the shield to two time intervals (1470–1525, 1580–1625, at 68% level of probability), compatible with the date of the Battle of Lepanto. Optical and Scanning Electron Microscopy, Attenuated Total Reflectance, External Reflection Fourier Transform Infrared, micro‐Raman and Energy Dispersive X‐ray Spectroscopy, and histochemical and shrinking temperature tests were used to characterize the artefact. The shield resulted to be made of softwood panels, covered by a stratigraphy of flax fibres, cattle skin, and a painting preparation layer of hydromagnesite. Adhesion was ensured by bone glue. Oblique, red, and cinnabar stripes alternating on the white preparation are visible on the front of the shield, whereas a homogeneous dark paint covers the back. None of the raw materials was informative by themselves, but the unusual presence of the hydromagnesite could be crossed with the geographical context, resulting in a promising outcome for future isotopic and trace element studies. [ABSTRACT FROM AUTHOR]

Hexavalent chromium (Cr6+) is a toxic carcinogenic pollutant that might be released by the mining and processing of ultramafic rocks and nickel laterites and which requires permanent removal from the contaminated biosphere. Ultramafic material can also serve as a feedstock for the sequestration of CO2 resulting from the growth of new minerals, raising the intriguing proposition of integrated sequestration of both pollutants, CO2 and chromium, into magnesium carbonates. Such a synergistic process downstream of ore recovery and mineral processing could be an elegant proposition for more sustainable utilisation and management of the Earth's resources. We have therefore carried out an experimental and microanalytical study to investigate potentially suitable carbonate minerals. Uptake of chromium in carbonate phases was determined, followed by identification of the crystalline phases and characterisation of the local structural environment around the incorporated chromium centres. The results suggest that neither nesquehonite nor hydromagnesite have the structural capacity to incorporate Cr6+ or Cr3+ significantly at room temperature. We therefore propose that further research into this technology should focus on laboratory assessments of other phases, such as layered double hyroxides, that have a natural structural capacity to uptake both chromium and CO2. [ABSTRACT FROM AUTHOR]

Achieving high mechanical performance in a polymer composite is one of the primary concerns when using mineral-based flame retardants. Mineral-based flame retardants should be added to the matrix at 50%–70% to achieve a high level of flame retardancy. High mineral content increases the modulus of elasticity and reduces the ductility of the material. This can be disastrous in applications where flexibility is critical. To overcome this problem, minerals can be used in combination with other effective flame retardants. In this study, the combination of halogen-free mineral-based huntite/hydromagnesite (H/H) and phosphorus-nitrogen-based intumescent flame retardants was investigated to improve the flame retardancy and maintain the flexibility of polypropylene (PP). For this purpose, PP carrier composites were prepared by extrusion by decreasing the mineral concentration in the active formulations and increasing the ratio of phosphorus-based flame retardants. These composites were subjected to mechanical, morphological (SEM), color, and flammability (UL94) tests. As a result, composites with UL94 V0 flammability and 18% to 104% higher tensile strength than equivalent H/H-based flame retardants were obtained. [ABSTRACT FROM AUTHOR]

Binders based on blends of reactive magnesia and hydromagnesite are low‐CO2 or even carbon‐negative cements in case the MgO is generated from magnesium silicates or Mg‐containing brines. When MgO is hydrated with water alone, a well crystalline brucite forms, but no measurable strength is developing. In the presence of hydromagnesite, the hydration of MgO is accelerated and the hydrate assemblage is changed. A poorly‐crystalline brucite containing some carbonate is the main hydration product. Furthermore, a kind of "gel‐water" is observed, which is lost at temperatures above 40‐60°C. This brucite‐like phase is responsible for strength formation in MgO‐hydromagnesite blends. After 28 days, a mortar compressive strength is reached similar to the one of a Portland cement of strength class 32.5. [ABSTRACT FROM AUTHOR]

The article presents the results of studying novel bursts of ULF emissions within 0.5-2.5 Hz (Pc1 geomagnetic pulsations) with a simultaneous increase (~0.01 Hz/min) and decrease in frequency (~0.01 Hz/min) lagging by ~5÷10 min from the initial mean frequency (~0.6 Hz). From their formal resemblance with the stripes on military officer uniforms, these emissions are called Pc1 chevrons. The bursts were observed during strong short-term geomagnetic disturbances (~1 hour): substorms with high gradient of ionospheric currents+ intensification (dD/dt or dH/dt ~1-1.3 nT/s). We present the main parameters and couplings of these emissions with substorm geophysical phenomena of 11.02.1985, 06.04.1997 and 18.07.2013 events. Polar satellite observations of auroras are compared with LANL-91, 94 and THEMIS-A observations of charged particle fluxes. Possible mechanisms generating oscillations are also discussed. [ABSTRACT FROM AUTHOR]

Lake Salda has a unique environment due to the presence of hydromagnesite stromatolites. Samples collected from the stromatolites contained a new diatom species, Gomphonella saldanensis sp. nov., which is described based on light and scanning electron microscopy. The main character of the taxon is its linear-lanceolate outline with a rostrate headpole and narrowly rounded footpole. The striae are strongly radiate mid-valve, becoming parallel to slightly radiate towards the poles. Elliptical Fourier and Linear Discriminant analyses indicate that there are different shape forms within the same taxon. Based on LM observations, G. saldanensis is similar to Gomphonema lagerheimii f. simplex, G. geisslerae, Gomphonella calcarea, G. olivacea, G. acsiae and G. coxiae. However, G. lagerheimii f. simplex differs with slightly radiate striae throughout the valve. Gomphonella geisslerae has a slightly undulate raphe and linear apices. Larger specimens of G. acsiae are lanceolate, clavate in smaller specimens. The remaining Gomphonella species, G. calcarea, G. coxiae and G. olivacea have broader outlines and lower stria densities. [ABSTRACT FROM AUTHOR]

Jezero Crater on Mars is a paleolacustrine environment where Mg-carbonates may host evidence of ancient life. To elucidate the environmental and mineralogical controls on biosignature preservation, we examined samples from five terrestrial analogs: Lake Salda (Turkey), Lake Alchichica (Mexico), Qinghai-Tibetan Plateau (China), Mg-carbonate playas (British Columbia, Canada), and a mine with fine-grained ultramafic tailings (Yukon, Canada). The mineralogical compositions of the samples varied, yet were often dominated by either aragonite (CaCO3) or hydromagnesite [Mg5(CO3)4(OH)2·4H2O]. Aragonite-rich samples from Alchichica, Mg-carbonate playas, and the ultramafic mine contained an abundance of entombed microbial biomass, including organic structures that resembled cells, whereas hydromagnesite-rich samples were devoid of microfossils. Aragonite often precipitates subaqueously where microbes thrive, thereby increasing the likelihood of biomass entombment, while hydrated Mg-carbonates typically form by evaporation in subaerial settings where biofilms are less prolific. Magnesite (MgCO3), the most stable Mg-carbonate, forms extremely slowly, which may limit the capture of biosignatures. Hydrated Mg-carbonates are prone to transformation via coupled dissolution–precipitation reactions that may expose biosignatures to degradation. Although less abundant, aragonite is commonly found in Mg-carbonate environments and is a better medium for biosignature preservation due to its fast precipitation rates and relative stability, as well as its tendency to form subaqueously and lithify. Consequently, we propose that aragonite be considered a valuable exploration target on Mars. [ABSTRACT FROM AUTHOR]

Carbonate minerals are major contributors to carbon sequestration in geological deposits; however, their nature and behavior remain unclear. Amorphous magnesium carbonate (AMC) is formed as a precursor to crystalline magnesium carbonates and as a product of thermal decomposition of nesquehonite (NSQ). In this study, the AMCs formed during the crystallization and decomposition of NSQ were investigated using X-ray diffraction (XRD) and atomic pair distribution function (PDF) methods. An AMC with a hydromagnesite-like structure (AMC-I) was formed immediately after mixing MgCl2 and Na2CO3 solutions. After 5 min of stirring, no change was observed in the XRD pattern; however, the PDF pattern changed. This suggests that the medium-range ordered structure of AMC-I transformed into an intermediate structure (AMC-II) between AMC-I and NSQ. After 10 min of stirring, the AMC-II crystallized into NSQ. In the case of Rb2CO3, the AMC-II structure was formed immediately after the mixing of solutions and was stable for three days. AMC-II in the Rb2CO3 solution appeared to be in equilibrium with energetic local minima, indicating the existence of polyamorphism in AMC. When Cs2CO3 solution was used, the first precipitate had an AMC-I structure. By stirring for 5 min, the AMC-I was transformed to AMC-II, and after 10 min of stirring, a few quantities crystallized into NSQ. After three days, NSQ dissolved and transformed back into AMC-I. Thus, it is inferred that the crystallization of NSQ is significantly influenced by alkali cations in aqueous solutions. The AMC formed during the thermal decomposition also possesses the AMC-I structure. [ABSTRACT FROM AUTHOR]

In this work, the flame retardancy influences of colemanite and boron oxide are investigated n huntite-hydromagnesite (HH) containing plasticized poly (lactic acid) (PLA) biocomposites. The composites are characterized using limiting oxygen index (LOI), horizontal (UL 94 HB) and vertical (UL-94 V) burning tests, mass loss calorimeter (MLC), and thermogravimetric analysis (TGA). The addition of colemanite causes enhanced fire retardancy performance with higher UL-94 V rating and LOI value and lower peak heat release rate (pHRR) value. V0 rating and the highest LOI value are observed with the addition 1 wt% colemanite. The samples get V1 rating for the loading amount of 3 and 5 wt% colemanite. When the concentration of colemanite reaches to 5 wt%, pHRR and average heat release rate (avHRR) values are lower than those of the reference sample. Addition of boron oxide causes improvement in LOI value. The highest LOI value (33.2) is observed in 5 and 10 wt% boron oxide containing samples. [ABSTRACT FROM AUTHOR]

Naturally occurring layered double hydroxide mineral, brucite (BRU), was compared with hydromagnesite (HYD) and a commercial Mg-rich mineral adsorbent (trade name AQM PalPower M10) to remove antimony (Sb) from synthetic and real wastewaters. The BRU and HYD samples were calcined prior to the experiments. The adsorbents were characterized using X-ray diffraction, X-ray fluorescence, and Fourier transform infrared spectroscopy. Batch adsorption experiments were performed to evaluate the effect of initial pH, Sb concentration, adsorbent dosage, and contact time on Sb removal from synthetic wastewater, mine effluent, and textile industry wastewater. Several isotherm models were applied to describe the experimental results. The Sips model provided the best correlation for the BRU and M10. As for the HYD, three models (Langmuir, Sips, and Redlich–Peterson) fit well to the experimental results. The results showed that the adsorption process in all cases followed the pseudo-second-order kinetics. Overall, the most efficient adsorbent was the BRU, which demonstrated slightly higher experimental maximum adsorption capacity (27.6 mg g-1) than the HYD (27.0 mg g-1) or M10 (21.3 mg g-1) in the batch experiments. Furthermore, the BRU demonstrated also an efficient performance in the continuous removal of Sb from mine effluent in the column mode. Regeneration of adsorbents was found to be more effective under acidic conditions than under alkaline conditions. [ABSTRACT FROM AUTHOR]

We prepared solution-precipitated amorphous calcium carbonate by two similar methods and tracked structural changes over time as they crystallized. By cross-referencing mid-range infrared (400–4000 cm−1) with far-infrared (100–400 cm−1) spectral features, and by comparing with powder X-ray diffraction data for the aged crystallized products, we provide guidelines for—and potential limitations of—using far-infrared spectroscopy to assess multi-phase Mg-containing calcium carbonate samples that include amorphous or poorly crystallized components. [ABSTRACT FROM AUTHOR]

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Hydratable alumina (HA) was premixed with hydromagnesite (BMC) to investigate the BMC impact on the hydration behavior of HA and the thermo-mechanical properties of HA bonded (HAB) castables. The phase composition, microstructure and mass changes of dried HA samples, were characterized by XRD, SEM, and TG, respectively. Flow ability, microstructure, and thermo-mechanical properties of HAB castables were studied. Results indicated that BMC effectively lowered HA hydration rate due to the decreased specific surface area. The hot modulus of rupture strength of castables was improved because the sintering of Al 2 O 3 was enhanced by the MgO from BMC decomposition. [ABSTRACT FROM AUTHOR]

Light-burned MgO cement has a lower calcination temperature than ordinary Portland cement. It has been widely studied as a measure to reduce carbon dioxide because of its property of absorbing carbon dioxide during curing. This study investigated the effects of calcination temperature on the physical properties of light hydrated magnesium carbonate and calcined MgO hydrated in moisture and CO2 at 25 °C and 60 °C. The crystal size of light-burned MgO increased with increasing calcination temperature, and carbonates were formed through carbonation curing. Further, nesquehonite and hydromagnesite were formed in the 25CC and 60CC specimens, respectively, and the carbonate formation reduced with increasing crystal size. The highest compressive strength of 3.5 MPa was obtained for the 25CC specimen in which nesquehonite was formed; however, hydromagnesite exhibited better CO2 sequestration capacity.

This study demonstrated the use of nesquehonite (MgCO 3 ×3 H 2 O) in producing plasterboard. Nesquehonite (NQ) can be activated via thermal treatment under 50–175°C, after which the dehydrated nesquehonite can hydrate to form a plaster-like product, like gypsum-based plaster. Here, we developed different blends of NQ and hemihydrate (dehydrated gypsum) to produce plaster. In addition, the influence of trace elements (i.e., sulfate and ammonium) on the reaction kinetics, mechanical properties, microstructure, phase assemblage, and evolution of the developed plasters were investigated. The target was to achieve comparable manufacturing process and compressive strength with the conventional gypsum plaster process using NQ as a major precursor. Data revealed that the mixes containing 75 % NQ obtained from 2 different synthesis routes (i.e., wet carbonation or chemically-synthesis route) attained 6–8 MPa compressive strength after 28 days of curing, which was comparable to those observed in gypsum-based products. The chemically synthesized NQ achieved the stability of NQ in the plasterboard sample, while the use of wet-carbonated NQ led to the formation of dypingite. Furthermore, the water resistance of plasterboard mixes with 100 % NQ outperformed that of gypsum, without any decline in compressive strength after water exposure. • Nesquehonite MgCO 3 ·3 H 2 O can be used as a precursor to produce plaster-like materials as an alternative to gypsum. • Nesquehonite can partially or completely replace gypsum in the plaster. • The presence of impurities like sulfate originated from nesquehonite synthesis did not negatively change the strength of plaster but altered its phase formation. • The presence of magnesium carbonates in the plaster outperformed the water resistance as compared the plain gypsum plaster. [ABSTRACT FROM AUTHOR]

The economic challenges arising from resource depletion and uneven distribution of Mg in land reserves have spurred interest in technologies for Mg extraction from seawater. Thus, this study introduces a novel method to recover high-purity magnesium carbonate, specifically hydromagnesite, from seawater through carbonation using Ca(OH)₂ as an alkali source. A significant achievement of this method is the production of high-purity hydromagnesite despite the presence of large amounts of Ca, realized only through carbonation without the need for expensive additives such as NaOH. Experimental results reveal that the Ca(OH)₂ content predominantly influences the Mg dissolution efficiency during carbonation and the overall Mg recovery rate. Maximizing the efficiency of this technology relies on using an appropriate amount of Ca(OH)₂. Notably, the appropriate amount of OH− supply enhances CO₂ dissolution, facilitating Mg dissolution. Conversely, an excess of Ca(OH)₂ impedes Mg dissolution. The overall Mg recovery rate reaches a peak of 67 %, enabling the recovery of 3.4 kg of hydromagnesite from one ton of seawater. The findings highlight the effectiveness of the proposed technology as a promising and cost-effective method for extracting high-purity Mg from seawater, utilizing Ca(OH) 2 , previously considered an impurity, as a cost-effective alkali source. • Recovered high-purity hydromagnesite from seawater via carbonation using Ca(OH) 2 • Used Ca(OH)₂, previously deemed an impurity, as a cost-effective alkali • Optimal amounts of Ca(OH)₂ enhanced Mg dissolution but excess content inhibited it • Mg recovery peaked at 67 %, yielding 3.4 kg hydromagnesite per ton of seawater [ABSTRACT FROM AUTHOR]

In this study, performances of three different natural minerals, namely magnesite, hydromagnesite, and huntite, were investigated as regenerable Ca-Mg-based sorbents for high-temperature CO2 capture. XRD analyses results showed that these raw minerals contained different forms of Ca and Mg oxides, such as Mg5(CO3)4(OH)2.4H2O-hydromagnesite, MgCO3-magnesite, CaMg3(CO3)4-huntite, and CaMg(CO3)2-dolomite. In order to obtain CaO as an active phase, natural minerals were calcined at different temperatures (900–950 °C) as defined by TGA-DTA analyses. After activation, CaO and MgO phases were obtained in all of these sorbents. High-temperature CO2 removal tests were performed in a continuous-flow fixed-bed reactor system in the temperature range of 400–700 °C with a GHSV of 3600 cm3h−1 g−1. CO2 sorption capacities were determined using breakthrough curves and TGA-DTA analyses. Following CO2 sorption, CaCO3 formation via carbonation of the CaO phase was determined by TGA-DTA and XRD analyses. CO2 sorption capacities of huntite, hydromagnesite, and magnesite-derived sorbents were found as 3.5, 2.0, and 0.4 mmol CO2/gsorbent at 550 °C, respectively. In the experiments carried out at different temperatures using huntite-derived sorbent, the highest CO2 sorption was observed in the pre-breakthrough region at 550 °C. Huntite-derived sorbent was found to be highly stable after 10 cycles and its stability was attributed to the presence of MgO as an inert phase in the sorbent. After ten cycles, the huntite-derived sorbent maintained about 73% of its first cycle sorption capacity. It was concluded that the huntite-derived material is a promising sorbent for high-temperature CO2 capture based on using 83% of its CaO content in its structure. [ABSTRACT FROM AUTHOR]

In this study, hydromagnesite, a rare natural hydrated alkaline magnesium carbonate, was used to synthesize magnesium hydroxide (MH) as a flame retardant for ethylene-vinyl acetate (EVA) to enhance its fire resistance and smoke suppression. Various concentrations of sodium hydroxide (NaOH) were used to alter the morphology and the flame-retardant efficiency of synthesized MH. EVA/MH composites were prepared through melt blending, and the influence of NaOH on the flame retardancy and mechanical properties was investigated by means of the limiting oxygen index (LOI), cone calorimeter test (CCT) and tensile test. The flame retardancy results demonstrated that composites exhibited remarkably improved flame retardant properties after introducing MH, reflected by an increase in the LOI value from 20% for neat EVA to roughly 38%. Additionally, the peak of heat release rate (pHRR), the total heat release (THR) and the peak of the smoke production rate for EVA3 were decreased by 37.6%, 20.7% and 44.4% compared with neat EVA, respectively. In the meantime, increasing char residues were also observed. The incorporation of different MH concentrations had a limited effect on the mechanical properties of the EVA/MH composites. [ABSTRACT FROM AUTHOR]

We prepared solution-precipitated amorphous calcium carbonate by two similar methods and tracked structural changes over time as they crystallized. By cross-referencing mid-range infrared (400–4000 cm−1) with far-infrared (100–400 cm−1) spectral features, and by comparing with powder X-ray diffraction data for the aged crystallized products, we provide guidelines for—and potential limitations of—using far-infrared spectroscopy to assess multi-phase Mg-containing calcium carbonate samples that include amorphous or poorly crystallized components.

This study refers to the application of in-situ electrochemical synthesis as an alternative method to improve the properties of porous materials against harmful external agents that deteriorate them. It is oriented to an understanding of the effects of crystallisation on the pore structure of different compounds commonly used in the restoration and conservation of porous materials (historical ceramics, building walls, sculptures, or biomedical applications). It analyses the microstructural, chemical details, and stability of the neo-formed phases that modify the pore network. The electrochemical synthesis was carried out at ambient temperature (20 °C), over high porous sandstone for crystallising Ca carbonate, Mg carbonate, Ca phosphate, and Ca oxalate compounds. Based on the neo-formed minerals, a comparison was made depending on their specific properties defining how they affected the pore structure. The characterisation included polarised light optical microscopy, environmental and field-emission scanning electron microscopy, digital image analysis, cathodoluminescence (CL-ESEM),energy-dispersive X-ray spectroscopy, and X-ray microdiffraction. Aragonite, hydromagnesite, hydroxyapatite, and whewellite were identified as the majority phases depending on the treatment. Phase transformation, dehydration, and dissolution-re-precipitation processes suggested different degrees of stability, including aragonite/calcite (CaCO3 treatment) and hydromagnesite/magnesite (MgCO3 treatment) transformations and simultaneous crystallisation of brushite/hydroxyapatite ((Ca 3 (PO 4) 2 treatment). Electrocrystallisation induced changes in inter-granular porosity, the development of secondary porosity inherent to the minerals, and differences in pore cementation depending on its mineralogy. Among the treatments, Mg carbonate reduced porosity most effectively, followed in descending order by calcium carbonate and calcium phosphate, being the calcium oxalate the less effective. [ABSTRACT FROM AUTHOR]

In this study we present the first Mg isotope data that record the fate of Mg during mineralisation of atmospheric CO 2 in ultramafic mine tailings. At the Woodsreef Asbestos Mine, New South Wales, Australia, weathering of ultramafic mine waste sequesters significant amounts of CO 2 in hydromagnesite [Mg 5 (CO 3) 4 (OH) 2 ·4H 2 O]. Mineralisation of CO 2 in above-ground, sub-aerially stored tailings is driven by the infiltration of rainwater dissolving Mg from bedrock minerals present in the tailings. Hydromagnesite, forming on the surface of the tailings, has lower δ 26Mg (δ 26Mg Hmgs = −1.48 ± 0.02‰) than the serpentinised harzburgite bedrock (δ 26Mg Serpentinite = −0.10 ± 0.06‰), the bulk tailings (δ 26Mg Bulk tailings = −0.29 ± 0.03‰) and weathered tailings containing authigenic clay minerals (δ 26Mg Weathered tailings = +0.28 ± 0.06‰). Dripwater (δ 26Mg Dripwater = −1.79 ± 0.02‰) and co-existing hydromagnesite (δ 26Mg Hmgs = −2.01 ± 0.09‰), forming in a tunnel within the tailings, and nodular bedrock magnesite [MgCO 3 ] (δ 26Mg Mgs = −3.26 ± 0.10‰) have lower δ 26Mg than surficial fluid (δ 26Mg = −0.36‰) and hydromagnesite. Complete dissolution of source minerals, or formation of Mg-poor products during weathering, is expected to transfer Mg into solution without significant alteration of the Mg isotopic composition. Aqueous geochemical data and modelling of saturation indices, along with Rayleigh distillation and mixing calculations, indicate that the 26Mg-depletion in the drip water, relative to surficial water, is the result of brucite dissolution and/or precipitation of secondary Mg-bearing silicates and cannot be assigned to bedrock magnesite dissolution. Our results show that the main mineral sources of Mg in the tailings (silicate, oxide/hydroxide and carbonate minerals) are isotopically distinct and that the Mg isotopic composition of fluids and thus of the precipitating hydromagnesite reflects both isotopic composition of source minerals and precipitation of Mg-rich secondary phases. The consistent enrichment and depletion of 26Mg in secondary silicates and carbonates, respectively, underpins the use of the presented hydromagnesite and fluid Mg isotopic compositions as a tracer of Mg sources and pathways during CO 2 mineralisation in ultramafic rocks. [ABSTRACT FROM AUTHOR]

Interpreting the environmental conditions under which ancient microbialites formed relies upon comparisons with modern analogues. This is why we need a detailed reference framework relating the chemical and mineralogical compositions of modern microbialites to the physical and chemical parameters prevailing in the environments where they form. Here, we measured the chemical, including major and trace elements, and mineralogical composition of microbialites from ten Mexican lakes as well as the chemical composition of the surrounding waters. Saturation states of lakes with different mineral phases were systematically determined and correlations between solution and solid chemical analyses were assessed using multivariate analyses. A large diversity of microbialites was observed in terms of mineralogical composition, with occurrence of diverse carbonate phases such as (Mg-)calcite, monohydrocalcite, aragonite, hydromagnesite, and dolomite as well as authigenic Mg-silicate phases (kerolite and/or stevensite). All lakes harbouring microbialites were saturated or supersaturated with monohydrocalcite, suggesting that such a saturation state might be required for the onset of microbialite formation and that precursor soluble phases such as amorphous calcium carbonate and monohydrocalcite play a pivotal role in these lakes. Subsequently, monohydrocalcite transforms partly or completely to aragonite or Mg-calcite, depending on the lake (Mg/Ca) aq. Moreover, lakes harbouring hydromagnesite-containing microbialites were saturated with an amorphous magnesium carbonate phase, supporting again the involvement of precursor carbonate phases. Last, authigenic Mg-silicates formed by homogenous or heterogenous nucleation in lakes saturated or supersaturated with a phase reported in the literature as "amorphous sepiolite" and with a H 4 SiO 4 concentration superior to 0.2 mM. A strong correlation between the alkalinity and the salinity of all the lakes was observed. The observed large variations of alkalinity between the lakes relate to varying concentration stages of an initial alkaline dilute water, due to a varying hydrochemical functioning. In all cases, the size of microbialites in the lakes correlated positively with salinity, (Mg/Ca) aq ratio and alkalinity. The trace element compositions of the microbialites also varied significantly between the lakes. Detrital contamination of the studied microbialites was the major factor affecting their rare earth elements (REE) + Y patterns. In particular, the microbialites highly affected by detrital contamination showed a high (REE + Y) content and flat (REE + Y) patterns. In contrast, some microbialites poorly affected by detrital contamination showed (REE + Y) patterns with features commonly reported for marine microbialites, such as a superchondritic Y/Ho ratio, enrichment in heavy REE and a negative Ce anomaly. This last observation questions the possibility to infer the marine versus lacustrine origin of a microbialite based on (REE + Y) patterns only. Overall, while microorganisms can impact nucleation processes and textural arrangements in microbialites, we observe that the hydrogeochemical evolution of lakes exerts a primary control over the onset of microbialite formation and the evolution of their chemical and mineralogical composition. Moreover, while changes of all these chemical and mineralogical features upon diagenesis and metamorphism will need to be assessed, the present study, together with recent meta-analyses of modern microbialites, broadens the set of modern references available for comparisons with geological archives. [ABSTRACT FROM AUTHOR]

The Cariboo Plateau in central British Columbia, Canada, hosts the largest aerial extent of low temperature lacustrine magnesite in the world. Magnesite was the most abundant carbonate mineral in 21 of 33 sampled alkaline lakes and 20 of 22 sampled playas. Calcite, hydromagnesite and very high magnesium calcite (VHMC, also called disordered dolomite) were also found in closed basin alkaline lakes on the plateau. Carbonate mineralogy was strongly associated with lake water geochemistry, particularly total alkalinity. VHMC was the dominant carbonate mineral in lakes with the highest alkalinities (>1000 mEq/kg), magnesite was the dominant carbonate mineral in lakes in the middle range of alkalinities (generally 70 to 800 mEq/kg) and hydromagnesite with minor amounts of magnesite was the phase assemblage in lakes with lower relative alkalinities (<110 mEq/kg). The geochemical trends for the activity of CO 3 2−, Ca2+ and Mg2+ suggest that these constituents in all Cariboo water bodies are controlled by the precipitation of Ca- and/or Mg‑carbonate minerals. The waters are near equilibrium with metastable crystalline Ca‑carbonate and Mg‑carbonate minerals. We hypothesize that carbonates present in the lakes likely formed via non-classical crystallization pathways based on the relationship between aqueous geochemistry and mineralogy and the chemical composition of the Ca-bearing magnesites. Non-classical crystallization offers new models for thinking about magnesite formation in lacustrine environments. • Cariboo Plateau has largest aerial extent of low temperature lacustrine magnesite • Alkalinity likely controls carbonate mineralogy (VHMC, magnesite, hydromagnesite) • Lake water geochemistry is controlled by Ca- and/or Mg-carbonate precipitation • Non-classical crystallization offers new models for natural carbonate formation [ABSTRACT FROM AUTHOR]

We investigated crystal structure and the local structure changes during the thermal decomposition of hydromagnesite by using in situ high-temperature XRD and ex situ high-temperature X-ray total scattering measurements. Hydromagnesite displayed anisotropic thermal expansion up to 220 °C. The a and c lattice parameters exhibited an increase trend with temperature, whereas the b lattice parameter and β angle did not show a regular trend with temperature. The relative expansion between 25 and 220 °C followed the c/c0 > a/a0 ≫ b/b0. At 260 °C, the a, b, and c lattice parameters significantly decreased. Above 280 °C, hydromagnesite underwent a structural collapse with dehydration and dehydroxylation reactions, but was never accompanied by nucleation and growth of crystal phases up to 425 °C. During the thermal decomposition from hydromagnesite to periclase, the Mg atoms maintained the octahedral coordination environments in the structure. [ABSTRACT FROM AUTHOR]

In this study, hydromagnesite, a rare natural hydrated alkaline magnesium carbonate, was used to synthesize magnesium hydroxide (MH) as a flame retardant for ethylene-vinyl acetate (EVA) to enhance its fire resistance and smoke suppression. Various concentrations of sodium hydroxide (NaOH) were used to alter the morphology and the flame-retardant efficiency of synthesized MH. EVA/MH composites were prepared through melt blending, and the influence of NaOH on the flame retardancy and mechanical properties was investigated by means of the limiting oxygen index (LOI), cone calorimeter test (CCT) and tensile test. The flame retardancy results demonstrated that composites exhibited remarkably improved flame retardant properties after introducing MH, reflected by an increase in the LOI value from 20% for neat EVA to roughly 38%. Additionally, the peak of heat release rate (pHRR), the total heat release (THR) and the peak of the smoke production rate for EVA3 were decreased by 37.6%, 20.7% and 44.4% compared with neat EVA, respectively. In the meantime, increasing char residues were also observed. The incorporation of different MH concentrations had a limited effect on the mechanical properties of the EVA/MH composites.

Calcium aluminate cement was premixed with hydromagnesite having different particle sizes, as a calcium magnesium aluminate cement precursor, to investigate the influence of particle size of hydromagnesite on the volume stability and thermo-mechanical properties of corundum based castables after firing at 1550 °C. The impact of particle size of hydromagnesite on the phase composition and microstructure evolution of fired castables matrix were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results demonstrate that hydromagnesite with smaller particle size has a better volumetric stability and thermo-mechanical properties of castables, because of the more homogeneously distributed micro-pores, and smaller size MA and CA 6 resulted from the more uniformly distributed hydromagnesite in castables. [ABSTRACT FROM AUTHOR]

• Hierarchically MgO-based mesoporous adsorbent (CH-5) derived from hydromagnesite was obtained. • CH-5 showed superior adsorption efficiency (69.60 mg/g) towards fluoride in aqueous. • The adsorption performance of CH-5 kept stable from pH=5 to 11. • The prepared adsorbent can be recycled easily at least five times by re-calcination. Hierarchically mesoporous adsorbent derived from natural hydromagnesite with remarkable adsorption capacity for fluoride (F−) has been synthesized through the facile one-step calcination. Various characterizations confirmed that calcination temperature was directly correlated with their crystallite structures, surface properties, and adsorb ability. The hydromagnesite calcined at 500 °C (CH-5) exhibits the highest adsorption capacity and rate towards F−. The F− adsorption isotherm data was explored by both Langmuir and Freundlich model and the maximum adsorption capacity of CH-5 was 69.6 mg/g. Kinetic study revealed that the adsorption kinetics follows the pseudo-second-order model, suggesting the chemisorption mechanism. Regeneration study showed that the adsorption ability for fluorine can be completely recovered by calcining at 500 °C, and after five cycles, the adsorption ability is not significantly weakened. After detailed evaluation, the enhancement mechanism of the calcined product can be attributed to relatively higher specific surface area and exposed more magnesite active sites due to the maintenance of the intrinsic three-dimensional layer structure during the dehydration and decarbonation, which were responsible for the striking adsorption abilities. The green adsorbent originated from natural mineral with the characteristics of low cost, high adsorption ability and recoverability is advantageous for fluoride-containing wastewater treatment. [ABSTRACT FROM AUTHOR]

Modern hydrated Mg rich stromatolites are actively growing along the shallow shorelines of Lake Salda (SW Turkey). An integrated approach involving isotopic, mineralogical, microscopic, and organic/geochemical techniques along with culture-independent molecular methods were applied to various lake samples to assess the role of microbial processes on stromatolite formation. This study further explores the biosignature preservation potential of fossil stromatolites by comparing with textures, lipid profiles and isotopic composition of the modern stromatolites. Similar lipid profile and δ13C isotope values in active and fossil stromatolites argue that CO2 cycling delicately balanced between photosynthetic and heterotrophic (aerobic) activity as in the active ones may have regulated stromatolite formation in the lake. A decrease in the exopolymeric substances (EPS) profile of the mat and concurrent hydromagnesite precipitation imply a critical role for EPS in the formation of stromatolite. Consistently, a discrete, discontinuous lamination and clotted micropeloidal textures with cyanobacterial remnants in the fossil stromatolites likely refer to partial degradation of EPS, creating local nucleation sites and allowing precipitation of hydrated Mg minerals and provide a link to the active microbial mat in the modern stromatolites. Our results for the first time provide strong evidence for close coupling of cyanobacterial photosynthesis and aerobic heterotrophic respiration on hydromagnesite textures involved in the stromatolite formation of Lake Salda. The creation of photosynthesis induced high-pH conditions combined with a change in the amount and properties of the EPS and the repetition of these processes over time seems to be a possible pathway for stromatolite growth in the lake. Understanding these microbial symbioses and their mineralized records may provide new insights on the formation mechanism of Mg-rich carbonates not only for terrestrial geological records but also for planetary bodies like Mars, where hydrated Mg-carbonate deposits have been identified in possible paleolake deposits at Jezero crater, the landing site of the NASA Mars 2020 rover. [ABSTRACT FROM AUTHOR]

Hydromagnesite with different morphologies has been synthesized using magnesite as raw materials. The influences of pyrolysis temperature, the pH value of the magnesium bicarbonate solution and pyrolysis time on the preparation process of hydromagnesite were investigated. The synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and fully automatic specific surface meter (BET). The phase transition process and mechanism of hydrated magnesium carbonate with different crystal structures were further explored. The results showed that porous rod-like, porous flower-like and lamellar hydromagnesite was prepared at above 70 °C, 90 min and different pH values. The specific surface area of porous rod-like, porous follower-like and lamellar hydromagnesite is 59.292m2/g, 34.568m2/g and 27.973m2/g, respectively. While the average pore sizes of the corresponding hydromagnesite are 15.172 nm, 15.368 nm and 17.132 nm, respectively. Based on the crystal assembly method, the structural stability of hydromagnesite is the highest compared with lansfordite and nesquehonite. Unlabelled Image Typical SEM micrographs of porous rod-like, porous flower-like and lamellar hydromagnesite; Triangular plot of compositional characters of magnesite carbonates; Crystal structure of landsfordite, nesquehonite and hydromagnesite. • Porous rod-like, porous flower-like and lamellar hydromagnesite was prepared using natural magnesite. • The texture properties and shape features of hydromagnesite were characterized. • Hydromagnesite morphology has a significant effect on the texture properties. • Hydromagnesite owns the best structural stability compared with landfordite and nesquehonite. • The phase transformation mechanism of hydrated magnesium carbonate was investigated. [ABSTRACT FROM AUTHOR]

Understanding low temperature carbon sequestration through serpentinite-H2O-CO2 interaction is becoming increasingly important as it is considered a potential approach for carbon storage required to offset anthropogenic CO2 emissions. In this study, we present new insights into spontaneous CO2 mineral sequestration through the formation of hydromagnesite + kerolite with minor aragonite incrustations on serpentinite walls of the Montecastelli copper mine located in Southern Tuscany, Italy. On the basis of field, petrological, and geochemical observations coupled with geochemical modeling, we show that precipitation of the wall coating paragenesis is driven by a sequential evaporation and condensation process starting from meteoric waters which emerge from fractures into the mine walls and ceiling. A direct precipitation of the coating paragenesis is not compatible with the chemical composition of the mine water. Instead, geochemical modeling shows that its formation can be explained through evaporation of mine water and its progressive condensation onto the mine walls, where a layer of serpentinite powder was accumulated during the excavation of the mine adits. Condensed water produces a homogeneous film on the mine walls where it can interact with the serpentinite powder and become enriched in Mg, Si, and minor Ca, which are necessary for the precipitation of the observed coating paragenesis. The evaporation and condensation processes are driven by changes in the air flow inside the mine, which in turns are driven by seasonal changes of the outside temperature. The presence of "kerolite", a Mg-silicate, is indicative of the dissolution of Si-rich minerals, such as serpentine, through the water-powder interaction on the mine walls at low temperature (~17.0 to 18.1 °C). The spontaneous carbonation of serpentine at low temperature is a peculiar feature of this occurrence, which has only rarely been observed in ultramafic outcrops exposed on the Earth's surface, where instead hydromagnesite predominantly forms through the dissolution of brucite. The high reactivity of serpentine observed, in this study, is most likely due to the presence of fine-grained serpentine fines in the mine walls. Further study of the peculiar conditions of underground environments hosted in Mg-rich lithologies, such as that of the Montecastelli Copper mine, can lead to a better understanding of the physical and chemical conditions necessary to enhance serpentine carbonation at ambient temperature. [ABSTRACT FROM AUTHOR]

The genesis of hydromagnesite [Mg5(CO3)4(OH)2·4H2O] has attracted great interest as a pathway for sequestering anthropogenic CO2 and because of its importance to Mg carbonate depositional environments; however, there remain uncertainties regarding the chemical environment for hydromagnesite precipitation in modern and ancient geologic systems. Trace and rare earth element (REE) concentrations in hydromagnesite from Dujiali Lake, central Qinghai–Tibetan Plateau, China identified the formation conditions in the context of the depositional environment. The analyzed hydromagnesite samples had low total REE concentrations, varying from 0.62 to 3.11 ppm, with an average ∑REE value of 1.75 ppm. Comparisons of Ce/Ce* with LaN/SmN, DyN/SmN, and ∑REE showed no correlation indicating preservation of the original redox conditions during hydromagnesite precipitation. Redox-sensitive trace element ratios (U/Th, Ni/Co, V/Cr and V/V + Ni), negative Mn* values, and low authigenic uranium (Ua) values all indicate oxic conditions at the time of hydromagnesite formation. Furthermore, the Post-Archean Australian Shale-normalized REE patterns of the hydromagnesite display slight heavy REE enrichment, a slightly negative Ce anomaly, and a consistently positive Eu anomaly, which are consistent with precipitation in a predominantly oxidizing environment. Data indicate that hydromagnesite precipitated from waters influenced by both Mg-rich hydrothermal fluids and meteoric water with a similar composition to the lake water. This study provides new insights into the conditions of hydromagnesite formation at Dujiali Lake with implications for the understanding of the genesis of modern and ancient Mg carbonate deposits. [ABSTRACT FROM AUTHOR]

Recently, a hydrous carbonate-containing brucite (HCB) with an approximate composition of MgCO 3 ·35 Mg(OH) 2 ·H 2 O was postulated as hydration product of binders based on mixtures of reactive magnesia and hydromagnesite (Mg 5 (CO 3) 4 (OH) 2 ·4H 2 O). X-ray diffraction showed that the 001 reflection of this phase was split into two reflections at 20 °C, whereas at 60 °C only one reflection occurred. Rietveld refinement revealed that the X-ray pattern of the HCB-phase could be fitted well when a random displacement (stacking faults) and a movement of the layers in x, y and z direction were allowed. The presence of water and/or carbonate leads to different distances between the layers, thus causing the splitting of the 001 reflection. At 60 °C, only carbonate is included in the brucite structure, leading to a similar distance between all brucite layers. The assignment of the additional reflection appearing at 20 °C to a separate, unknown hydrated magnesium carbonate, as suggested in earlier studies, can be excluded. [ABSTRACT FROM AUTHOR]

The formation of dolomite is very challenging in the laboratory under ambient conditions due to kinetic inhibition. The goal of this study was to test the impact of pH cycling and zinc ions on the formation of magnesium-rich carbonates in saline fluids at a low temperature. Batch reactor experiments were conducted in two series of pH cycling experiments, one without and one with zinc ions, at 43 °C. The results after 36 diel pH cycles indicate a reaction product assemblage of hydromagnesite, aragonite and magnesite in the experiments without zinc ions, and of magnesite and minor aragonite in the experiments with zinc ions. The presence of zinc ions leads to a decrease in the pH in the acid phase of the cycling experiments, which likely plays a role in the reaction product assemblage. Moreover, the hydration enthalpy and other specific ion effects could be additional factors in the formation of magnesium-rich carbonate. The results show a clear evolution towards increasing incorporation of magnesium in the carbonate phase with cycle number, especially in the experiments with zinc ions, reflecting a ripening process that is enhanced by pH cycling. Hence, repeated pH cycling did not lead to more ordered dolomite (from protodolomite), but rather to the formation of magnesite with 92 mol% MgCO3 after 36 cycles, even though geochemical models indicate a higher saturation index for dolomite than for magnesite.

In order to study the morphology, chemical composition, symbiotic relationship among major minerals and the typical embedding characteristics of the hydromagnesite from Bangor Lake in Tibet, polarizing microscope, scanning electron microscope, transmission electron microscope, electron probe and MLA mineral analysis system were used.The results indicate that the hydromagnesite is cryptocrystalline aggregate which consists of small crystal particles.The single crystal which has characteristic striation on the (001) is monoclinic plate-like.The hydromagnesite crystal usually grow into layered polysynthetic twin and contact twin along (100).Aragonite, dolomite and other carbonate minerals are often associated with hydromagnesite in disseminated symbiotic relationship in raw ores.It might be difficult to use physical mineral processing to get rid of them.

This paper provides strong evidence for the contribution of the phylum Firmicutes in mediating the primary precipitation of Mg-rich carbonates (hydromagnesite, dolomite, magnesite, and nesquehonite) in recent microbialites from a highly alkaline and ephemeral inland lake (Las Eras, Central Spain). The carbonate mineral precipitation occurs sequentially as the microbial mats decay. Scanning electron microscopy (SEM) provided solid proof that hydromagnesite nucleation is initiated on the exopolymeric substances (EPS) and the microbial cells associated to the microbial mat degradation areas. The progressive mineralization of the EPS and bacterial cells by hydromagnesite plate-like crystals on their surface, results in the entombment of the bacteria and formation of radiating aggregates of hydromagnesite crystals. The hydrous phases, mostly hydromagnesite, were produced at a high percentage in the first stages of the microbial degradation of organic matter. When the availability of organic substrates declines, the heterotrophs tend to reduce their number and metabolic activity, remain dormant. At this stage, the anhydrous phases, dolomite and magnesite, nucleate on bacterial nanoglobules and/or collapsed cells. Evidence for the sequential formation of the Mg-rich carbonates trough the decay of organic matter by a fermentative EPS-forming bacterium isolated from the microbialites, Desemzia incerta, is drawn through a comparative analysis of carbonate formation in both natural and experimental settings. This study will help to constrain potential mechanisms of carbonate formation in natural systems, which are of fundamental importance not only for understanding modern environments but also as a window into the geologic past of Earth and potentially Mars.

Hydromagnesite with different morphologies has been synthesized using self-made nesquehonite whiskers as raw materials. The synthesized samples have been characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that porous rod-like hydromagnesite are generated at 328~353K and in the pH value of 9.30+0.2, while irregular flower-like and flat layered ones are synthesized in the pH values of 10.0+0.05 and 11.0+0.05, respectively. The yield of hydromagnesite improved linearly with the increase of the temperatures and solution pH values. Porous rodlike hydromagneiste crystals with good crystalline and uniform morphology are obtained when the pyrolysis time is over 60 min. Furthermore, the apparent activation energy of phase transformation is calculated to be 3.4080 kJ/mol. According to the results, the experimental data can be well described by the kinetic model, suggesting that the phase transfer rate is dependent on the temperature. [ABSTRACT FROM AUTHOR]

Silica derived from sol-gel approach was used as co-filler in aluminium hydroxide (ATH) or hydromagnesite (HM) ethylene-vinyl acetate (EVA) formulations. In this bottom-up approach, the silica network is formed inside the extruder in a first process step. The 3D silica structure is investigated by solid-state nuclear magnetic resonance (ssNMR) spectroscopy and a combined TGA-FTIR analysis revealing a partially incomplete reaction to form silica particles. In addition, 29Si CPMAS NMR showed the formation of chemical Si–C bonds between EVA and silica networks. The performance enhancement of the fire retardant properties was established by comparing with composites prepared by a top-down approach, i.e by adding preformed silica. It is confirmed that the preformed silica acts as a synergistic agent with both ATH or HM, increasing the flame retardant properties (limiting oxygen index, UL-94 and cone calorimeter tests) of EVA composites, however, the sol-gel silica shows the best performances in reducing both heat release and smoke production. The role of the sol-gel structure was deeply investigated in correlation with combustion test. • EVA-based flame retarded composites have been successfully prepared with silica as co-filler. • Silica formed in-situ during the extrusion process is compared with preformed silica. • NMR spectroscopy shows that the sol-gel silica lattice is not completely condensed and it reacts with the polymer. • Preformed and in-situ prepared silica does not lead to any change in the polymer degradation mechanism. • In-situ formed silica based composite forms a consistent and compact residue with better flame retardant properties. [ABSTRACT FROM AUTHOR]

The hydromagnesite–magnesite playas near Atlin, British Columba, Canada are unique Mg-carbonate depositional environments that have formed at Earth's surface since the end of the last deglaciation. This study elucidates the mechanisms, pathways, and rates of magnesite (MgCO 3) formation in these near-surface environments, which are challenging to study in short-duration laboratory experiments because magnesite precipitation is extremely slow at low temperature. The Atlin playas, having formed over millennia, contain abundant magnesite as well as a suite of other Mg- and Ca-carbonate minerals. Mineralogical and textural evidence demonstrate that hydromagnesite [Mg 5 (CO 3) 4 (OH) 2 ·4H 2 O] forms at least in part through transformation of more hydrated phases, e.g., lansfordite (MgCO 3 ·5H 2 O). Deposition of these hydrated Mg-carbonate minerals is limited by the evaporative flux, and thus, is effectively transport-controlled at the scale of the playas. Magnesite is a spatially distinct phase from hydromagnesite and its crystal morphology varies with depth indicating variable crystal growth mechanisms and precipitation rates. Particle size distributions and mineral abundance data indicate that magnesite formation is nucleation-limited. Furthermore, mineralogical data as well as stable and radiogenic isotope data support magnesite formation starting after the majority of hydromagnesite had been deposited likely resulting from long induction times and slow precipitation rates. Hydrated Mg-carbonate minerals precipitate relatively rapidly and control pore water chemistry while magnesite remains highly supersaturated, and thus, is reaction-controlled. This difference in controlling regime allows for magnesite abundance to increase over time without the loss of hydromagnesite such as through its transformation, which the data also does not support. We estimate rates of magnesite formation (nucleation + crystal growth) in the range of 10−17 to 10−16 mol/cm2/s over approximately 8000 years. This study helps to elucidate the geochemical conditions needed to form Mg-carbonate minerals in ancient and modern sedimentary environments and provides insights into facilitating long-term storage of anthropogenic CO 2 within Mg-carbonate minerals. [ABSTRACT FROM AUTHOR]

Flower-shaped hydromagnesite (MgO-P) and magnesium oxide (MgO) micro spheres were synthesized via precipitation-aging method with and without calcination. The disassociated nano flaky petal of around 50 nm (~ 20 nm in thickness) of MgO-P and MgO by sonication were used as flame-retardants for the electrospun cellulose acetate (CA)/MgO-P or CA/MgO nanofiber. SEM, XRD, FTIR, HRTEM were used to characterize all related products. TGA was used to analyze the detailed decomposition procedures of bare, MgO-P, and MgO doped CA nanofiber. Three evaluation methods including broken time after heating, thermography during heating, and combustion observation were adopted to compare the flame-retardancy of composited nanofibers. Doped CA nanofibers showed best flame-retardancy by extending the heat-induced broken time from 0.46 to 12.09 s, which is 26.3 times that of CA nanofiber. The flame of CA/MgO-P and CA/MgO could be self-extinguished once the ignition source was removed. No smoke and size shrinkage could be observed during the combustion of CA/MgO-P or CA/MgO nanofibers. Corresponding flame-retarding mechanism of Mg-based nano flaky petal was proposed. [ABSTRACT FROM AUTHOR]

Transition metal ions impurities in microbialites have been previously suggested as paleoenvironmental and/or paleobiological proxies. However, how metals are incorporated into microbialites remains poorly known. Here, in order to assess the distribution and speciation of Fe in modern microbialites, we conducted bulk X-ray diffraction, infrared spectroscopy, X-ray absorption near-edge structure (XANES) spectroscopy, electron microscopy and X-ray microscopy analyses on samples collected from shallow depths in five alkaline lakes in Mexico: Lake Alchichica, Lake La Preciosa, Lake Atexcac, Lake La Alberca de Los Espinos and Lake Pátzcuaro. A range of Fe contents were measured in these microbialites, from low (∼0.12 wt.%) to relatively high (∼2.2 wt.%). Fe was distributed heterogeneously in microbialites, mostly localized as hotspots or sometimes arranged as discrete laminae. Fe was mostly trivalent in all microbialites and was incorporated into diverse authigenic phases, the proportion of which varied among microbialites. Authigenic phases included Fe-bearing Mg-phyllosilicates such as kerolite and/or stevensite, i.e. (Mg,Fe(II),Fe(III)) 3 Si 4 O 10 (OH) 2 ·nH 2 O and Fe-Mg layered double hydroxides (LDH), i.e. pyroaurite [Mg(II) 6 Fe(III) 2 (CO 3)(OH) 16 ·4H 2 O] and/or iowaite [Mg(II) 6 Fe(III) 2 Cl 2 (OH) 16 ·4H 2 O], together with Fe-(oxyhydr)oxides. Carbonate phases were negligible carriers of Fe in all lakes. Iowaite/pyroaurite phases, which have often been found as low temperature alteration products in serpentinites, were surprisingly observed in Lake Alchichica, whereas Fe-bearing kerolite/stevensite was present in Lake La Preciosa, Lake Atexcac, Lake La Alberca de Los Espinos and Lake Pátzcuaro. Fe present in these shallow water microbialites may be originated from groundwater seepage derived from adjacent Fe-rich volcanic rocks. We suggest that the occurence of iowaite/pyroaurite vs authigenic Fe-bearing kerolite/stevensite depends on the orthosilicic acid concentration in the lakes. Pyroaurite/iowaite may form and stay preserved in Lake Alchichica because of the low [H 4 SiO 4 ] as well as high alkalinity, [Mg2+], [CO 3 2−], [Cl−] and pH (∼9) prevailing in this lake, while in other lakes, where [H 4 SiO 4 ] is higher (above ∼0.4 mM), Fe-bearing kerolite or smectite phases are formed. Overall, the message carried by the Fe-bearing mineral phases in these modern microbialites is multifold: Fe-bearing phases within microbialites may contain some information on environmental conditions (e.g., [H 4 SiO 4 ]) as well as on geochemical processes implicated during their formation (e.g., seepage of anoxic Fe-bearing groundwater). Considering the reactivity of pyroaurite/iowaite and kerolite/stevensite and their possible transformation into diverse mineral phases upon microbial activity, burial and diagenesis, a careful appraisal of Fe speciation using a similar combination of bulk and microscopy analyses is required in order to better assess the origin of Fe in ancient microbialites. [ABSTRACT FROM AUTHOR]

Rare earth element (REE) and strontium isotope data (87Sr/86Sr) are presented for hydromagnesite and surface waters that were collected from Dujiali Lake in central Qinghai-Tibet Plateau (QTP), China. The goal of this study is to constrain the solute sources of hydromagnesite deposits in Dujiali Lake. All lake waters from the area exhibit a slight LREE enrichment (average [La/Sm] PAAS = 1.36), clear Eu anomalies (average [Eu/Eu*] PAAS = 1.31), and nearly no Ce anomalies. The recharge waters show a flat pattern (average [La/Sm] PAAS = 1.007), clear Eu anomalies (average [Eu/Eu*] PAAS = 1.83), and nearly no Ce anomalies (average [Ce/Ce*] PAAS = 1.016). The REE+Y data of the surface waters indicate the dissolution of ultramafic rock at depth and change in the hydrogeochemical characteristics through fluid-rock interaction. These data also indicate a significant contribution of paleo-groundwater to the formation of hydromagnesite, which most likely acquired REE and Sr signatures from the interaction with ultramafic rocks. The 87Sr/86Sr data provide additional insight into the geochemical evolution of waters of the Dujiali Lake indicating that the source of Sr in the hydromagnesite does not directly derive from surface water and may have been influenced by both Mg-rich hydrothermal fluids and meteoric water. Additionally, speciation modeling predicts that carbonate complexes are the most abundant dissolved REE species in surface water. This study provides new insights into the origins of hydromagnesite deposits in Dujiali Lake, and contributes to the understanding of hydromagnesite formation in similar modern and ancient environments on Earth. [ABSTRACT FROM AUTHOR]

Mineral carbonation is an important technical option for the effective reduction of CO2 emissions. Natural magnesium‐containing minerals, such as serpentine and blast furnace slag (BFS), have recently been used for CO2 storage with an indirect carbonation route using ammonium sulfate. In this study, the effects of the feeding mode and process parameters on the magnesium conversion, product phase, and morphology during the aqueous carbonation of MgSO4 with ammonium carbonate solution were investigated in detail. The results showed that the carbonation ratio with a parallel feed was higher than the forward and reverse feed by about 3–5% with a limited reaction time, and the product size was more uniform. The phase and morphology of the products were affected significantly by the temperature. The highest carbonation ratio appeared at 40°C because only 75% of magnesium was carbonated if hydromagnesite was produced. When the mole ratio of (NH4)2CO3 to MgSO4 was 2:1 and the concentration of magnesium sulfate was higher than 0.4 mol·L−1, the carbonated products contain ammonium magnesium carbonate, and the ammonia should be recovered by selective thermal decomposition. When the mole ratio decreased to 1.5:1, only nesquehonite appears with high crystallinity and uniform size. The optimized conditions were therefore selected as 40°C, a mole ratio of 1.5:1, and magnesium sulfate concentration of 0.7 mol·L−1. Under these conditions the carbonation ratio reached 88%. Additionally, the optimal initial pH of MgSO4 solution was 9.5 (the product was nesquehonite at low pH) while the hydromagnesite will be produced at higher pH (pH more than 10). © 2019 Society of Chemical Industry and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Solid‐state NMR measurements coupled with density functional theory (DFT) calculations demonstrate how hydrogen positions can be refined in a crystalline system. The precision afforded by rotational‐echo double‐resonance (REDOR) NMR to interrogate 13C–1H distances is exploited along with DFT determinations of the 13C tensor of carbonates (CO32−). Nearby 1H nuclei perturb the axial symmetry of the carbonate sites in the hydrated carbonate mineral, hydromagnesite [4 MgCO3⋅Mg(OH)2⋅4 H2O]. A match between the calculated structure and solid‐state NMR was found by testing multiple semi‐local and dispersion‐corrected DFT functionals and applying them to optimize atom positions, starting from X‐ray diffraction (XRD)‐determined atomic coordinates. This was validated by comparing calculated to experimental 13C{1H} REDOR and 13C chemical shift anisotropy (CSA) tensor values. The results show that the combination of solid‐state NMR, XRD, and DFT can improve structure refinement for hydrated materials. [ABSTRACT FROM AUTHOR]

This paper provides strong evidence for the contribution of the phylum Firmicutes in mediating the primary precipitation of Mg-rich carbonates (hydromagnesite, dolomite, magnesite, and nesquehonite) in recent microbialites from a highly alkaline and ephemeral inland lake (Las Eras, Central Spain). The carbonate mineral precipitation occurs sequentially as the microbial mats decay. Scanning electron microscopy (SEM) provided solid proof that hydromagnesite nucleation is initiated on the exopolymeric substances (EPS) and the microbial cells associated to the microbial mat degradation areas. The progressive mineralization of the EPS and bacterial cells by hydromagnesite plate-like crystals on their surface, results in the entombment of the bacteria and formation of radiating aggregates of hydromagnesite crystals. The hydrous phases, mostly hydromagnesite, were produced at a high percentage in the first stages of the microbial degradation of organic matter. When the availability of organic substrates declines, the heterotrophs tend to reduce their number and metabolic activity, remain dormant. At this stage, the anhydrous phases, dolomite and magnesite, nucleate on bacterial nanoglobules and/or collapsed cells. Evidence for the sequential formation of the Mg-rich carbonates trough the decay of organic matter by a fermentative EPS-forming bacterium isolated from the microbialites, Desemzia incerta , is drawn through a comparative analysis of carbonate formation in both natural and experimental settings. This study will help to constrain potential mechanisms of carbonate formation in natural systems, which are of fundamental importance not only for understanding modern environments but also as a window into the geologic past of Earth and potentially Mars. [ABSTRACT FROM AUTHOR]

Abstract In this paper, a review is offered on lacustrine sites in Australia where both dolomite and magnesite are found to precipitate today (but not through diagenesis) and have been doing so during the Holocene. Information thus far obtained in the Coorong region of SE South Australia is provided and this follows with a discussion of the published literature from the volcanic plains of Western Victoria where modern dolomite and magnesite are also precipitating in saline playa lakes today. The work presented here clearly identifies that the volcanic rises (and topographic highs) in Western Victoria, such as Warrion Hill, play a vital role in groundwater recharge first, with subsequent flow paths towards the low plains in the region where saline playas are located. In the topographically-high recharge areas, the waters are predominantly rich in Ca and, as water progressively flows towards the playas, Ca is lost through calcite precipitation in scorias and soils, and is also presumably exchanged for K, with a considerable enrichment of the groundwaters with respect to Mg/Ca and K/Ca. This cationic exchange is thought to occur on clays in the subsurface, which are the by-product of weathering of the local Quaternary basalts. Eventually, the lake waters become supersaturated with respect to Mg-rich minerals and both dolomite and magnesite precipitate naturally in the lake waters to finally settle on the lake floors. It is more than likely that a brine pool occurs below each lake and it is from this that precipitation occurs once that pool waters reaches the lake surface. High levels of bicarbonate are found in the groundwaters principally as a result of carbonic acid supplied through rain and also from hydrolysis of silicate minerals that originally occurred in the basalts. The high concentrations of bicarbonate contribute to the precipitation of the carbonate minerals. Chemical analyses of the groundwaters and the lakes are tabled and help understand the chemical changes along the groundwater flow paths. In addition, examination of the δ18O and δ13C of the Western Victorian lake sediments, by analogy with those of Coorong lakes, indicates that although the carbonates in the Coorong lakes are precipitated as a result of sulphate bacterial reduction, this appears not to be the case for the Western Victorian lakes. Due to the high turbidity of the Western Victorian dolomite-precipitating lakes as well because of their very high salinities (often ≫50TDS), there are no macrophytes, and little if any algal growth. [ABSTRACT FROM AUTHOR]

Abstract: In this study, cotton fiber fabrics were coated with polyacrylate-based polymer paste containing huntite-hydromagnesite by knife over roll process. The flame retardancy properties of the cotton fabrics were tested according to vertical flame test, limit oxygen index (LOI) test, smoldering cigarette and match flame tests. Thermal behavior of cotton fabrics was investigated by DTA-TG analysis. The antibacterial activities of coated cotton fabrics were qualitatively determined according to AATCC Method 147. Mechanical properties of cotton fabrics were evaluated with the help of tear strength, abrasion strength, thickness and stiffness measurements. Air permeability, UV protection and color properties were also investigated. Free formaldehyde content of the coatings on cotton fabrics were determined according to Japanese Law 112. The microstructural properties of cotton fabrics were characterized using SEM and FTIR (ATR) analysis. Furthermore, the mineral content and mean particle size value of huntite-hydromagnesite were also determined with X-ray fluorescence spectrometer and particle size analyzer. The flame retardancy and antibacterial properties of the cotton fabrics were improved after coating with acrylate-based polymer paste containing huntite-hydromagnesite. Mechanical properties such as tear strength of the cotton fabric decreased after coating process however huntite-hydromagnesite addition into coating paste did not lead to further increase on the tear strength loss of cotton fabric. Despite this, the resultant maximum abrasion cycles significantly increased following coating process application. Furthermore, colorimetric properties such as whiteness and lightness of cotton fabric did not significantly change after applied coating processes. SEM and FTIR analyses confirmed the constancy of microstructure of cotton fibers after coating process.Graphical abstract: [ABSTRACT FROM AUTHOR]