Magnesium-rich Minerals Research Articles

The Role of Operating Conditions in the Precipitation of Magnesium Hydroxide Hexagonal Platelets Using NaOH Solutions.

Magnesium hydroxide, Mg(OH)2, is an inorganic compound extensively employed in several industrial sectors. Nowadays, it is mostly produced from magnesium-rich minerals. Nevertheless, magnesium-rich solutions, such as natural and industrial brines, could prove to be a great treasure. In this work, synthetic magnesium chloride and sodium hydroxide (NaOH) solutions were used to recover Mg(OH)2 by reactive crystallization. A detailed experimental campaign was conducted aiming at producing grown Mg(OH)2 hexagonal platelets. Experiments were carried out in a stirred tank crystallizer operated in single- and double-feed configurations. In the single-feed configuration, globular and nanoflakes primary particles were obtained, as always reported in the literature when NaOH is used as a precipitant. However, these products are not complying with flame-retardant applications that require large hexagonal Mg(OH)2 platelets. This work suggests an effective precipitation strategy to favor crystal growth while, at the same time, limiting the nucleation mechanism. The double-feed configuration allowed the synthesis of grown Mg(OH)2 hexagonal platelets. The influence of reactant flow rates, reactant concentrations, and reaction temperature was analyzed. Scanning electron microscopy (SEM) pictures were also taken to investigate the morphology of Mg(OH)2 crystals. The proposed precipitation strategy paves the road to satisfy flame-retardant market requirements.

Mineralogy of the deposit of albitized granites Kanarata, Sakar Мountain, Southеastern Bulgaria

The Kanarata albitized granite deposit is located on the NE periphery of the Sakar Upper Carboniferous granite batholith, immediately adjacent to its contact with the metamorphic framework. The porphyroid two-mica granites of the batholith are replaced by albite and quartz, and by magnesium-rich minerals in small amounts (some enriched in sodium): Mg-actinolite to Mg-hornblende and Mg-tremolite, phlogopite, chlorite, talc, saponite, as well as accessory rutile, probably titanite and apatite. The only relict mineral is zircon. The newly formed minerals are products of alkaline (magnesium-sodium) metasomatosis of possibly Alpine age. This has led to a change in the chemical composition of the granite, resulting in a decrease in the amount of silicon, iron, calcium, and especially potassium, and an increase in the amount of titanium, aluminum, magnesium, and especially sodium.

Preferential groundwater flow directions in a pseudokarst system in Colombia, South America

The correct identification of recharge zones and understanding of groundwater flow through fractured and karstic media are some of the main issues in hydrogeology and are of utmost importance for managing groundwater resources. The Aburra Valley is an excellent example of a promising aquifer system for future water supply in Colombia, with recharge zones and hydrogeological units (HGU) still not well understood. One of those HGUs, the Medellin Dunite, is an ultramafic igneous rock with significant secondary porosity due to the high grade of fracturing and the presence of a pseudokarst system formed by the dissolution of magnesium-rich minerals. In this study, we aimed for the first approximation of this unit's hydrogeologic characteristics by performing a hydro-structural analysis based on a systematic collection of structural data in rock outcrops and secondary information available on karstic landforms. In order to improve the understanding of the relations between the analyzed HGU and the other units in the Aburra Valley, the main objective of our investigation was to calculate the hydraulic weight of discontinuities and determine the associated preferential groundwater flow directions. The results showed that the predominant directions of groundwater flow are WNW and ENE, which imply that lateral flows from the Dunite not only recharge the alluvial aquifers of the Aburra Valley Aquifer System (AVAS) but could also be recharging the San Nicolas Valley Aquifer System (SNVAS), another region with high rates of population growth. However, detailed studies are needed to establish the connections of the Dunite with the Medellin Amphibolite and other hard rocks of the SNVAS. Our findings show the importance of studying the Medellin Dunite and implementing management and protection measures for this aquifer unit.

Analysis of Some Heavy Metals and Organic Acids in Ficus carica Growing Adjacent in the Serpentine Soil in Sulaimani/Kurdistan, Iraq

Serpentine soil refers to soil having magnesium-rich minerals such as chrysotile, lizardite, and antigorite. Fig (Ficus carica) is a plant of major importance in the world. It is a nourishing food and is used in industrial productions. This study analyzes the distribution of heavy metal contents such as nickel, cobalt, zinc, and manganese in different parts of this plant, such as its leaves, stems, and fruits. Furthermore, the organic acid content, including citric acid, fumaric acid, malic acid, and oxalic acid, was estimated. The studied area is known as Kunjirin which is a village located in the northwestern extension of the Zagros-fold-thrust belt in Mawat town, northeast of Sulaimani Province, Kurdistan Region of Iraq. The results show that there are significant differences in heavy metal contents among the plant organs except for zinc. The lowest level of heavy metals is in the fruits, while the highest amount of the heavy metal content is in the leaves. Moreover, the organic acids were unequally distributed in the plant organs. However, the lowest level of organic acids are found in the stem, while the highest concentration of the organic acids are found in the fruit part of the plant. The present study concludes that the level of heavy metals in the fruit part is within the legally admitted limits. This indicates that it is normal for human beings and animals to consume such fruits.

CO2 sequestration by wollastonite-amended agricultural soils – An Ontario field study

Terrestrial enhanced weathering is the spreading of silicate powder on land, and can sequester atmospheric CO2 through carbonation of calcium- and magnesium-rich minerals. When applied to soils, at suitable geochemical conditions, alkaline minerals lead to accumulation of soil inorganic carbon as pedogenic carbonates. Agricultural land covers 37 % of the Earth’s land surface, thus offering a natural sink for atmospheric CO2. Wollastonite (CaSiO3) has been commercialized in Ontario as an agricultural soil amendment for several years, but it is not known if or how much CO2 is sequestered annually. In the present study, wollastonite-amended soils were collected from three commercial-scale fields located in Ontario: a leafy vegetables farm in the town of Paris (Field 1); a potato farm in the settlement of Alliston (Field 2); and a soybean farm in the city of Woodstock (Field 3). Chemical and mineralogical assessments of these soils were conducted to determine their carbonate content and the fate of the wollastonite. The soil inorganic carbon (SIC) content was higher in all soils that had been amended with wollastonite, compared to controls, and in Field 1 the SIC content was 2.6 times higher in soil amended thrice with wollastonite compared to a single application. Mineralogical analysis showed the presence of additional polymorphs of SiO2, which may originate from the wollastonite after the dissolution of calcium. This work provides further evidence, here for the first time at a regional scale, that soil amendment with alkaline silicates can be an effective geoengineering tool for climate change mitigation.

The discovery of Lake Hephaestus, the youngest athalassohaline deep-sea formation on Earth

Hydrated, magnesium-rich minerals and subglacial brines exist on the martian surface, so the habitability of high-Mg2+ environments on Earth has extraterrestrial (as well as terrestrial) implications. Here, we report the discovery of a MgCl2-dominated (4.72 M) brine lake on the floor of the Mediterranean Ridge that underlies a 3500-m water column, and name it Lake Hephaestus. Stable isotope analyses indicated that the Hephaestus brine is derived from interactions between ancient bishofite-enriched evaporites and subsurface fluids. Analyses of sediment pore waters indicated that the Hephaestus depression had contained the MgCl2 brine for a remarkably short period; only 700 years. Lake Hephaestus is, therefore, the youngest among currently known submarine athalassohaline brine lakes on Earth. Due to its biologically hostile properties (low water-activity and extreme chaotropicity), the Hephaestus brine is devoid of life. By contrast, the seawater-Hephaestus brine interface has been shown to act as refuge for extremely halophilic and magnesium-adapted stratified communities of microbes, even at MgCl2 concentrations that approach the water-activity limit for life (0.653).

Atmospheric Carbon Mineralization in an Industrial-Scale Chrysotile Mining Waste Pile.

Magnesium-rich minerals that are abundant in ultramafic mining waste have the potential to be used as a safe and permanent sequestration solution for carbon dioxide (CO2). Our understanding of thermo-hydro-chemical regimes that govern this reaction at an industrial scale, however, has remained an important challenge to its widespread implementation. Through a year-long monitoring experiment performed at a 110 Mt chrysotile waste pile, we have documented the existence of two distinct thermo-hydro-chemical regimes that control the ingress of CO2 and the subsequent mineral carbonation of the waste. The experimental results are supported by a coupled free-air/porous media numerical flow and transport model that provides insights into optimization strategies to increase the efficiency of mineral sequestration at an industrial scale. Although functioning passively under less-than-optimal conditions compared to laboratory-scale experiments, the 110 Mt Thetford Mines pile is nevertheless estimated to be sequestering up to 100 tonnes of CO2 per year, with a potential total carbon capture capacity under optimal conditions of 3 Mt. Annually, more than 100 Mt of ultramafic mine waste suitable for mineral carbonation is generated by the global mining industry. Our results show that this waste material could become a safe and permanent carbon sink for diffuse sources of CO2.

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.

Magnesium-rich gangue dissolution in column bioleaching of chalcopyrite

This study is to assess the effect of magnesium-rich gangue dissolution on the ore containing chalcopyrite and carrollite from Democratic Republic of the Congo (DRC). The ore contains 11.0 wt% magnesium and 2.4 wt% copper. When Mg2+ concentration is over 20 g·L−1 after 2 months in column bioleaching, the redox potential of leachate keeps stable, much less than that when Mg2+ concentration is below 20 g·L−1. Meanwhile, the extractions of Cu2+ and Co2+ from chalcopyrite with carrollite obviously inhibited by Mg2+ concentration of over 20 g·L−1 were investigated. The extraction of cobalt depends on chalcopyrite dissolved in column reactor. The growth and oxidation activity of bacteria are affected by high Mg2+ concentration (>20 g·L−1) in leachate of column reactor. As the rate of gangue dissolution depends on the amount of sulfuric acid, the decrease of the particle size relates to the increase of the total particle surface area and acid consumption. Therefore, the operation of pre-leaching for magnesium removal is not the most effective and economical way to solve the problem of magnesium-rich gangue minerals dissolution.

A geobotanical investigation of the Koedoesfontein Complex, Vredefort Dome, South Africa

The Vredefort Dome (impact structure) in South-Africa hosts several unique peralkaline to ultramafic intrusive bodies that were emplaced into its core and collar. Because of the broad difference in mineral composition, between the intrusions as well as the abundant siliciclastic country rocks, abrupt transitions in soil chemical characteristics are produced that can ultimately affect the floristic and physiognomic characteristics of the associated vegetation in the dome. Despite various efforts to study the ultramafic-adapted vegetation of southern Africa, little is known about the effect these particular intrusions have on the soil and floristical components of the dome. The aim of the present study was to investigate the geobotany of one of the ultramafic-peralkaline complexes in the dome, namely the Koedoesfontein Complex. The complex hosts several different intrusions that collectively form a mineralogical gradient ranging from mafic- to felsic-dominant silicates. Three primary components (geology, soil and woody vegetation) were sampled along this gradient and analysed to determine the effect of these intrusions on the residual soil chemistry, and the floristics and physiognomy of the supported vegetation type. Weathering of the peralkaline to ultramafic lithologies greatly affected the soil’s chemical properties and related species composition and physiognomy. Major chemical variation in the soil, including the total chromium and nickel content, is correlated with the difference in abundance of sodium-, calcium-, iron- and magnesium-rich silicate minerals. Significant variations in woody species composition (particularly across Senegalia and Vachellia genera) have been established between soils originating from ultramafic–mafic, intermediate and felsic rock types and can be utilised as indicators for different geological substrates within the dome.

Extended vegetation histories from ultramafic karst depressions

Solutional landforms (karst) can form on old surfaces on ultramafic rocks in the tropics because of the solubility of some magnesium-rich minerals under warmth and high CO2. The radiocarbon dating of organic pond deposits in several tropical ultramafic karst hollows demonstrates that very slow sediment accumulation has occurred, relative to other tropical shallow lakes. Some sites have gaps in their records, whereas others appear continuous. Sections of organic lake muds from the Indonesian sites Wanda and Hordorli provide sequences of ages from modern back to >35 000 years ago at depths of 3–4 m. In New Caledonia, no Holocene record has been obtained, and dates of 17 000–30 000 years ago are found near the top of deep organic layers that, in some cases, are buried by inorganic muds derived from an erosion event. These ages, pollen analyses and the increasingly compressed organic sediments with depth mean that deeper levels should be well beyond radiocarbon dating limits. Only at one New Caledonian lake were deeper sediments beyond detectable 14C measurement. Other sites returned finite dates at all levels tested, suggesting that some mechanism is moving small amounts of younger organics down profile. The slow sediment-accumulation rates provide an explanation why high concentrations of pollen relative to tropical peats and limnic sediments derived from high nutrition substrates are preserved in the sediments. This makes them attractive targets for studying the palaeoecology and forest stability of the surrounding vegetation. The sites are sensitive to disturbance because the poor nutrition impedes successional recovery after disturbances such as fire and landslips.

Internal 26Al–26Mg isotope systematics of a Type B CAI: Remelting of refractory precursor solids

High precision SIMS 26Al–26Mg isotope analyses were performed for a pristine Type B1 CAI Leoville 3535-1 on multiple mineral phases that include aluminum-rich zoned melilite mantle (Åk20–Åk60), magnesium-rich melilite (Åk70), fassaite, spinel and anorthite in the core. The data yield a well-defined internal isochron with an inferred initial 26Al/27Al ratio of (5.002±0.065)×10−5, which is lower than those of bulk CAIs and pristine fine-grained CAIs. Assuming homogeneous distribution of 26Al in the early solar system, Leoville 3535-1 formed ∼50ka after the time corresponding to the bulk CAI isochron. One anorthite analysis near the grain boundary adjacent to melilite shows sub-μm-scale heterogeneous magnesium distribution, though the 26Al–26Mg data plot on the isochron regression. Thus, the internal 26Al–26Mg system of the CAI remained closed since the last melting event that crystallized anorthite.High precision magnesium isotope analyses of magnesium-rich minerals (fassaite, magnesium-rich melilite, and spinel) show a small amount of scatter from the regression line (∼0.1‰) beyond analytical uncertainties. Most spinel and some fassaite data are systematically displaced below and above the regression line, respectively. It is likely that spinel remained unmelted at the time of the last melting event, while fassaite and anorthite crystallized from a partial melt. The regression line made by fassaite and anorthite data shows an initial 26Al/27Al ratio indistinguishable from that using all data. Mass dependent fractionation of magnesium isotopes in spinel is the same as those in melilite and fassaite (δ25Mg∼5‰), indicating that magnesium isotope fractionation, presumably from evaporative magnesium loss, predated the last melting event. Thus a majority of the CAI appears to have escaped magnesium isotope exchange with nebular gas during the last melting event, except for the outer rim of the melilite mantle that shows slightly lower δ25Mg values.The 26Al–26Mg systematics of this Leoville Type B CAI suggest it experienced a final melting event at ∼50ka after the initial Al–Mg fractionation event. This may correspond to melting during the active protostar stage (known as “Class I”), during which sporadic high temperature heating events repeatedly affected refractory solids in the solar nebula.

Activation of magnesium rich minerals as carbonation feedstock materials for CO 2 sequestration

Mineral carbonation, the reaction of magnesium-rich minerals such as olivine and serpentine with CO 2 to form stable mineral carbonates, is a novel and promising approach to carbon sequestration. However, the preparation of the minerals prior to carbonation can be energy intensive, where some current studies have been exploring extensive pulverization of the minerals below 37 μm, heat treatment of minerals up to 650 °C, prior separation of CO 2 from flue gases, and carbonation at high pressures, temperatures and long reaction times of up to 125 atm, 185 °C and 6 h, respectively. Thus, the objective of the mineral activation concept is to promote and accelerate carbonation reaction rates and efficiencies through surface activation to the extent that such rigorous reaction conditions were not required. The physical activations were performed with air and steam, while chemical activations were performed with a suite of acids and bases. The parent serpentine, activated serpentines, and carbonation products were characterized to determine their surface properties and assess their potential as carbonation minerals. The results indicate that the surface area of the raw serpentine, which is approximately 8 m 2/g, can be increased through physical and chemical activation methods to over 330 m 2/g. The chemical activations were more effective than the physical activations at increasing the surface area, with the 650 °C steam activated serpentine presenting a surface area of only 17 m 2/g. Sulfuric acid was the most effective acid used during the chemical activations, resulting in surface areas greater than 330 m 2/g. Several of the samples produced underwent varying degrees of carbonation. The steam activated serpentine underwent a 60% conversion to magnesite at 155 °C and 126 atm in 1 h, while the parent sample only exhibited a 7% conversion. The most promising results came from the carbonation of the extracted Mg(OH) 2 solution, where, based on the amount of magnesium recovered in the precipitate after the 3.5 h reaction, the carbonation efficiency was estimated to be at least 53%. Reaction products included several hydrated magnesium carbonate compounds. The carbonation reaction was conducted at ambient temperature, 20 °C, and low pressure, 45 atm.