Fe-rich Minerals Research Articles

A Review of the Mineral Chemistry and Crystallization Conditions of Ediacaran–Cambrian A-Type Granites in the Central Subprovince of the Borborema Province, Northeastern Brazil

Ediacaran–Cambrian magmatism in the Central Subprovince (Borborema Province, NE Brazil) generated abundant A-type granites. This study reviews published whole-rock and mineral chemistry data from thirteen Ediacaran–Cambrian A-type intrusions and a related dike swarm. It also presents new mineral chemistry and whole-rock data for one of these intrusions, along with zircon trace element data for five of the intrusions. Geochronological data from the literature indicate the formation of these A-type intrusions during a 55 Myr interval (580–525 Ma), succeeding the post-collisional high-K magmatism in the region at c. 590–580 Ma. The studied plutons intruded Paleoproterozoic basement gneisses or Neoproterozoic supracrustal rocks. They are ferroan, metaluminous to peraluminous and mostly alkalic–calcic. The crystallization parameters show pressure estimates mainly from 4 to 7 kbar, corresponding to crustal depths of 12 to 21 km, and temperatures ranging from 1160 to 650 °C in granitoids containing mafic enclaves, and from 990 to 680 °C in those lacking or containing only rare mafic enclaves. The presence of Fe-rich mineral assemblages including ilmenite indicates that the A-type granites crystallized under low ƒO2 conditions. Zircon trace element analyses suggest post-magmatic hydrothermal processes, interpreted to be associated with shear zone reactivation. Whole-rock geochemical characteristics, the chemistry of the Fe-rich mafic mineral assemblages, and zircon trace elements in the studied granitoids share important similarities with A2-type granites worldwide.

Late-stage microstructures in Chang’E-5 basalt and implications for the evolution of lunar ferrobasalt

Abstract This study investigates silicate liquid immiscibility (SLI) microstructures in the Chang’E-5 (CE-5) lunar ferrobasalt sample, the youngest recovered mare basalt (ca. ~2.0 Ga). Employing advanced high-resolution imaging techniques and chemical analysis, we examined a subophitic fragment, revealing two distinct types of microstructures indicative of multi-stage SLI events. The first type is observed in the mesostasis pockets and exhibits both “sieve” and “maze” textures, where the Si-K rich glassy phases are interconnected with Fe-rich minerals, e.g., fayalite. This type of microstructure, similar to previous observations in Apollo and Luna samples, is the product of a stable SLI event. The second type is characterized by K-free but high-Si melt inclusions occurring as emulsions in the rims of plagioclase. The entrapment of these emulsions followed a metastable SLI event, with the Fe-rich liquids serving as precursors to subsequent stable SLI processes. Additionally, the Fe-rich droplets within the emulsions underwent coarsening via Ostwald ripening, a phenomenon in which smaller particles in solution dissolve and deposit on larger particles. Our simulation of this coarsening process suggests a duration of at least 15-32 days for the SLI processes, alongside a slow cooling rate (< 0.3 °C/hour) of the late-stage CE-5 lava. We propose that metastable SLI may have influenced the effusive signature of the CE-5 lava flow during its late-stage evolution. The metastable SLI process can potentially lead to the formation of various phases during the late-stage evolution of lunar ferrobasaltic magmas, thereby contributing to the diversity of lunar rock types.

Amphibole fractionation as a key driver for oxidation of magmas in convergent margins

During the process of differentiation, the magmas in convergent margins undergo an increase of oxidized nature, accompanied by a decreased Fe content and concentration of heavy Fe isotopes. Garnet and amphibole are both Fe-rich minerals, which can be responsible for this phenomenon through fractional crystallization. One prevailing hypothesis suggests that Fe2+-rich garnet cumulates in the arc root as a "crustal redox filter." However, the stability of garnets is highly dependent on pressure conditions. In contrast, amphibole can crystallize under a broader range of temperature and pressure conditions and is a more common mineral phase in magmas. As such, the contribution of amphibole might have been underappreciated. Here, we conducted elemental composition, zircon trace element, and high-precision Fe isotope analyses on Miocene magmatic rocks from the Gangdese arc to trace the evolution of magmatic oxidation. The results indicate that the enrichment of heavy Fe isotopes in these magmas is primarily controlled by amphibole-dominated fractional crystallization rather than garnet. This also implies that amphibole fractional crystallization may play a role in enhancing the oxygen fugacity of the magmas. Taking a global perspective, we found a pervasive correlation between amphibole fractional crystallization and Fe isotope fractionation in magmatism at convergent plate margins, indicating its widely applicable influence on oxidation. The influence of garnet cannot be entirely neglected in some specific scenarios, such as within thickened continental arcs, but its impact is generally limited. Continuous amphibole fractional crystallization increases oxidation, facilitating the mobilization and concentration of Cu within the magma, thereby enhancing the potential for porphyry deposit formation. This impact is especially notable in spatiotemporally related magmatic events and could be decisive in determining the magmatic mineralization potential.

Melting and Rapid Solidification of Lunar Regolith Particles Returned by Chang'E-5 Mission.

Melting and solidification of lunar regolith are pivotal for comprehending the evolutionary dynamics of lunar volcanism, geology, and impact history. Additionally, insights gained from these processes can contribute to the advancement of insitu resource utilization technologies, for instance additive manufacturing and resource extraction systems. Herein, we conduct the direct observation of the melting and rapid solidification of lunar particles returned by the Chang'E 5 mission. The melting temperature and melting sequence were obtained. Bubble generation, growth, and release were clearly observed, with a maximum bubble diameter of 5 µm, which is supposed to be according to the release of volatiles that embedded in the particles. During the solidification process, evident crystallization occurred with incremental crystal growth rate approximately of 27 nm/s. Scanning electron microscopy and energy-dispersive x-ray spectroscopy results verified that the Fe-rich mineral crystalizes first. These results would improve the understanding of the evolution of lunar volcanism, geology, and impact history.

Depositional Architecture and Reservoir Quality Assessment of Abu Roash (G) Dolomite, as a Low Resistivity Pay, Razzak Oilfield, Western Desert, Egypt.

Abu-Roash (G) dolomite (A/R-G) is considered one of the most prolific oil-bearing reservoirs in the Western desert. In this study, Razzak Oilfield was selected to investigate the geological and petrophysical findings of these successions that were assigned stratigraphically to the upper Cretaceous (Late Cenomanian), in the Alamein basin. Sedimentologically, three main facies groups have been recognized in the studied successions of A/R-G and include dolostone, limestone and mudstone facies; and depositional; are divided into three parts: A) Back-reef to reef crest (Reefal depositional setting) - dolostone facies group; graded to B) Inner-ramp of limestone / dolomitic limestone facies types; then C) mud-flat (intertidal setting) of calcareous mudstone facies type. Petrophysically, most of the recorded pore sizes in the dolomite zone are mesopores (≈1 micron), with scattered macropores (2-10 microns), as identified by MICP on some selected 1.5” core plugs. In general; A/R-G dolomite displays low resistivity pay (high conductivity) due to: A) The heterogeneity of moldic and vuggy pore size distributions (macropores); associated with the dominant intercrystalline-dolomite mesopores; and B) Fe-rich minerals that increase the conductivity; where most of the identified dolomite crystals are secondary, late-diagenetic stage ferroan dolomite; and confirmed that petrographically by thin-section analysis.

Nitrogen-Rich Organic Matter Formation and Stabilization in Iron Ore Tailings: A Submicrometer Investigation.

Organic matter (OM) formation and stabilization are critical processes in the eco-engineered pedogenesis of Fe ore tailings, but the underlying mechanisms are unclear. The present 12 month microcosm study has adopted nanoscale secondary ion mass spectrometry (NanoSIMS) and synchrotron-based scanning transmission X-ray microscopy (STXM) techniques to investigate OM formation, molecular signature, and stabilization in tailings at micro- and nanometer scales. In this system, microbial processing of exogenous isotopically labeled OM demonstrated that 13C labeled glucose and 13C/15N labeled plant biomass were decomposed, regenerated, and associated with Fe-rich minerals in a heterogeneous pattern in tailings. Particularly, when tailings were amended with plant biomass, the 15N-rich microbially derived OM was generated and bound to minerals to form an internal organo-mineral association, facilitating further OM stabilization. The organo-mineral associations were primarily underpinned by interactions of carboxyl, amide, aromatic, and/or aliphatic groups with weathered mineral products derived from biotite-like minerals in fresh tailings (i.e., with Fe2+ and Fe3+) or with Fe3+ oxyhydroxides in aged tailings. The study revealed microbial OM generation and subsequent organo-mineral association in Fe ore tailings at the submicrometer scale during early stages of eco-engineered pedogenesis, providing a basis for the development of microbial based technologies toward tailings' ecological rehabilitation.

Iron Isotope Systematics of the Skaergaard Intrusion and Implications for its Liquid Line of Descent

Abstract The Skaergaard intrusion is one of the most thoroughly studied layered mafic intrusions on Earth and an exceptional example of (near) closed-system magmatic differentiation. We report new Fe isotope data for whole rocks, and magnetite and ilmenite separates through the layered series (LS) and upper border series (UBS) of the intrusion. δ56Fe values for gabbroic rocks range from 0.033‰ to 0.151‰ with an abrupt step increase at the base of Lower Zone c (LZc) within LS with the appearance of cumulus magnetite and subsequent decline accompanying FeTi oxide fractionation. The lowest δ56Fe values are found near the Upper Zone b (UZb) to c (UZc) boundary followed by a sharp rise across UZc approaching the Sandwich Horizon. Magnetite–ilmenite separates straddle bulk rock compositions with fractionation factors (Δ56Femt-ilm) of 0.081‰ to 0.239‰, consistent with subsolidus equilibration. Granophyric rocks occurring as pods, sheets and wispy layers from the upper zone and UBS equivalents and having unradiogenic Sr similar to gabbroic rocks of Skaergaard, are isotopically heavier than their host ferrodiorites (Δ56Fegranophyre-ferrodiorite ≥ 0.1‰) reaching a maximum δ56Fe of 0.217‰ in UBS. A fused xenolith from UBS has δ56Fe = 0.372‰. This range in δ56Fe spans much of that reported for terrestrial igneous rocks, and like the global dataset, shows a pronounced increase in δ56Fe with inferred silica content of modeled Skaergaard liquids. Forward modeling of closed system fractional solidification was undertaken to account for Fe isotope systematics, first by testing published liquid lines of descent (LLD), and then by exploring improvements and considering the impacts of liquid immiscibility, crustal contamination, fluid exsolution and diffusional processes. Our modeling relies on published Fe+2 and Fe+3 force constants for magmatic minerals and silicate glasses, and the most reliable estimates of the average bulk composition and mass proportions of the well-defined subzones of the intrusion. We show that the increase in δ56Fe across the LZb–LZc boundary is readily explained by the increased incorporation of Fe+3 into the crystallizing solid including magnetite. We further demonstrate that the classic Fenner LLD, involving strong Fe enrichment at nearly constant silica, does not lead to a rise in δ56Fe toward the end stages of evolution, while a Bowen-like LLD, with little Fe enrichment and strong Si enrichment, also underestimates enrichment in heavy Fe isotopes in the ferrodiorites of UZc. A LLD following an intermediate path involving modest Fe and Si enrichment, followed by Fe depletion best explains the observations. We predict ~3.5% (by mass) residual liquid after crystallization of UZc having a composition similar to felsic segregations in pegmatitic bodies found in the intrusion. While liquid immiscibility may have been encountered within fractionating mush at the margins of the intrusion, the Fe isotope systematics do not support liquid phase separation of the bulk magma. Crustal contamination, fluid exsolution, hydrothermal alteration and thermal diffusion are also shown to have no resolvable effect on the Fe isotope composition of the gabbroic and granophyric rocks. We conclude that the Fe isotope systematics documented in the Skaergaard intrusion reflect the dominant role of fractionating Fe-rich minerals from gabbroic through ferrodioritic to rhyolitic liquids. The success of our model to account for the observed Fe isotope systematics for Skaergaard demonstrates the utility of Fe+2 and Fe+3 force constants determined at ambient conditions to model magmatic conditions and gives critical insights into plutonic processes fractionating Fe isotopes complementary to the volcanic record.

Tourmaline chemistry, boron, and strontium isotope systematics trace multiple melt–fluid–rock interaction stages in deeply subducted continental crust

The generation, transport, and recrystallization of slab-derived melts/fluids play a critical role in the deep recycling of elements in subduction zones. While boron (B) isotope systematics have been invoked as an important tracer of these processes, its behavior during metamorphic dehydration and partial melting of deeply subducted continental slabs, and the partitioning of B isotopes between minerals and melts/fluids is not fully understood. Here, we investigate these processes through an in situ study of the major, trace-element, and B-Sr isotope variations in different occurrences of tourmaline in migmatite from the Yuka terrane, North Qaidam orogen (China), which resulted from partial melting of a continental slab at different stages during subduction and exhumation. Based on textural and detailed high-resolution X-ray mapping studies, tourmaline was classified into four paragenetic generations (Tur-I, Tur-II, Tur-III, and Tur-IV). Dravitic Tur-I occurs in melanosomes and shows increasing Fe, Ca, and Ti contents from the core to the outer rim. In addition, it has relatively homogeneous Sr isotope values (0.7407–0.7416) and decreasing δ11B values (-3.8 to −8.6 ‰) and XMg (Mg/(Mg + Fe)) ratios, indicating formation in a rock buffered by an aqueous fluid during the prograde to peak metamorphism. Schorlitic Tur-II occurs within selvage zones between melanosomes and leucosomes, and yields high-Fe values and low δ11B (-13.5 to −10.9 ‰) and more variable 87Sr/86Sr (0.7343–0.7418) values, indicating crystallization in the presence of a hydrous melt external derived from breakdown of Fe-rich mineral(s) during partial melting of the subducted slab. Dravitic Tur-III formed in the matrix and also enveloped Tur-II. It shows homogeneous 87Sr/86Sr values (0.7411–0.7420) and decreasing δ11B values (-6.8 to −9.9 ‰) and XMg ratios as well as increasing Fe and Ti contents from core to outer rim. Formation of Tur-III reflects a transitional stage from hydrous melt to aqueous fluid during exhumation. Tur-IV in the leucosomes is essentially a schorl-dravite solid solution with small amounts of Ca. Its 87Sr/86Sr values (0.7402–0.7416) and δ11B values (-11.4 to −8.5 ‰) are intermediate between the respective values of Tur-II and Tur-III. The formation of Tur-IV likely results from interaction between melt and fluid and, based on its chronological sequence, and is interpreted to have formed during the exhumation stage of the Yuka terrane.Overall, the variable XMg ratios and δ11B values in the different generations of tourmalines are a consequence of the evolution of the melt/fluid at different depths within the deeply subducted slab. Decreasing δ11B values from Tur-I to Tur-II and Tur-III are controlled by the breakdown of different minerals during partial melting or metamorphic dehydration of the subducted slab, while the co-variations of the elemental geochemistry and B isotopic compositions of tourmaline reflect different depths of formation during subduction and exhumation of the lithosphere. These observations suggest tourmaline may serve as a useful tracer of multiple melt/fluid–rock interactions and of boron cycling in continental subduction zones. The heterogeneity of δ11B in melts/fluids at different depth levels of the continental subducted slab may also result in locally variable B isotope values in syn- and post-collisional magmas.

Gallium isotope constraints on the intense weathering of basalt

Gallium (Ga) isotopes are potentially advantageous for characterizing the surficial biogeochemical cycles of Ga and tracking the geochemical behavior of the monoisotope element aluminum (Al) during chemical weathering. To test the potential of Ga and its isotopes to trace continental weathering, we studied Ga isotope compositions of a strongly-weathered latosol profile developed from basalt on the Leizhou Peninsula, Guangdong Province, South China. In the profile, δ71GaSRM-994 values of latosol samples decrease from ∼0.69‰ in unweathered basalts at the bottom to ∼0.51‰ in shallow sections, while the Ga concentration increases from ∼19.9 µg/g in the bedrock to ∼42.7 µg/g in soil, indicating significant enrichment of Ga and its isotope fractionation triggered by weathering processes. Bulk samples analysis and sequential leaching of selected weathering products suggest that the majority of Ga in these samples occurs in the crystal lattice of Al-rich and Fe-rich minerals, with the light Ga isotope (69Ga) enriched in latosol samples relative to the coexisting solution. Interestingly, our results show δ71Ga values of latosols display a bigger variation (about 3–4 times of its analytical uncertainty) than their Ga/Al ratios (about 1.5 times of its analytical uncertainty) throughout the profile, indicating δ71Ga values may be a more sensitive proxy to track the loss of Al3+ and involvement of Fe3+ during weathering process. Based on a simple Rayleigh model, the upper limit of Ga isotope fractionation between solution and weathered basalt (Δ71Gasolution-weathered rocks) is estimated to be ∼1.50‰, implying that heavy Ga isotopes may be enriched in surface river systems. This study highlights the potential of Ga elemental and isotope geochemistry to trace continental weathering and global Ga cycling.

The role of Fe dissolution in olivine-hydroxylamine-induced Fenton reaction for enhanced oxidative degradation of organic pollutant

In this study, a dye pollutant (methyl orange, MO) was effectively oxidized in a hydroxylamine (HA)-assisted Fenton system using various Al/Si/Fe- and Fe-containing minerals. The fastest degradation kinetics of MO were observed in the olivine-HA Fenton system, whereas other Al/Si/Fe and Fe-rich minerals (magnetite and lepidocrocite) demonstrated much slower degradation kinetics. The degradation rate constants were proportional to dissolved Fe(II) quantities in mineral suspensions (R2 = 0.98), indicating the crucial role of dissolved Fe(II) quantity in HA-assisted Fenton reactions. Radical scavenging and electron spin resonance results revealed that MO was dominantly oxidized by ·HO produced in the olivine-HA Fenton system. The continuous production of aqueous Fe(II) via direct Fe(II) dissolution at a pH of 3 and further Fe dissolution from the reductive dissolution of surface Fe(III) by HA was the main driving force for efficient MO degradation. Furthermore, lowering the pH by the addition of hydroxylamine hydrochloride resulted in the effective removal of MO under various pH conditions (3–9), indicating the additional advantage of HA use in Fenton reactions. Liquid chromatography-mass spectroscopy analysis revealed that the cleavage of C–N and C–C bonds, demethylation, hydroxylation, and dehydroxylation were the main processes for MO oxidation in the olivine-HA Fenton system.

Copper mobilization via seawater-volcanic rock interactions: New experimental constraints for the formation of the iron oxide Cu-Au (IOCG) mineralization

The involvement of modified seawater and/or basinal brine in IOCG mineralization was documented by numerous geologic studies. Still, this process remains poorly understood. In this study, we reacted seawater with andesite and basalt, the most common arc volcanic rocks, at 150 and 240 °C and water vapor-saturated pressure to assess the Cu leaching capacity and fluid evolution due to the seawater-volcanic rock interactions. The experimental results show that Mg, Fe, Al, and SO42− in the modified seawater decreased significantly, accompanied by an increased Ca concentration. The fluid pH also decreased from 8.3 of the initial seawater to ∼7.5 and ∼7.0 after 150 and 240 °C experiments, respectively. The incongruent calcsilicate (e.g., plagioclase) dissolution and anhydrite precipitation were observed in the reacted rock samples. Thermodynamic modeling confirmed the trends of those concentration changes but predicted several alteration minerals that were not observed in the experiments. Our experiments show that Cu-leaching efficiency is much higher from the andesite than from the basalt, also higher at 240 °C than at 150 °C. The andesite contains more Cu than the basalt samples, and abundant Cu-rich (average 106 ppm) plagioclase-hosted silicate melt inclusions and Cu-bearing (average 59 ppm) groundmass are identified in the original andesite sample but not in the original basalt sample, which is considered to be the cause of the Cu-leaching differences between the two rock samples. The δ34S value of resultant fluids is dominated by seawater sulfate with a minor contribution of magmatic sulfur from the rock. Thermodynamic modeling combined with recent experimental studies show that deposition of Cu sulfides is the most effective when Cu-bearing fluids react with early-stage pyrite. Our results suggest that the seawater/(brine)-volcanic rock reactions at 150–250 °C can produce Cu-bearing fluids, which in turn can form IOCG deposits, and that Cu mineralization with sulfides zonation can result from replacement reactions between the Cu-bearing fluids and Fe-rich minerals in precursor iron ore bodies.

Bioaugmentation with Acidithiobacillus species accelerates mineral weathering and formation of secondary mineral cements for hardpan development in sulfidic Pb-Zn tailings

The development of hardpan caps has great potential in rehabilitating sulfidic and metallic tailings, which may be accelerated by using exogenous Acidithiobacillus species. The present study aims to establish a bioaugmentation process with exogenous Acidithiobacillus species for accelerating the weathering of sulfidic minerals and formation of secondary mineral gels as precursors for hardpan structure development in a microcosm experiment. Exogenous Acidithiobacillus thiooxidans (ATCC 19377) and A. ferrooxidans (DSM 14882) were inoculated in a sulfidic Pb-Zn tailing containing negligible indigenous Acidithiobacillus species for accelerating the weathering of pyrite and metal sulfides. Microspectroscopic analysis revealed that the weathering of pyrite and biotite-like minerals was rapidly accelerated by exogenous Acidithiobacillus species, leading to the formation of secondary jarosite-like mineral gels and cemented profile in the tailings. Meanwhile, approximately 28% Zn liberated from Zn-rich minerals undergoing weathering was observed to be re-immobilized by Fe-rich secondary minerals such as jarosite-like mineral. Moreover, Pb-bearing minerals mostly remained undissolved, but approximately 30% Pb was immobilized by secondary Fe-rich minerals. The present findings revealed the critical role of exogenous Acidithiobacillus species in accelerating the precursory process of mineral weathering and secondary mineral formation for hardpan structure development in sulfidic Pb-Zn tailings.

Enzymes, Manganese, or Iron? Drivers of Oxidative Organic Matter Decomposition in Soils.

Oxidative decomposition of soil organic matter determines the proportion of carbon that is either stored or emitted to the atmosphere as CO2. Full conversion of organic matter to CO2 requires oxidative mechanisms that depolymerize complex molecules into smaller, soluble monomers that can be respired by microbes. Current models attribute oxidative depolymerization largely to the activity of extracellular enzymes. Here we show that reactive manganese (Mn) and iron (Fe) intermediates, rather than other measured soil characteristics, best predict oxidative activity in temperate forest soils. Combining bioassays, spectroscopy, and wet-chemical analysis, we found that oxidative activity in surface litters was most significantly correlated to the abundance of reactive Mn(III) species. In contrast, oxidative activity in underlying mineral soils was most significantly correlated to the abundance of reactive Fe(II/III) species. Positive controls showed that both Mn(III) and Fe(II/III) species are equally potent in generating oxidative activity, but imply conventional bioassays have a systematic bias toward Fe. Combined, our results highlight the coupled biotic-abiotic nature of oxidative mechanisms, with Mn-mediated oxidation dominating within Mn-rich organic soils and Fe-mediated oxidation dominating Fe-rich mineral soils. These findings suggest microbes rely on different oxidative strategies depending on the relative availability of Fe and Mn in a given soil environment.

Variations of diatom opal Ge/Si in Prydz Bay, East Antarctica

Diatom frustules record the variations of seawater Ge/Si, a ratio related to continental weathering, early diagenesis, and biological productivity. To investigate the geochemistry of germanium and the possibility of using opal Ge/Si as a proxy of changes in Ge and Si budgets in high latitude Antarctic marginal sea, nine surface sediment samples and one sediment core were collected from Prydz Bay during Chinese Antarctic Expeditions from 2005 to 2009. Diatom opal was separated and purified from sediment samples, and opal Ge/Si was determined. The results showed that (Ge/Si)opal ranged from 0.48 to 0.78 μmol mol−1 (0.60 ± 0.09 μmol mol−1) in Prydz Bay. (Ge/Si)opal from sites in the open sea and at the edge of the Amery Ice Shelf are comparable to modern oceanic seawater, while it is relatively low on the continental shelf. The variations in (Ge/Si)opal were not significantly influenced by biological fractionation during diatom uptake, but were related to changes in external input or authigenic loss of Ge. An authigenic sink of Ge was detected at all sites and accounted for 20% ~ 52% of Ge in bulk sediments. Authigenic precipitation of Ge is generally controlled by precipitation of Fe-rich minerals, but is also influenced by biogenic opal delivery in the centre of the bay where siliceous productivity is extremely high. The higher (Ge/Si)opal relative to those on the continental shelf is suggested to be caused by Ge supplementation from oceanic water in the open sea area and by high input of glacial debris from ice-melting at the edge of the Amery Ice Shelf.

“Deep-ultraviolet Raman spectra of Mars-relevant evaporite minerals under 248.6 nm excitation”

We have measured the deep-ultraviolet (DUV) Raman spectra of a number of evaporite minerals that are relevant to lacustrine and fluvial environments found on Earth and Mars, and show that DUV Raman can provide detailed information on elemental composition and crystal structure. The minerals included three borates, eight carbonates, and seven sulfates, with each class of mineral exhibited very distinct spectra under 248.6 nm excitation, dominated by the various internal vibrations of the borate, carbonate or sulfate oxyanion. Peak positions were shifted by markedly lower wavenumbers vs positions reported in the literature for longer wavelengths, a phenomenon we ascribe to the effect of pre-resonance with the oxyanion. Within each class, minerals consisting different metallic cations could be distinguished by the position of the dominant vibrational mode and the relative intensities of the minor modes, ascribed to the electrostatic impact of the cation on the vibrational behavior of the oxyanion. There was also evidence that DUV Raman can reveal minor metallic components even if they are not apparent in XRD, as two of three calcite (CaCO3) samples exhibited a shoulder on the dominant peak consistent with perturbation by ~19% Mg. UV absorption by Fe2+/3+ was a major factor in determining measurable signal, with Fe-rich minerals exhibiting weak/undetectable spectra. Understanding the spectra of these evaporite minerals will be essential to interpreting and identifying complex mineral samples using this technique, and represents an important addition to the spectral standards library of DUV Raman spectroscopy.

Hydrochemical and isotopic characteristics of thermal groundwater at Farafra Oasis, Western Desert, Egypt

Groundwater from the Nubian sandstone aquifer at Farafra Oasis in the Western Desert of Egypt has been investigated using chemical tracers and environmental isotopes to clarify the hydrochemical features of this aquifer. The majority of the collected samples are characterized by Ca-Cl water type, which may be attributed to dissolution of the carbonate-rich sediments. Calculated saturation indices show that the main hydrogeochemical processes were the dissolution of carbonates and evaporites and the precipitation of Fe-rich minerals. Temperatures calculated using the K-Mg geothermometer show that the reservoir temperature ranges from 58°C to 121°C. The groundwater samples have δD and δ 18 O values similar to the isotopic content of the Nubian aquifer palaeowater in the Western Desert. Additionally, the isotopic composition suggests that there is no active potential current recharge from the local precipitation. Gamma spectrometry of 226 Ra, 232 Th and 40 K activities indicates that the groundwater of the Nubian aquifer is safe with respect to radioactivity. Groundwater in Farafra Oasis is the main source of irrigation and drinking water for local residents. All the samples are excellent for irrigation uses for all types of crops. It is suggested that the water samples may require some treatment regarding the high iron content before usage as drinking water. Supplementary material: Locations of the collected groundwater samples and their Na-K-Mg ternary cation plot is available at https://doi.org/10.6084/m9.figshare.c.4938252

Identification of Opaque Sulfide Inclusions in Rubies from Mogok, Myanmar and Montepuez, Mozambique

The red variety of corundum owes its color and strong fluorescence to the presence of Cr, as well as traces of Fe. The latter can reduce the fluorescence and thus impact the appearance of the final gem. Gem quality rubies are rarely available for scientific study and even less common in their rough form. Opaque inclusions in rubies are often removed during faceting and remain unidentified. This study aims to identify opaque inclusions in rubies from the two most common origins seen in the high end market today: Mogok, Myanmar and Montepuez, Mozambique. Using electron probe microanalaysis (EPMA) the inclusions were identified as sphalerite and pyrrhotite in Mogok rubies. The paragenesis of Myanmar, marble-related rubies is fairly well understood and no Fe-rich minerals apart from sulfides have been identified. Opaque inclusions in Mozambican rubies are a complex mix of Fe-Cu-Ni sulfides with exsolution textures. These inclusions are interpreted to be small amounts of sulfide melt trapped during corundum formation. The different sulfide phases crystallized from this entrapped melt and some phases experienced later exsolution during cooling. The formation of amphibole-related, Mozambican rubies is not well understood, but it is obvious that very different processes are at work compared to the marble-related Myanmar ruby deposits.

Color analysis and detection of Fe minerals in multi-mineral mixtures from acid-alteration environments

Iron electronic transitions produce intense colors in minerals with moderate to high Fe content, which phenomenon allows using color and visible spectroscopy to identify and quantify Fe-bearing minerals. This approach is especially useful for Mars investigation due to the limited analytical instruments available at Mars. The challenges are: (1) overlap between Fe electronic absorption bands of Fe-bearing minerals, (2) band wavelengths are modified by chemical and structural variables, and (3) particle size effects can modify mineral color. The magnitude of these difficulties, however, depends on the mineral mixture investigated. In this study, we further probe the potential of visible-range data for investigation of Fe-minerals. The samples were mineral mixtures from the Riotinto area (SW Spain) that resulted from acidic alteration and contained Fe in oxy-hydroxides, hydroxysulfates and phyllosilicates. Samples were studied using visible/near-infrared spectroscopy, X-ray diffraction and thermogravimetry. The wavelength range 320–1000 nm was investigated using spectral second derivatives and CIELAB color parameters. Hematite was readily detected, below X-ray diffraction detection limits, from the band at 520–560 nm. Jarosite was readily identified by the combined presence of bands at ~390 and ~ 440 nm. Thermogravimetric and CIELAB color analyses indicated goethite as the most common Fe-rich mineral in the samples. Correlations between CIELAB color parameters were controlled by goethite, where jarosite-rich specimens aligned with the correlations but some samples containing hematite plotted outside them. Quantification of goethite was attempted as calibrated against thermogravimetric data. Calibration was not possible below ~10 wt% goethite. Above this value, results from spectral second derivatives allowed quantification up to the maximum goethite content of ~35 wt% (linear correlation with R2 = 0.89). Goethite quantification with CIELAB color parameters was hindered by hematite. In samples with very low or no hematite, goethite quantification was possible with parameters L*10 (R2 = 0.83) and hab,10 (R2 = 0.65), also up to ~35 wt% goethite. Consequently, if accurate calibration is possible and depending on mineral mixture components, spectroscopic and CIELAB color parameters should allow goethite quantification in a wide concentration range.

Evidence for Silicate–Liquid Immiscibility in Monzonites and Petrogenesis of Associated Fe–Ti–P-rich rocks: Example from the Raftsund Intrusion, Lofoten, Northern Norway

Abstract The 1800 Ma monzonitic to syenitic Raftsund intrusion is the largest intrusive body of the Lofoten–Vesterålen anorthosite–mangerite–charnockite–granite (AMCG) suite. It is composed of three units that can be differentiated based on their textures. This study focuses on the most voluminous, predominantly equigranular, unit consisting of a pigeonite–augite syenite and a fayalite–augite monzonite. The pigeonite–augite syenite is associated with centimeter-scale to hundred-meter scale occurrences of Fe–Ti–P-rich rocks that display sharp to gradational contacts with the surrounding syenite. Iron–Ti–P-rich rocks consist of augite, Fe-rich olivine ± partly inverted pigeonite, apatite, ilmenite, titanomagnetite and sparse pyrrhotite, hornblende and biotite. Partly resorbed ternary feldspar crystals are common toward the contact with the syenite. Microtextures, such as symplectites, encountered at the contact between the syenite and the Fe–Ti–P-rich rocks indicate local disequilibrium between the two rock types. The Fe–Ti–P-rich rocks show large compositional variations but overall are enriched in Ca, Zn, Sc and rare earth elements in addition to Fe, Ti and P compared with the host syenite. Field evidence, whole-rock compositions and textural relationships all suggest that that silicate–liquid immiscibility was involved in the genesis of the Fe–Ti–P-rich rocks. These are interpreted to represent Fe-rich unmixed melts, whereas the syenite is inferred to originate from the crystallization of conjugate Si-rich immiscible melt. The existence of an Fe-rich melt is further supported by the high trace element content of augite from the Fe–Ti–P-rich rocks, showing that they grew from a melt enriched in elements such as Sc and Ti. The fayalite–augite monzonite also displays textural and chemical evidence of silicate liquid immiscibility resulting in unusually variable Zr contents (few hundred ppm to more than 3000 ppm) and the presence of abundant zircon and allanite restricted to millimeter- to centimeter-scale Fe-rich mineral clusters. The most Fe-rich and Si-poor rocks are interpreted to represent the larger proportion of the Fe-rich melt. Liquid immiscibility can be identified at various scales in the pigeonite–augite syenite, from millimeter-size clusters to large-scale bodies, up to hundreds of meters in size, indicating various degrees of separation and coalescence of the Fe-rich melt in the intrusion. The immiscible liquids in the fayalite–augite monzonite consist of an emulsion, with small millimeter- to centimeter-scale droplets of Fe-rich melt, whereas in the pigeonite–augite syenite, Fe-rich melt pockets were able to coalesce and form larger pods. The difference between the two units either results from earlier onset of immiscibility in the pigeonite–augite syenite or reflects a difference in the degree of polymerization of the melt at the time of unmixing. This study emphasizes the importance of silicate–liquid immiscibility in the evolution of intermediate to felsic alkalic ferroan systems and provides a series of arguments that can be used to identify the process in such systems.

Organic Matter Amendment and Plant Colonization Drive Mineral Weathering, Organic Carbon Sequestration, and Water-Stable Aggregation in Magnetite Fe Ore Tailings.

The formation of water-stable aggregates in finely textured and polymineral magnetite Fe ore tailings is one of the critical processes in eco-engineering tailings into soil-like substrates as a new way to rehabilitate the tailings. Organic matter (OM) amendment and plant colonization are considered to be effective in enhancing water-stable aggregation, but the underlying mechanisms have not yet been elucidated. The present study aimed to characterize detailed changes in physicochemistry, Fe-bearing mineralogy, and organo-mineral interactions in magnetite Fe ore tailings subject to the combined treatments of OM amendment and plant colonization, by employing various microspectroscopic methods, including synchrotron-based X-ray absorption fine structure spectroscopy and nanoscale secondary ion mass spectroscopy. The results indicated that OM amendment and plant colonization neutralized the tailings' alkaline pH and facilitated water-stable aggregate formation. The resultant aggregates were consequences of ligand-promoted bioweathering of primary Fe-bearing minerals (mainly biotite-like minerals) and the formation of secondary Fe-rich mineral gels. Especially, the sequestration of OM (rich in carboxyl, aromatic, and/or carbonyl C) by Fe-rich minerals via ligand-exchange and/or hydrophobic interactions contributed to the aggregation. These findings have uncovered the processes and mechanisms of water-stable aggregate formation driven by OM amendment and plant colonization in alkaline Fe ore tailings, thus providing important basis for eco-engineered pedogenesis in the tailings.