Abstract The thermal decomposition and dehydration mechanism of kurnakovite MgB3O3(OH)5·5H2O, a hydrated Mg borate mineral with potential industrial applications, were investigated using thermal analysis, in-situ high-temperature synchrotron single-crystal X-ray diffraction (30–140 °C), and ex-situ high-temperature synchrotron powder X-ray diffraction (20–1000 °C). Kurnakovite exhibited anisotropic thermal expansion up to 120 °C, attributed to the increase in lengths of the hydrogen bonds and thermal expansion of the Mg octahedra. The linear thermal expansion coefficients obtained between 30 and 120 °C for lattice parameters a, b, and c were αa = 3.73 × 10–5 K–1, αb = 2.14 × 10–5 K–1, and αc = 3.13 × 10–5 K–1, respectively. Therefore, the thermal expansibilities were largest along the a axis, followed by the c and b axes. Subsequently, a rapid mass loss of 25.6% occurred at 142 °C, corresponding to dehydration of four H2O molecules. Analyses of isotropic and anisotropic displacement parameters indicated that the dehydration of two distinct types of H2O molecules exhibiting pronounced thermal vibrations led to the disconnection of hydrogen bonds and significantly disrupted the electrostatic balance between the Mg-centered interstitial complex and borate fundamental building blocks (FBBs), resulting in rapid amorphization. Upon further heating, the amorphous material recrystallized at 761 °C, and ex-situ XRD revealed that the anhydrous Mg borate phases derived from kurnakovite evolved through four distinct stages containing m-Mg2B2O5, t-Mg2B2O5, MgB4O7, and the unidentified phases X, Y1, Y2, and Z. Stage I between 700 and 850 °C consisted of m-Mg2B2O5, MgB4O7, and phase X; stage II between 850 and 900 °C of t-Mg2B2O5, MgB4O7, and phase Y1; stage III between 900 and 950 °C of t-Mg2B2O5, MgB4O7, and phase Y2; and stage IV between 950 and 1000 °C of t-Mg2B2O5 and phase Z. The endothermic event at 993 °C is attributed to partial melting of MgB4O7 and/or phase Y2 during the transition from stage III to stage IV.
Abstract Scandium holds significant importance in modern technology and is recognized as a strategic or critical metal in many countries. Scandium-rich clinopyroxene is important in forming various Sc resources, whereas the favorable conditions in forming Sc-rich clinopyroxene remain unclear. By compiling available experimental data on Sc partition coefficients between clinopyroxene and melt (DSccpx/melt), we demonstrate that DSccpx/melt values increase exponentially with the alkali contents in melts. Using first-principle calculations combined with a simple thermodynamic model, we propose that this trend arises from simultaneous competitive formation of AlM1AlT, NaM2AlM1 and NaM2ScM1 pairs in clinopyroxene. This mechanism differs from conventional models such as charge-coupled substitutions, which would increase Sc partitioning with Al through ScM1AlT pairing. The incorporation of Sc into clinopyroxene is highly energetically unfavorable compared to Al, suggesting that Sc uptake may be governed mainly by configurational entropy rather than enthalpic factors. Entropy-driven competition model predicts a pronouced temperature dependence of Sc partitioning, with lower temperature and alkali-richer melts tending to yield larger DSccpx/melt values, thereby potentially promoting Sc enrichment in clinopyroxene. Our model demonstrates that Sc partitioning may reflect a complex interplay of different components in both the melt and solid phases, and previous regressions of experimental data may have not fully captured these effects. To complement the thermodynamic approach, we develop a predictive machine-learning model trained on experimental data, achieving a high coefficient of determination (R2 = 0.98). Together, the thermodynamic framework and data-driven model quantify how melt alkalis and temperature regulate Sc partitioning, provide new constraints on the petrogenic conditions and magma compositions that promote Sc-rich clinopyroxene and, by extension, Sc enrichment in alkaline magmatic systems.
Abstract The behavior of iron-bearing magnesium carbonate (Mg,Fe)CO3 at high pressures has significant implications for deep carbon cycling in Earth’s mantle, as this composition more realistically reflects the carbonates expected in the Earth’s deep interior. In this study, we have investigated high-pressure vibrational properties of Fe-bearing magnesite using synchrotron Mössbauer, infrared (IR), and Raman spectroscopies in diamond anvil cells up to ∼71 GPa at room temperature. Above 29 GPa, Raman and IR spectra reveal signatures of pressure-induced lattice distortion, including splitting of lattice-coupled (T) and in-plane bending (ν4) modes of internal CO32- vibrations, changes in the pressure dependence of the wavenumbers of both Raman and IR modes, and a variation in the L/ν4 mode intensity ratio. While lattice distortion has been proposed in pure MgCO3, even minor Fe substitution alters the response of internal modes. This result suggests that low Fe content can modify the local bonding environment, which may influence the high-pressure and -temperature stability of the MgCO3-FeCO3 solid solution. Mössbauer spectra show that a high-spin to low-spin transition occurs between ∼45–50 GPa in Fe-bearing magnesite. Raman and IR measurements display discontinuous shifts in both lattice (L) and internal vibrational (ν1 and ν4) modes across the spin transition. Previous studies on the MgCO3 endmember have reported additional lattice distortion near 50 GPa, close to the pressure at which the spin transition occurs in Fe-rich compositions. The relative changes in the Grüneisen parameters (Δγ) of the lattice mode across the spin transition for Fe-containing compositions and the further distortion for MgCO3 endmember exhibit a linear dependence on Fe content, whereas those of the internal modes show a second-order, non-linear trend, particularly for the ν4 mode. Both lattice and internal modes may reflect the combined effect of lattice distortion and the spin transition, but the two cannot be distinguished from the behavior of the lattice mode. While lattice distortions have been discussed for the Mg-endmember, they have not been reported for Fe-bearing or Fe-rich compositions, possibly because the lattice distortions may be masked by the dominant effects of the spin transition, particularly in Fe-rich compositions. Since lattice modes are expected to primarily govern elastic wave propagation and the contribution of internal-mode variations is limited, the Fe-content dependence of sound velocities after the spin transition is expected to be approximately linear. This suggests that Fe-bearing carbonates may lower seismic wave velocities in the deep mantle where they are sufficiently abundant.
Abstract Uranyl oxy-hydroxy-hydrate minerals are common alteration products of uraninite (UO2+x) which is chemically and structurally analogous to uranium dioxide nuclear fuel. Therefore, structural and spectroscopic investigations of these alteration minerals and their analogous anthropogenic counterparts can provide insight into the environmental behavior of nuclear fuel cycle materials. Previously, we compiled available vibrational spectroscopic data for uranyl minerals in the Compendium of Uranium Raman and Infrared Experimental Spectra (CURIES) and found that only 37% of known uranyl oxy-hydroxy-hydrate minerals had spectra readily available in the literature and existing databases for inclusion therein. Furthermore, no available infrared spectra for this mineral group were included in CURIES. To expand our understanding of the spectroscopic features of uranyl oxy-hydroxy-hydrates and the structural origins thereof, we collected, and now include in CURIES, Raman and infrared spectra for an additional 12 uranyl hydroxide phases. To better understand the impact of structural and compositional variations of these phases on their spectroscopic features, we compare Raman spectra of different anion sheet topological groups and of analogous phases hosting different counter cations. We identify spectroscopic variations related to differences in equatorial bonding and structural changes as a result of cation substitution. We also prepare a uranyl hydroxide phase via hydrolysis of uranyl fluoride (UO2F2) as an analog of hydrolysis reactions that occur in nuclear fuel cycle materials; and we find that the alteration product of UO2F2, despite chemical and structural similarities to uranyl oxy-hydroxy-hydrate minerals, is readily distinguishable from related mineral phases using Raman spectroscopy. In this work, we provide new insights into the structural origins of spectroscopic features in uranyl oxy-hydroxy-hydrate minerals, improve the average Raman spectrum for this group of minerals, and thereby improve capabilities for identifying these mineral species and related anthropogenic phases using Raman spectroscopy.
Abstract Emulsion textures between sulfide and silicate minerals provide insight into the complex mingling behaviors between immiscible sulfide and silicate melts. The Sora gabbroic intrusion in eastern Germany not only hosts exceptionally well-preserved emulsion textures but also displays previously undescribed spherical silicate droplets within a sulfide matrix. These droplets are completely enclosed by thin coats of partly skeletal magnetite, ilmenite, apatite and/or biotite. The mineral assemblage and crystal morphologies of these coats suggest the involvement of a third immiscible Fe-Ti-P-rich melt (besides silicate and sulfide melts) during the formation of the emulsion textures. Supporting evidence includes the absence of reaction textures at the droplet interfaces, the independence of the composition of hosting sulfides and the enrichment of V and Cr in magnetite and Mn in ilmenite, hence of elements that are generally incompatible in both sulfide and silicate melts. This dense, rapidly crystallized Fe-Ti-P melt enclosing silicate droplets could have acted as geological emulsifier, increasing the effective density of the coated silicate melt and preventing its ascent through the hosting denser sulfide melt. This prevention of coalescence could have likely contributed to the exceptionally well preserved emulsion textures at the Sora intrusion. This study provides new evidence for three-liquid immiscibility involving silicate, sulfide, and Fe-Ti-P melts in a natural example and highlights how their interaction can preserve complex multi-melt processes in magmatic systems.
Abstract Mafic-ultramafic intrusions represent the primary source of global scandium (Sc) resources worldwide. These intrusions typically originate from fertile mantle sources in arc systems. While fluid- and melt-driven metasomatism are widely recognized as crucial mechanisms for Sc enrichment in the mantle, key aspects, including the precise composition of these metasomatic agents and their operating conditions remain poorly constrained. To address these knowledge gaps, we conducted a comprehensive investigation of Sc enrichment and release using clinopyroxene and orthopyroxene chemistry from charnockites and mafic granulites in East Antarctica’s Prydz Bay Belt. We reveal a multi-stage Sc evolution pathway within the accretionary belt that is initiated by dehydration of carbonate rocks, generating reduced carbonatite melts which trigger mantle metasomatism and significantly enrich Sc in the pyroxene. This enrichment is followed by post-peak decompression melting at 800-1000 °C and 6.5-12.8 kbar, which triggers metasomatism by an oxidized silicate melt, facilitating Sc release from both orthopyroxene and clinopyroxene. The subsequent arc accretion stage involves interaction with an external, oxidized, and likely phosphorus-rich aqueous fluid, leading to Sc release. The findings of Sc-rich pyroxene in mafic granulite and charnockite, together with the presence of CO₂-rich fluid inclusions in East Gondwana (Antarctica), collectively indicate that scandium enrichment may be a regional event. This study provides further implications for the exploration of Sc deposits in metamorphic rocks.
Abstract The bond valence (BV) method is a semiempirical description of bonding in ionic crystal structures based on Pauling’s second rule, the electrostatic valence principle. However, calculated (unitless) BV bond strengths are often uncorrelated with measured bond energies because conventional BV descriptors, R0 and B, are exquisitely sensitive to the structures and methods used for training. In this work, we show that global bond graph optimization of pyroxene-type compounds enables the prediction of their absolute formation energies. Furthermore, a threshold of -2.31 volts/atom of formation potential - the derivative of formation energy with respect to the standard deviation of the valence - is shown to delineate ambient-pressure from high-pressure phases for this class of structures. Experiments confirm that high pressures are required to form α spodumene, a major mineral resource for lithium production, despite its thermodynamic stability under ambient conditions. All known α-spodumene-type clinopyroxene structures containing lithium, comprising 16 types of cation-anion pairs, exhibited two irreducible BV parameters. We show that R0, which is unique to each structure, is the covalent bond radius and is nearly equal to the observed bond length when the coordination number equals the cation formal charge, such as in BO3 and SiO4 polyhedra. We derive a new parameter β and show that it depends on the formal charge, coordination number, and electron spin, capturing the effects of electronic interactions in ionic bonds. Our findings elucidate the physical underpinnings of valence in minerals and its role in defining thermochemical properties by extending Pauling’s second rule.
Abstract The type material of davemaoite, CaSiO3-perovskite, contains several at-% of Fe, Al, Na, and K which has potential implications for the distribution of alkalines and the Urey number of the deep mantle. Previous experiments have not shown such high alkaline contents in davemaoite and it was questioned if the type material actually formed in the deep mantle. However, these studies did not explore compositions similar to that material. We show that glass of a composition close to that of type davemaoite transforms to a homogenous perovskite-phase at 22 GPa. This finding supports correlation of Na+K with Ca and ferric Fe in deep mantle rock. The perovskite-type solid solution is volumetrically favored by ∼2% over a mixture of calcium-ferrite type oxides and endmember davemaoite.
Abstract Shock-melt veins in ordinary chondrites record ultrafast, high-pressure mineral transformation reactions. However, resolving the nano- to microscale mineral assemblages that form and quench during these events remains challenging. Here we demonstrate that near-axis transmission Kikuchi diffraction (NA-TKD), combined with scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), can reliably resolve crystal structures and Fe–Mg zoning in sub-micron, high-pressure olivine phases across 10–15 μm fields of view, providing new insights into shock transformation mechanisms. We apply this approach to shock-melt veins in the Catherwood L6 chondrite, where host olivine along the shock-vein margin transforms into dense, randomly oriented clusters of ringwoodite crystallites. These textures indicate rapid solid-state transformation by homogeneous intracrystalline nucleation and interface-controlled growth under strongly overstepped conditions. Olivine fragments entrained within the melt preserve similar ringwoodite-dominated cores but develop Fe-rich reaction zones and Mg-rich wadsleyite rims at melt-wetted grain boundaries, accompanied by interstitial majoritic garnet. These features record brief melt infiltration, partial dissolution, and melt-assisted recrystallization during shock events. Together, the observed microstructures define a two-stage, but spatially heterogeneous transformation sequence: initial solid-state ringwoodite formation followed by localized melt-mediated overprinting and wadsleyite crystallization. This demonstrates that pressure–temperature conditions vary substantially across a single shock-melt vein, allowing multiple transformation mechanisms to operate sequentially or simultaneously within the same system. By enabling phase discrimination, orientation mapping, and coupled chemical–structural analysis at ∼10–30 nm spatial resolution, NA-TKD combined with EDS provides nanoscale crystallographic mapping within the SEM. This approach allows shock transformation sequences to be reconstructed across micrometer-scale fields of view that are difficult to access using SEM and TEM alone, providing a powerful framework for interpreting high-pressure reaction pathways in planetary materials.
Abstract Understanding the factors that control the co-precipitation of Sb and Au is critical for guiding exploration in epizonal orogenic systems. Stibnite is a common ore mineral that is closely associated with the Au mineralization in such systems, serving as an ideal mineral target for micro-analytical studies to better constrain the hydrothermal controls of coupled Au-Sb deposition. This study focuses on the newly-discovered, large Qukulekedong orogenic Au-Sb deposit in the westernmost East Kunlun orogenic belt. Antimony mineralization predominantly occurs as quartz-stibnite veins in the deposit. Two generations of stibnite mineralization were identified, including (1) early quartz-stibnite (Sbn1) veins with abundant inclusions of visible Au, andorite, jamesonite and sphalerite, and (2) late quartz-stibnite (Sbn2) stockwork. Comparable in-situ S and Pb isotope compositions of both generations of stibnite suggest a common fluid and metal source. However, Se concentrations from both stibnite generations vary significantly, with Sbn1 containing 310 to 580 ppm Se, which is about one order of magnitude higher than in Sbn2 (Se <50 ppm). The high Se contents in Sbn1 are related to increasing ∑Se/S ratios and decreasing fS2 in the hydrothermal fluids due to accompanied sulfide precipitation. This ultimately led to the strong Au enrichment associated with Sbn1, which is controlled by two mechanisms that are not mutually exclusive, including (1) the fS2-driven destabilization of Au-HS complexes and (2) the substitution of Se2- for S2- in Sbn1 that may have induced lattice distortions and defects enhancing the Au adsorption potential. These processes highlight the critical role of Se in stibnite as a pathfinder towards high-grade Au-Sb mineralization, offering new insights into the geochemical mechanisms that control the formation of epizonal orogenic Au-Sb deposits.
Abstract The timing of plate tectonics onset is still debated. It may have developed gradually since the Archean, evolving from warm (ancient) to cold (modern) regimes. However, there is still a lack of robust petrologic evidence, such as Archean subduction products like eclogite, and it is unclear when and how such a tectonic mode transition occurred. Here we examine the well-preserved Belomorian eclogites from Fennoscandia, in which we identify two high-pressure metamorphic events in the Neoarchean (2.72-2.70 Ga) and Paleoproterozoic (1.92-1.84 Ga). The Neoarchean event is inferred from the omphacite-associated inclusions within the metamorphic zircon cores, reflecting likely warm tectonic conditions (333-425 °C/GPa). The Paleoproterozoic metamorphism, recorded by zircon rims, represents eclogite facies. The petrologic-mineralogical data of these eclogites indicate cold subduction conditions (300 ± 12 °C/GPa) consistent with our previous conclusions. Each event has global counterparts, marking ancient and modern style plate tectonics. In summary, the Belomorian eclogite records the transition in plate tectonics from ancient (warm) to modern (cold) styles through two episodes of HP metamorphism.
Abstract Modraite, ideally Ca19Fe2+Al4(Al7Fe2+)(SiO4)10(Si2O7)4O(OH)9, is a new member of the vesuvianite group. It was found at Modra, Harmónia skarn rock in the Malé Karpaty Mountains, Pezinok District, Slovakia. Modraite formed as a high-temperature and low-pressure metamorphic mineral on the contact zone between Devonian limestones and Carboniferous granitic rocks, in association with diopside-hedenbergite, rarely grossular, calcite, titanite, and clinozoisite-epidote. The new mineral forms dark brown microscopically transparent euhedral to subhedral prismatic crystals or radiating aggregates (up to 5 cm long) in diopside-hedenbergite groundmass. Modraite crystals are characterized by dominant {100} and minor {110} prism faces; the mineral is brittle, has an irregular (uneven) fracture without cleavage or parting. The Mohs’ hardness is 6−7. The measured and calculated densities are 3.35(2) and 3.42 g/cm3, respectively. The new mineral is microscopically transparent to translucent, optically uniaxial, negative, ω = 1.7221(2), ε = 1.7151(3). The mean chemical composition of modraite (wt%, electron microprobe, REE and Li by LA-ICP-MS, Fe2O3 and FeO by Mössbauer spectroscopy, H2O calculated from stoichiometry) is: SiO2 36.42, TiO2 1.79, Al2O3 16.37, V2O3 0.03, Cr2O3 0.03, La2O3 0.03, Ce2O3 0.08, Pr2O3 0.01, Nd2O3 0.04, Sm2O3 0.01, Gd2O3 0.01, Fe2O3 1.32, FeO 3.65, MnO 0.23, ZnO 0.05, MgO 0.87, CaO 34.92, BaO 0.26, Li2O 0.03, Na2O 0.09, H2O 1.80, F 1.49, Cl 0.16, O=F -0.63, O=Cl -0.04, total 99.02. The empirical formula based on 50 cations (excluding H) per formula unit is: X1–X4(Ca18.60Na0.09Ba0.05Ce0.02La0.01Nd0.01)Σ18.78Y1(Fe2+0.81Fe3+0.13Al0.09)Σ1.03Y2Al4Y3(Al5.50Fe2+0.71Ti4+0.67Mg0.65Fe3+0.36Mn2+0.10Li+0.06Zn2+0.02 V3+0.01Cr3+0.01)Σ8.09T1(□4) T2□ (Z1–Z2Si1.01O4)10 (Z3Si2O7)4O10–O11(OH5.97F2.34O1.55Cl0.14)Σ10.00. Mössbauer spectroscopy indicates a dominant proportion of Fe2+ (76 %) over Fe3+ (24 %), with the dominancy of Fe2+ over Fe3+ at both the Y1 and Y3 sites. The crystal structure was refined to R1 = 0.0215 for 3970 unique reflections. The mineral is tetragonal, space group P4/nnc, single-crystal X-ray diffraction data of a modraite are: a = 15.559(2) Å, c = 11.804(2) Å, V = 2857.4(9) Å3, Z = 2. The seven strongest lines of the powder X-ray diffraction pattern are [d(Å)−I(%)−hkl]: 3.235−19−402, 2.949−29−004, 2.749−100−432, 2.593−59−522, 2.458−37−620, 1.625−16−526, 1.621−30−922. Modraite is the Y1Fe2+-dominant member, ideally with 7 Al3+ and 1 Fe2+ cations at the Y3 site, and a strong O10-H10…O10 hydrogen bond. Modraite is named after the type locality, Modra, western Slovakia.
Abstract A new member of the pavonite homologous series, sunshuite, ideally FeBi2S4, was discovered in the Jiawula-Chaganbulagen intermediate-sulfidation epithermal Ag-Pb-Zn ore-field, Inner Mongolia, China. The mineral occurs as anhedral grains, 5-20μm in size, in association with pyrrhotite, sphalerite, galena, chalcopyrite, native bismuth, bismuthinite, quartz, calcite, chlorite, and sericite, etc. Sunshuite forms from bluish-gray to light gray with metallic luster. The calculated density is 6.13 g·cm-3. Reflectance values for the four COM wavelengths of sunshuite in air [Rmax, Rmin (%) (λ in nm)] are 34.6, 36.4 (470); 33.2, 35.1 (546); 33.5, 35.1 (589); 33.7, 35.4 (650), respectively. The average composition (in wt%) of sunshuite was 1.44 Mn, 21.22 S, 7.46 Fe, 0.3 Pb, 0.11 Cu, 68.84 Bi, 0.41 Zn, 0.31 Ag, total 100.09, analyzed by electron microprobe analyses. The empirical formula is (Fe0.81Mn0.16Zn0.04Cu0.01Pb0.01Ag0.02) Σ1.05Bi1.99S4, on the basis of 4 S apfu. The crystal structure was determined by single-crystal X-ray diffraction. Sunshuite is monoclinic, with space group C2/m (#12). The unit-cell parameters are a = 12.576 (2) Å, b = 3.9100 (5) Å, c = 14.717 (2) Å, β = 115.04 (2)°, V = 655.7 (2) Å3 and Z = 4. The strongest X-ray diffraction lines [d (I%)] of sunshuite are 3.637 Å (54), 3.622 Å (59), 3.444 Å (100), 3.333 Å (47), 3.057 Å (63), 2.842 Å (74), 2.720 Å (87), 2.523 Å (46). Sunshuite is the Fe analogue of graţianite (MnBi2S4), 3P members of the pavonite homologous series. Sunshuite comprises two types of slabs, parallel to (001), namely type I thinner slabs have Fe-centered octahedra, flanked by Bi-centered square-pyramidal five-fold coordination; and type II thicker slabs comprise Fe-, Bi-centered octahedra. Both slabs are interconnected via common sulfur atoms. The Fe in sunshuite is divalent (Fe2+), and its association with pyrrhotite and native bismuth suggests that sunshuite has the potential to serve as an indicator of a reducing hydrothermal environment. The mineral is named after the late professor Shu Sun, a member of the Chinese Academy of Sciences on sedimentary geotectonics.
Abstract Crystal size distribution (CSD) slopes are used to calculate magma residence times, based on the principle that the slope is inversely proportional to the product of crystal residence times and growth rate. Most CSD studies are based on two-dimensional (2D) data, relying on statistical calculations and stereological corrections of the analyzed data to estimate three-dimensional (3D) size distributions using specialized software. However, the effect of this estimation on the actual CSD distributions and their slopes remains unclear. To evaluate the effect of this CSD slope calculation/estimation on crystal residence times, we compare CSD slopes estimated from 2D and directly measured from 3D data. A 2D data set of pyroxene microlites from a glassy lava sample from Mount Ruapehu (New Zealand) was used to generate CSD (2D-CSD) applying three different aspect ratios. These aspect ratios were calculated using known databases (i.e., CSDSlice and ShapeCalc), and the average aspect ratios from a 3D dataset. The CSD slopes between 10 – 20 μm were extracted and compared to the slope obtained from a true CSD using synchrotron radiation X-ray computational tomography (3D-CSD). Our results show differences in the shape determination by the two databases compared to the average 3D aspect ratio, mainly impacting the intermediate/long axes ratio (I/L) showing a I/L ratio difference of 0.18 between both databases, that translate in slope differences of ±0.03 μm-1 compared to the 3D-CSD. The slopes were applied to the determination of crystal residence times using a known growth rate (i.e., 1.80 × 10−11 m/s), finding differences of approximately 31 h between those CSD determined with the databases, and nearly 17 h comparing the 2D-CSD to the 3D-CSD. Our results contribute to the discussion about which is the best shape estimate to use for 2D stereological conversions, highlighting the uncertainties derived from the statistical calculations of the aspect ratios, and the difficulties in replicating true 3D crystal distributions. We conclude that limitations can be circumvented by using 3D datasets.
Abstract The abundance of volatiles profoundly impacts the interior evolution and surface habitability of Earth and other terrestrial planets. Infrared spectroscopy, with exceptional sensitivity, holds a central responsibility for such detection and quantification. Here, we revisit the physical principles and measurement methodologies of quantitative infrared analysis, and find that a commonly used mathematical formulation of absorption in arbitrarily oriented crystalline samples is subject to significant errors in non-principal planes. A new description of infrared absorption in crystals is derived from the concepts of electric dipole, optical indicatrix, and crystal symmetry. The model’s validity is examined through a case study of O-H stretching bands in a non-principal plane of olivine, using both polarized and unpolarized infrared analysis. Building on this foundation and performing Monte Carlo simulations, we evaluate the performance of four analytical strategies that employ unoriented sample grains under polarized or unpolarized radiation, highlighting their robustness, limitations, and uncertainties. We show that (1) measurements of maximum and minimum polarized absorbances in three perpendicular planes are equivalent to measuring principal polarized absorbances, regardless of crystal symmetry or refractive indices; (2) measuring three mutually perpendicular polarization directions, on the other hand, may introduce substantial systematic errors; (3) averaging unpolarized spectra consistently underestimates the total absorbance; and (4) the latter two approaches yield greater potential errors for low-symmetry systems or for bands that are highly anisotropic or intense, whereas increasing the number of absorption peaks may mitigate these errors. Our model holds for both high- and low-symmetry crystal systems and can provide guidance for accurately quantifying infrared-absorbing species in minerals.
Abstract Manganese is an important minor element in the Earth’s mantle, and its metal–silicate partitioning behavior has previously been used to constrain the conditions of planetary core formation. Therefore it is important to understand the properties of manganese in lower mantle phases like (Mg,Mn)O, including its electronic spin state. We performed optical transmittance spectroscopy on a sample of Mg0.8Mn0.2O up to 84 GPa, finding a change in the spectra at ∼58 GPa, beyond which the sample gradually became opaque. We also performed synchrotron X-ray diffraction measurements on Mg0.8Mn0.2O up to 136 GPa. The compression data exhibit discontinuities at 58 GPa and 102 GPa, which we interpret as the onset and completion of a gradual spin crossover in Mn2+ from high spin to low spin, analogous to that of Fe2+ in (Mg,Fe)O. We fit equations of state to our pressure–volume data for both the high spin and low spin phases of Mg0.8Mn0.2O, finding a similar V0 but different K0 and K0′ for the two phases. Using 1 bar thermodynamic data and the equations of state of the endmembers, combined with constraints from our new data, we modeled the Gibbs free energy of a ternary solid solution of MgO, high spin MnO, and low spin MnO, allowing construction of pressure–temperature–composition phase diagrams across the Mn2+ spin transition in (Mg,Mn)O. These calculated phase diagrams suggest that this transition should occur more gradually (over a wider range of pressures) for lower XMnO and higher temperatures, such that all (Mg,Mn)O in Earth’s mantle may be mixed spin, with its low spin fraction increasing gradually but considerably with depth along a geotherm. If the Mn2+ spin state evolves similarly with pressure–temperature–composition in other phases, it could affect the partitioning behavior and other properties of manganese, which may be useful to consider when interpreting and especially extrapolating experimental data.
Abstract The iron oxyhydroxide “Schwimmeisen” (or floating iron) plays an important role in biogeochemical iron cycling in surface waters. In a dewatering shaft in the Harz area, Germany, Schwimmeisen was observed as a floating iron oxyhydroxide film on water and subsequently analyzed. The precipitate was dominated by ferrihydrite, consistent with previous studies. In contrast to previous reports, no ferrous iron was detected, suggesting that Schwimmeisen should not be considered as a discrete mineral phase with a defined Fe(II)/Fe(III) ratio. Rather, it represents a transient interfacial phenomenon that occurs when anaerobic ferrous-iron bearing water contacts atmospheric oxygen in the presence of sufficient (and suitable) organic material that i. preserves the low crystallinity of the oxidation product and ii. hydrophobizes its surface. Oxidation was likely facilitated by microorganisms of the genus Gallionella, which were also characterized in this study. Schwimmeisen occurred both as iridescent thin films and as brown, voluminous masses, a variation attributed to flowing water aggregating the films and to ascending bubbles adhering to the hydrophobic surfaces, thereby reducing aggregate density. The precipitate was commonly enriched with organic material, including potential contaminants, indicating that its formation and transport may influence the risk of contamination mobility in the surrounding biosphere.
Abstract This issue of New Mineral Names provides a summary of the newly described minerals from 2025, selected information for recently published descriptions from October 2025 to February 2026, and some of the new minerals approved during the period October 2025 to January 2026. New mineral name trends and observations are presented with an objective examination of new species and their broader implications. All minerals presented have been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA-CNMNC).
Abstract Nickel (Ni) and cobalt (Co) are critical elements necessary for the transition to carbon-free energy generation. Increasing global demand for these elements requires new sources to be found. Both Ni and Co are found in low concentrations in mafic and ultramafic silicate minerals. To systematically understand the partitioning behavior of these elements in silicate minerals and to provide starting materials for experiments, we synthesized forsterite and enstatite with Ni and Co contents between 1.2 and 8.6 wt.%. We find that >95% purity syntheses of large batches (tens of grams) of both minerals are possible and that purity decreases with increasing Ni or Co dopant levels in enstatite syntheses. Impurities in forsterite syntheses include cristobalite and enstatite, while impurities in enstatite syntheses include cristobalite and forsterite. Partitioning of Ni and Co dopants favors forsterite over enstatite, with a more pronounced effect observed in Ni than Co. The doped minerals serve as starting materials for experiments that seek to extract critical elements from ultramafic and mafic rocks, in which the primary carriers of Ni and Co are olivine and pyroxene.
