Volume 107, Issue 5

Using particle swarm optimization with density functional theory, we identify the positions of hydrogen in a hypothetical Mg-end-member of phase egg (MgSiO 4 H 2 ) and predict the most stable crystal structures with MgSiO 4 H 2 stoichiometry at pressures between 0 and 300 GPa. The particle swarm optimization method consistently and systematically identifies phase H as the energetically most stable structure in the pressure range 10–300 GPa at 0 K. Phase Mg-egg has a slightly higher energy compared to phase H at all relevant pressures, such that the energy difference nearly plateaus at high pressures; however, the combined efects of temperature and chemical substitutions may decrease or even reverse the energy difference between the two structures. We find a new MgSiO 4 H 2 phase with the P 4 3 2 1 2 space group that has topological similarities to phase Mg-egg and is energetically preferred to phase H at 0–10 GPa and 0 K. We compute the free energies for phase Mg-egg, phase P 4 3 2 1 2, and phase H at 0–30 GPa within the quasi-harmonic approximation and find that the efect of temperature is relatively small. At 1800 K, the stability field of phase P 4 3 2 1 2 relative to the other polymorphs increases to 0–14 GPa, while pure phase Mg-egg remains energetically unfavorable at all pressures. Simulated X‑ray diffraction patterns and Raman spectra are provided for the three phases. Additionally, the crystallographic information for two metastable polymorphs with the P 1 space group is provided. Our results have implications for the deep hydrogen cycle in that we identify two novel potential carrier phases for hydrogen in the mantles of terrestrial planets and assess their stability relative to phase H. We determine that further experimental and computational investigation of an extended compositional space remains necessary to establish the most stable dense hydrated silicate phases.

Brucite [Mg(OH) 2 ] is an important hydrous mineral in the MgO-SiO 2 -H 2 O system and also a key component in the process of hydrothermal metamorphism. Due to its large water storage capacity and presence within a sinking slab, the study of brucite’s physical properties under relevant extreme conditions could shed new light on its potential impacts on the slab’s thermal profile and geodynamics, as well as seismic anomaly observed around a subduction zone. For example, seismic tomography has revealed slab stagnation and low-velocity zones in the shallow lower mantle that is conventionally attributed, respectively, to large contrasts of physical properties between the slab and mantle as well as dehydration melt. However, the effect of hydrous minerals on slab dynamics and seismic anomalies remains poorly understood. Here we study thermal conductivity of brucite at high pressures and room temperature as well as at ambient pressure and elevated temperatures. We further model thermal conductivity of brucite along a representative geotherm and find an ~6-fold to 19-fold increase in the thermal conductivity as brucite decomposes to periclase in the shallow lower mantle (~800 km depth). This result implies that the subduction and decomposition of brucite-rich aggregate within a slab may create a local high-temperature anomaly that would both enhance the slab’s buoyancy, leading to stagnation, and facilitate dehydration melting, contributing to seismic low-velocity zones. Our findings offer mechanisms associated with brucite decomposition that could influence the slab dynamics and seismic structures in the shallow lower mantle.

The activity of volatile-rich fluids may be important in the evolution of basaltic magmatic systems and associated precious metal ore formation. There is evidence for Cl-rich fluids within the Stillwater Complex (Montana, U.S.A.), which have been linked to platinum and palladium mineralization in the economically important Johns-Manville (J-M) Reef ore body. We present the first data set for heavy halogens (Cl, Br, and I) and natural noble gas isotopes in bulk rock and mineral separates from the Peridotite Zone and the Olivine-Bearing Zone I of the Stillwater Complex, including samples from the J-M Reef and G Chromitite bodies. Our data reveal concentrations of 4 to 13 500 ppm for Cl, 26 ppb to 360 ppm for Br, and <1 ppb to 9 ppm I over the whole sample set. Cl, Br, and I correlate well with each other implying a shared process and/or distribution in mineral species. Br/Cl and I/Cl ratios span a range from 0.3 to 35 × 10 −3 and 5 to 900 × 10 −6 by weight, respectively, encompassing MORB-like to more enriched compositions, particularly for Br/Cl. High-Br/Cl ratios compared to MORB in some Stillwater samples suggest fractionation of halogens during the exsolution of a volatile-rich fluid to explain the most Br-enriched samples. More generally, the presence of minerals such as scapolite, hornblende, and apatite in the most halogen-enriched samples suggests that the halogen-bearing fluids were derived from the cooling of the intrusion rather than late-stage (low-temperature) metamorphism. The combined halogen abundance and noble gas isotope data set imply that crustal contamination may have played a limited role in the crystallization of pegmatoids and the G Chromitite but is not required to account for the halogen budget of the J-M Reef. High-halogen contents in the sulfide-bearing J-M Reef and associated lithologies are consistent with the influence of fluid-related activity during platinum-group element (PGE)-Reef formation, lending weight to the hydromagmatic model for mineralization in the Stillwater intrusion. Our new data also imply chalcophile tendencies of Br and I over Cl in sulfides in natural systems, hinting at the importance of sulfide liquid interaction with halogen-rich fluids in the formation of sulfide-hosted precious metal ore deposits.

Knowledge of volatile cycling is vital to understanding the evolution of the planet and the life it supports. Although it has been gradually accepted that the mantle is a vast storehouse of H 2 O and other volatiles, the impact of coexisting volatiles on the thermal stabilities of OH and the lattice of the host mineral is still poorly understood. Phlogopite is one of the few hydrous minerals capable of carrying both water and halogens into the mantle. Previous observations from both experiments and textural relationships in natural samples have indicated that F-rich phlogopite can be stable under ultrahigh-temperature conditions. Here, the impact of F on the thermal stability of phlogopite was investigated via XRD, Raman, and IR spectroscopy from room temperature to 1000 to 1200 °C. Based on the experimental results from F-poor and F-rich natural phlogopites, we show that about 4 wt% F can increase the breakdown temperature of phlogopite by 100 °C under ambient pressure. The impact mechanism mainly involves preventing OH and lattice softening at high temperatures. This study reveals the links between F and the behavior of OH and phlogopite lattice, which is important for constraining volatile cycling, as well as the role of F in the physical and chemical properties of the upper mantle.

Ferrous hydroxychlorides are geochemically important but less recognized mineral species due to their extreme sensitivity to oxidation and hydration in contact with air {typically they convert to akaganéite [Fe 3+ (O,OH,Cl)]}. Only the γ-form was previously known as the orthorhombic mineral hibbingite, associated with altered mafic intrusive rocks. In this study, we describe the β-polymorph of Fe 2 (OH) 3 Cl as a new mineral parahibbingite that was found in pyroxenite from the Karee platinum mine in the Bushveld Complex, South Africa. The two minerals were distinguished by a combination of Raman spectroscopy and FIB-SEM-TEM analytical techniques (TEM-EDX and TEM-SAED). They can be easily recognized by their distinct Raman spectra. Parahibbingite has two very strong vibration bands at ~3550 and 3560 cm –1 , accompanied by much weaker bands at ~124 and 160 cm −1 , while the Raman spectrum of hibbingite has a sharp, strong band at 3450 cm −1 and two moderate bands at 199 and 385 cm −1 . Parahibbingite was found as fine-grained reaction rims at the contact of orthopyroxene phenocrysts and talc inside a drill core. It has a trigonal space group [ R 3 m , a = 6.94(5) Å; c = 14.5(2) Å], with an empirical formula (Fe 2 1.98 + Mn 2 0.01 + Ca 0.01 )(OH) 3.08 Cl 0.92 . The origin of this mineral in the Bushveld Complex is most likely related to a late hydrothermal alteration of pyroxenite. Hibbingite forms as an abundant daughter mineral hosted by fluid inclusions and salt melt inclusions in hydrothermal quartz associated with granitic systems during cooling under reducing conditions. Such inclusions are common in Au-porphyry mineralization worldwide, such as the Biely Vrch (Slovakia) deposit studied in detail in this work. The lattice parameters obtained by TEM-SAED are a = 6.30 Å, b = 7.12 Å, and c = 9.89 Å. Hibbingite was recognized as the only phase that carries “water” (as a hydroxyl group) in otherwise water-free, salt melt inclusions. Furthermore, both hibbingite and parahibbingite should be considered as reservoirs for Cl and H 2 O in large volumes of altered basic and ultrabasic rocks. They can transport volatiles to shallow levels of subduction zones. Alternatively, their dissolution can fuel remobilization, transport, and deposition of sulfidic ores in saline fluids. Their detection, however, is difficult because of their sensitivity to oxidizing atmospheres. For example, in natural outcrops exposed to air, they may vanish, thus distorting estimates of their abundance and role in many processes that involve mineral-derived volatiles.

Chukochenite, (Li 0.5 Al 0.5 )Al 2 O 4 , is a new mineral species from the Xianghualing skarn, Hunan Province, southern China. It occurs as subhedral to euhedral crystals up to 200 μm across in the green rock of Xianghualing skarn, closely associated with fluorite, phlogopite, chrysoberyl, margarite, chlorite, ferronigerite-2 N 1 S , and zinconigerite-2 N 1 S . The crystals are colorless and transparent with a vitreous luster. Chukochenite is brittle with irregular fracture, has a Mohs hardness of 8, and shows light red fluorescence under 253.7 nm UV radiation and light green fluorescence under 365 nm UV radiation. The calculated density is 3.771 g/cm 3 . Chukochenite is optically biaxial (–) with α = 1.79(2), β = 1.82(2), and γ = 1.83(2) (589 nm). The calculated 2 V is 60°, with the optical orientations X , Y , and Z parallel to the crystallographic a , b , and c , respectively. Electron microprobe analysis (Li by LA-ICP-MS) yielded in wt% Al 2 O 3 80.70, Fe 2 O 3 8.16, Li 2 O 3.68, ZnO 3.25, MnO 2.49, MgO 1.70, Na 2 O 0.11, CaO 0.08, TiO 2 0.02, K 2 O 0.01, and Cr 2 O 3 0.01 (total 100.24 wt%), giving an empirical formula [(Li 0.355 Al 0.138 Na 0.005 Ca 0.002 ) Σ0.5 (Al 0.145 Fe + 0.147 3 Mg 0.061 Zn 0.058 Mn 0.051 Si 0.001 ) Σ0.463 ]Al 2 O 4 on a basis of 4 O atoms per formula unit. Chukochenite is orthorhombic, Imma , a = 5.659 (1), b = 16.898 (1), c = 7.994 (1) Å, V = 764.46 (8) Å 3 , and Z = 12. The nine strongest lines of powder XRD [ d in Å ( I ) ( hkl )] are: 2.405 (53) (231); 1.996 (29) (260); 1.535 (77) (303); 1.413 (100) (264); 1.260 (52) (2 12 0); 1.068 (36) (1 13 4); 1.039 (61) (503); 0.999 (59) (008); and 0.942 (35) (3 13 4). Chukochenite has a framework structure of spinel with low symmetry (orthorhombic Imma ) due to the ordering of Li cations over octahedrally coordinated sites, which has not been previously reported for synthetic (Li 0.5 Al 0.5 ) Al 2 O 4 . This structure type is based on a framework of AlO 4 tetrahedra, AlO 6 , and LiO 6 octahedra. AlO 6 edge-sharing octahedra form chains along the a axis. AlO 6 octahedra and LiO 6 octahedra in a 2:1 ratio share edges, forming octahedral chains along b . These octahedral chains are connected by AlO 4 tetrahedra and each corner of an AlO 4 tetrahedron shares with three AlO 6 octahedra or two AlO 6 + one LiO 6 octahedra. The discovery of chukochenite adds a new perspective on the cation ordering and the mechanism of luminescence and magnetism in (Li 0.5 Al 0.5 )Al 2 O 4 .

Pyrite trace element (TE) chemistry is now widely employed in studies of past ocean chemistry. Thus far the main proof of concept has been correlation between large data sets of pyrite and bulk analyses emphasizing redox sensitive TE data from ancient samples spanning geologic time. In contrast, pyrite TE data from modern settings are very limited. The sparse available data are averages from samples from the Cariaco Basin without stratigraphic resolution and from estuarine sediments. To fill this gap, we present TE data (Co, Ni, Cu, Zn, Mo, Ag, Pb, Bi) from the two largest euxinic basins on Earth today, locations where the majority of the pyrite formed within the water column, the Black Sea and Cariaco Basin. These locations have different water column TE contents due to their relative degrees of restriction from the open ocean, thus providing an ideal test of the relationship between pyrite precipitated under euxinic conditions from basins with different degrees of basin restriction and dissolved TE concentration. At each site we observed that down-core trends for pyrite increase before reaching relatively steady values for most TE. This observation suggests that instead of all the TE being sourced directly from the water column, some are incorporated from the sediments, presumably desorbing from detrital materials. However, since much of the adsorbed TE is adsorbed from the overlying water, the pyrite chemistry still seems to reflect the water chemistry at or near the surface. Indeed, for Mo, there is less variation in pyrite than in bulk sediment. Additionally, we found that pyrite formed during diagenesis due to sulfide difusion into iron-rich muds revealed low-TE contents, except for siderophile elements likely to have been adsorbed onto Fe (hydr)oxides, highlighting the risk of potential false negatives from pyrite formed under these conditions. This relationship highlights the need for detailed understanding of the full context, including the use of complementary geochemical data such as sulfur isotope trends, in eforts to use pyrite TE to interpret conditions in the global ocean.

In situ oxygen isotope compositions of white mica and quartz have been used to characterize the interplay of metamorphism and fluid events between a metasomatic whiteschist and its granite protolith in the Monte Rosa nappe, Western Alps. New natural muscovite and phengite reference materials were calibrated for in situ Secondary Ion Mass Spectrometry (SIMS) oxygen isotope measurement. White mica and quartz oxygen isotope compositions were measured in situ in one whiteschist and two metagranites. Based on microtextural observation, phengite composition of white mica, and phase petrology modeling, it is possible to identify two events of fluid infiltration and one event of fluid expulsion, all of which were responsible for forming this unique whiteschist occurrence and for tracing its metamorphic evolution from late Permian intrusion to Alpine subduction and finally to the present day, exhumed whiteschist. Metagranite samples contain three generations of white mica: igneous, high- P metamorphic, and late Alpine, retrograde compositions. In the whiteschist samples, we distinguish two distinct Alpine white mica generations: (1) prograde to peak generation and (2) retrograde generation. The δ 18 O VSMOW values of white mica and quartz from a whiteschist of 5.3 to 7.3‰ and 9.1 to 10.6‰ are significantly lower than in the metagranites, with 9.1 to 10.8‰ and 13.2 to 14.6‰, respectively. This indicates a complete recrystallization of the whiteschist protolith during intense fluid-rock interaction. Subsequent Alpine metamorphism transformed the protolith into the whiteschist. The isotopic composition of the whiteschist, fine-grained, retrograde white mica (5.3 to 6‰) is lower than that of the high-pressure phengite (6.2 and 7.5‰). The low δ 18 O values could be explained by infiltration of external fluids with δ 18 O values of 2 to 6‰. Such fluids would carry the isotopic signature of the serpentinites of the Piemonte-Liguria Ocean by either equilibration of fluids with or dehydration of serpentinites. Another, more simple explanation would be the infiltration of very small quantities of fluids leading to the breakdown of chloritoid. Local inheritance of the oxygen composition would then hide the origin of the fluids. Isotope exchange temperatures calculated from high- P phengite-quartz pairs in whiteschist give an average temperature of 440 ± 50 °C. These are lower than the best T -estimates from phase petrology of 570 °C, at 2.2 GPa. Igneous muscovite-quartz pairs in the metagranite yield 400 ± 40 °C. Only one high- P phengite-quartz pair was analyzed, resulting in 350 ± 40 °C. Greenschist facies, low silica phengites give an average temperature of 310 ± 10 °C. Propagation of analytical uncertainty suggests large errors of 60 to 120 °C, due to the relatively small T -dependence of the quartz-white mica fractionation factor for oxygen isotopes.

Hibonite (CaAl 12 O 19 ) is a common refractory mineral in Ca-Al-rich inclusions (CAIs) in primitive meteorites. Transmission electron microscope (TEM) studies have identified enigmatic planar defects in diferent occurrences of hibonite in the Allende meteorite that give rise to strong streaking along c * in electron diffraction patterns. Atomic resolution high-angle annular dark-field (HAADF) imaging and energy-dispersive X‑ray (EDX) analyses were used to determine the nature and origin of these planar features. HAADF images of hibonite grains reveal lamellar intergrowths of common 1.6 nm spacing, and less commonly 2.0 and 2.5 nm spacings, interspersed in stoichiometric hibonite showing 1.1 nm (002) spacing. Stoichiometric hibonite consists of alternating Ca-containing (“R”) and spinel-structured (“S”) blocks stacked in a sequence RS. In contrast, the 1.6 nm layers result from a doubled S block such that the stacking sequence is RSS, while in the widest defect observed, the stacking sequence is RSSSS. These intergrowths are epitaxial and have coherent, low-strain boundaries with the host hibonite Meteoritic hibonite shows common Ti and Mg substitution for Al in its structure. Atomic-resolution EDX maps of hibonite grains in the Allende CAI confirm the preferred site occupancy of Mg on tetragonal M3 sites in S blocks and of Ti on trigonal bipyramidal M2 and octahedral M4 sites in R blocks. Mg is highly concentrated, but Ti is absent in the planar defects where wider S blocks show Al-rich compositions compared to stoichiometric MgAl 2 O 4 spinel. Therefore, Mg likely played the major role in the formation and metastability of planar defects in hibonite. Electron energy loss spectroscopy data from the Ti L 2,3 edge show the presence of mixed Ti oxidation states with ~15–20% of Ti as Ti 3+ in hibonite, suggesting a direct substitution of Ti 3+ ↔ Al 3+ in hibonite. The remaining ~80–85% of Ti is present as Ti 4+ and corresponding EDX analyses are consistent with the well-known coupled substitution 2Al 3+ ↔ Ti 4+ + Mg 2+ being the major mechanism for Ti and Mg substitution in hibonite. The formation of planar defects in hibonite occurred during high-temperature nebular condensation or melting/crystallization processes. The occurrence of non-stoichiometric hibonite in the Allende CAI deviates from the mineral formation sequence predicted from equilibrium condensation models. Overall, our atomic resolution TEM observations signify non-equilibrium, kinetic-controlled crystal growth during the high-temperature formation of refractory solids in the early solar nebula.

Dense hydrous magnesium silicates (DHMSs) with large water contents and wide stability fields are a potential H 2 O reservoir in the deep Earth. Al-bearing superhydrous phase B (shy-B) with a wider stability field than the Al-free counterpart can play an important role in understanding H 2 O transport in the Earth’s transition zone and topmost lower mantle. In this study, a nominally Al-free and two different Al-bearing shy-B samples with 0.47(2) and 1.35(4) Al atoms per formula unit (pfu), were synthesized using a rotating multi-anvil press. Their single-crystal structures were investigated by X‑ray diffraction (XRD) complemented by Raman spectroscopy and Fourier-transform infrared spectroscopy (FTIR). Single-crystal XRD shows that the cell parameters decrease with increasing Al-content. By combining X‑ray diffraction and spectroscopy results, we conclude that the Al-poor shy-B crystallizes in the Pnn 2 space group with hydrogen in two different general positions. Based on the results of the single-crystal X‑ray diffraction refinements combined with FTIR spectroscopy, three substitutions mechanisms are proposed: 2Al 3+ = Mg 2+ + Si 4+ ; Mg 2+ = □ Mg2+ + 2H + (□ Mg2+ means vacancy in Mg site); Si 4+ = Al 3+ + H + . Thus, in addition to the two general H positions, hydrogen is incorporated into the hydrous mineral via point defects. The elastic stiffness coefficients were measured for the Al-shy-B with 1.35 pfu Al by Brillouin scattering (BS). Al-bearing shy-B shows lower C 11 , higher C 22 , and similar C 33 when compared to Al-free shy-B. The elastic anisotropy of Al-bearing shy-B is also higher than that of the Al-free composition. Such different elastic properties are due to the effect of lattice contraction as a whole and the specific chemical substitution mechanism that affect bonds strength. Al-bearing shy-B with lower velocity, higher anisotropy, and wider thermodynamic stability can help understand the low-velocity zone and the high-anisotropy region in the subducted slab located in Tonga.

In the field of biomaterials, octacalcium phosphate (OCP) and biocompatible elements doped with OCP-based materials are attractive materials for new bone substitution because they could be used to control the bone remodeling process in patients with bone diseases. Manganese (Mn) might be a good substitutional element candidate because of its regulation process of bone remodeling for controlling osteo-cellular activities. However, Mn strongly inhibits OCP formation. This study demonstrates that the sodium (Na)-induced OCP formation enhancing the HPO 4 -OH layer structure of the OCP can overcome this Mn inhibition effect. The Mn-substituted OCP can be fabricated from the coexisting solutions of Na and Mn. The results show that the Mn-substituted OCP-induced Na (OCP-Mn,Na) showed a 4.7° peak in the X‑ray diffraction pattern. The sub-peaks at 9.2° and 9.7° of the OCP disappeared, but an extra peak at 9.3° was observed. The thermal stability of the OCP-Mn,Na was significantly lower than that of the conventional OCP because the layer structure of the OCP-Mn,Na decomposed above ~70 °C. This ionic conjugation to Mn is a unique phenomenon for Na, unlike other cations.

Olivine grains from Songshugou mylonitized peridotite massif record δ 18 O both lower and higher than in pristine mantle samples in North Qinling Orogen, Central China. Olivines from dunites exhibit large variations in δ 18 O (4.03–7.07‰), and some porphyroclasts display negative correlations between δ 18 O and forsterite content {Fo; [100×Mg/(Mg+Fe 2+ )]}. The porphyroclast cores have low-δ 18 O values, indicating that they formed in the oceanic lithospheric mantle prior to subduction. We attribute low-δ 18 O values to seawater-peridotite interaction under high-temperature conditions. The porphyroclast rims and small olivines exhibit high-δ 18 O values. These features suggest that high-δ 18 O olivines formed during mylonitization in the exhumation process. Olivines reacted with 18 O-rich melt/fluids released from subducted altered oceanic basalts and continental sediments at low temperature (<610–680 °C). The 18 O-rich melt/fluids selectively affected porphyroclast rims and small olivine grains. Unlike the olivines in the dunites, the olivines and orthopyroxenes in the harzburgites show limited variations in δ 18 O (4.21–5.45‰ and 5.5–5.8‰, respectively), due to orthopyroxene exchange with melt/fluid at a slower rate than the coexisting olivine. The preservation of the low-δ 18 O signature in olivines indicates a short residence time (<20 Ma) for subducting peridotites to mantle depths.

This paper presents the results of an investigation of the incorporation of Au within pyritic ore from the Qiucun epithermal Au deposit, China. The new data provide insights into the mode of occurrence of Au during fluid-rock interactions within epithermal systems. The distribution and mode of occurrence of Au within arsenian pyrite were investigated using a chemical and structural characterization-based approach combining laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) trace element analysis, megapixel synchrotron X‑ray fluorescence analysis (MSXRF), and atom probe tomography (APT). The resulting data indicate that invisible Au is present at elevated concentrations in the form of a homogeneous solid solution within As-rich pyrite domains, which yields Au concentrations that positively correlate with As. Arsenic-induced lattice defects, such as stacking faults and the expansion of the pyrite unit cell, provide evidence of the effect of As on the incorporation of Au into pyrite. The nucleation and crystallization of electrum preferentially occurred at the fluid-pyrite reaction interface or along fractures and grain boundaries within the pre-existing pyrite. This study indicates that changes in physicochemical conditions (e.g., temperature, pH, and sulfur fugacity) during fluid-pyrite interactions are key controls on the development of nanometer- or micrometer-scale clusters of gold. The systematic compositional and textural observations documented in this study provide new insights into the mechanisms responsible for the different modes of occurrence of Au (ionic vs. particulate) and enable us to further understand the processes involved in the formation of Au mineralization.

Xuite, Ca 3 Fe 2 [(Al,Fe)O 3 (OH)] 3 , is a new member of the garnet supergroup discovered in basaltic scoria from Menan Volcanic Complex, Idaho, U.S.A. Oxidation of Fe-bearing pyroclasts at high temperatures led to the formation of xuite, together with luogufengite, valleyite, and hematite. The measured crystal size of xuite ranges from ~200 to 800 nm. The empirical chemical formula of xuite is (Ca 0.92 Mg 0.08 ) 3 (Fe 0.96 Ti 0.04 ) 2 [(AlO 4 H) 0.44 (FeO 4 H) 0.33 (SiO 4 ) 0.05 (◻O 4 H 4 ) 0.18 ] 3 . Xuite has a space group of Ia 3 d ; its unit-cell parameter refined from high-resolution synchrotron X‑ray diffraction (XRD) data is a = 12.5056(5) Å, with Z = 8 (calculated density = 3.53 g/cm 3 ). Fourier-transform infrared spectroscopy spectrum of xuite shows absorption bands at 3682 and 3579 cm −1 , indicating the presence of OH – in the hydrogarnet structure. In situ high-temperature synchrotron XRD combined with thermogravimetry and differential scanning calorimetry reveals that xuite undergoes dehydroxylation to form brownmillerite (Ca 2 FeAlO 5 ) from ~236 to ~396 °C. Xuite occurs in the form of nanocrystals with a soft magnetic property, which provides important insights into the origin of basaltic scoria and associated paleomagnetism. Xuite was also found in Wyoming paralava, suggesting the possibility of its wide occurrence in various geological environments. The mineral was named after Huifang Xu and Hongwu Xu in honor of their sustained contributions to mineral sciences.

Water content and oxygen isotopes in zircon provide crucial constraints on magma source and process, yet they can be significantly modified by zircon metamictization, which causes secondary water absorption into the zircon crystal and the concomitant oxygen isotope changes. Therefore, it is imperative to develop a screening scheme to select the least-metamict zircons for the analyses. We propose a screening scheme based on our study on the Suzhou A-type granite (South China) through integrating Raman spectroscopy, water and trace element measurements, and oxygen isotope analysis. The results show that the primary water content is retained in zircon when the full-width at half maximum (FWHM) is <8 cm −1 or the Raman shift is >1007 cm −1 of v 3 (SiO 4 ) vibration band, while the primary δ 18 O is preserved at <10 cm −1 FWHM or >1005.5 cm −1 Raman shift. Changes in trace element concentrations in Suzhou zircons are different from previous observations in metamict zircons but most likely related to magma evolution, which implies that trace elements are insensitive to metamictization. Primary δ 18 O in Suzhou zircons (4.5–6.0‰) fall into the mantle range, indicating a dominant mantle contribution to Suzhou granites. Primary water content was estimated at ca. 650–1400 ppm, significantly higher than those of typical I-type granite (400–736 ppm) and upper mantle-derived zircons (81–177 ppm). The high primary zircon water content was not controlled by the sub-solidus process, temperature, pressure, and cation charge balance but considered to reflect the high-water content in melts. This suggests a hydrous origin for the Suzhou A-type granite, which challenges the conventional view of anhydrous petrogenesis for A-type granites.

In spite of considerable efforts to understand the role of halogens (F, Cl, Br, I) in soil, concentration data for different soil size fractions is still sparse and information on the sorption behavior of halogens in natural soils is limited. We determined total halogen concentrations in different soil horizons and particle size fractions (i.e., coarse silt with 20–63 μm, medium and fine silt with 2–20 μm, coarse clay with 0.2 to <2 μm and medium clay with 0.02–0.2 μm) of a Cambisol from a temperate forest ecosystem in SW Germany. Furthermore, we estimated the minimum proportions of sorbed halogens onto clay minerals and pedogenic oxides for different soil horizons and different particle size fractions. Vertical depth profiles of halogens in the individual soil particle size fractions matched with the bulk soil vertical patterns. The lack of vertical differences of total halogens concentrations (F tot , Br tot , and I tot ) in the mineral soil during soil development may be due to steady state or equilibrium conditions between weathering, sorption processes, and surface input. In contrast, the vertical depth pattern of Cl tot tended to decrease, suggesting the process of Cl accumulation in the topsoil and nutrient uplift. While F was likely mainly incorporated into the crystal lattice of clay minerals and gibbsite occupying OH-sites, significant amounts of the halogens with larger ionic radii (Cl, Br, and I) were sorbed. The largest amounts (around 90% Cl and 70% Br and I, respectively) were sorbed on the smallest particle size fraction investigated (medium clay fraction; 0.02–0.2 μm), although this fraction only contributed about 1 wt% to the bulk soil. This is probably related to the highest sorption capacity of small particles due to their large surface area. Our study provides new data on sorption behavior of the various halogens in soils of forest ecosystems, which is different between F and the heavier halogens (Cl, Br, I) and further depends on soil particle sizes. The understanding of the chemical behavior of halogens in soils has implications for the retention processes of pollutants in landfills or radioactive waste disposal.

Quartz is abundant in the Earth’s continental crust and persistent throughout the geological record. Trace element signatures in silica minerals can be used to infer processes operating in magmatic and hydrothermal systems. Conventional analyses of trace elements in silica minerals are limited by either spatial or mass resolution [e.g., wavelength-dispersive X‑ray spectroscopy, micro-X‑ray fluorescence, laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), and secondary ion mass spectrometry (SIMS)]. Time-of-flight SIMS (TOF-SIMS) is a relatively new technique for geological applications and provides both high spatial and mass resolution. This minimally destructive, in situ technique rapidly acquires a full suite of elements down to tens of nanometers depth. No previous study has utilized TOF-SIMS to analyze quartz or silica. Four samples of silica minerals representing distinct environments in a magmatic-hydrothermal system were characterized with optical microscopy and qualitative cathodoluminescence (CL), quantitatively analyzed for trace elements with a 157 nm LA-ICP-MS, and qualitatively mapped for trace elements using TOF-SIMS. The novel technique produced maps of trace element distribution in silica minerals to a maximum resolution of 65 nm and consistently resolved light elements (including Li) to 195 nm. That makes this study the highest resolution geochemical characterization of silica minerals and places it among the highest resolution analyses by TOF-SIMS, or any technique, for that matter. TOF-SIMS isotope maps differentiate trace elements hosted in nano- and micro-inclusions from lattice incorporation in quartz and cryptocrystalline silica—an impossibility for lower resolution techniques, allowing insights into cations substituting for Si 4+ in the crystal lattice and their role in activating CL in low-temperature epithermal quartz. Further development of this technique could see TOF-SIMS become a routine tool for measuring diffusion profiles in a range of other geological materials. Quantification of TOF-SIMS would revolutionize mineral characterization, especially given its temporal efficiency and low-sampling volume.

Banded iron formations (BIFs) are iron-rich marine chemical sedimentary rocks, and their mineralogy and geochemistry can be used to gain insights into ancient ocean chemistry and biospheric evolution. Magnetite is the major iron-bearing mineral in many BIFs (particularly in the Archean) and is variably interpreted to be of primary, early diagenetic, or metamorphic origin. Different genetic interpretations for magnetite lead to divergent pictures of the Precambrian Earth system and its evolutionary models through time. The Baizhiyan Formation of the Neoarchean Wutai Group (Shanxi, North China) features magnetite-bearing, Algoma-type BIFs deposited ca. 2.52 Ga, in the lead-up to a major period of global iron formation deposition in the Paleoproterozoic. Abundant magnetite crystals found in the silica-rich bands of these BIFs show euhedral, hexagonal morphology. We suggest that this hexagonal magnetite likely represents pseudomorphs after green rust, a mixed-valence iron hydroxy-salt formed in the water column. The rare earth element composition of the BIFs shows negligible to slightly positive Ce anomalies (Ce SN /Ce SN * = 1.03 ± 0.07), which is characteristic of a dominantly anoxic water column. The presence of positive Eu anomalies (Eu SN /Eu SN * <3.9) suggests a substantial influence from proximal hydrothermal fluids. The co-occurrence of siderite layers associated with the magnetite-bearing strata may indicate iron cycling associated with ferruginous bottom seawater conditions. Geochemical signatures of the Baizhiyan BIFs are consistent with the interpretation that the magnetite was transformed from metastable green rust. This green rust could have formed via several processes, including the partial oxidation of Fe(II) by molecular oxygen/photoferrotrophs, the reaction of settling ferrihydrite with Fe(II)-rich hydrothermal fluids under anoxic conditions, or local dissimilatory iron reduction. In all cases, the contribution of primary green rust to BIF formation requires iron redox cycling, and similar pseudomorphs in the form of hexagonal magnetite may be more common in the geological record. Our findings support the models in which green rust was an important primary constituent of the Precambrian iron cycle, and the potential interactions of green rust with other elements (e.g., phosphorus) should be taken into consideration when reconstructing Precambrian biogeochemical cycles.