Volume 104, Issue 10

There are three major reservoirs for carbon in the Earth at the present time, the core, the mantle, and the continental crust. The carbon in the continental crust is mainly in carbonates (limestones, marbles, etc.). In this paper we consider the origin of the carbonates. In 1952, Harold Urey proposed that calcium silicates produced by erosion reacted with atmospheric CO 2 to produce carbonates, this is now known as the Urey reaction. In this paper we first address how the Urey reaction could have scavenged a significant mass of crustal carbon from the early atmosphere. At the present time the Urey reaction controls the CO 2 concentration in the atmosphere. The CO 2 enters the atmosphere by volcanism and is lost to the continental crust through the Urey reaction. We address this process in some detail. We then consider the decay of the Paleocene-Eocene thermal maximum (PETM). We quantify how the Urey reaction removes an injection of CO 2 into the atmosphere. A typical decay time is 100 000 yr but depends on the variable rate of the Urey reaction.

The geologic carbon cycle plays a fundamental role in controlling Earth’s climate and habitability. For billions of years, stabilizing feedbacks inherent in the cycle have maintained a surface environment that could sustain life. Carbonation/decarbonation reactions are the primary mechanisms for transferring carbon between the solid Earth and the ocean–atmosphere system. These processes can be broadly represented by the reaction: CaSiO3  (wollastonite)+CO2  (gas)↔CaCO3  (calcite)+SiO2  (quartz).$\text{CaSi}{{\text{O}}_{\text{3}\,\,\left( \text{wollastonite} \right)}}+\text{C}{{\text{O}}_{\text{2}\,\,\left( \text{gas} \right)}}\leftrightarrow \text{CaC}{{\text{O}}_{\text{3}\,\,\left( \text{calcite} \right)}}+\text{Si}{{\text{O}}_{\text{2}\,\,\left( \text{quartz} \right)}}.$This class of reactions is therefore critical to Earth’s past and future habitability. Here, we summarize their significance as part of the Deep Carbon Obsevatory’s “Earth in Five Reactions” project. In the forward direction, carbonation reactions like the one above describe silicate weathering and carbonate formation on Earth’s surface. Recent work aims to resolve the balance between silicate weathering in terrestrial and marine settings both in the modern Earth system and through Earth’s history. Rocks may also undergo carbonation reactions at high temperatures in the ultramafic mantle wedge of a subduction zone or during retrograde regional metamorphism. In the reverse direction, the reaction above represents various prograde metamorphic decarbonation processes that can occur in continental collisions, rift zones, subduction zones, and in aureoles around magmatic systems. We summarize the fluxes and uncertainties of major carbonation/decarbonation reactions and review the key feedback mechanisms that are likely to have stabilized atmospheric CO 2 levels. Future work on planetary habitability and Earth’s past and future climate will rely on an enhanced understanding of the long-term carbon cycle.

Calcium carbonate (CaCO 3 ) and particularly its stable phase, calcite, is of great geological significance in the deep carbon cycle since CaCO 3 from biomineralized shells and corals form sedimentary rocks. Calcite also attracts attention in medical science and pharmacy as a primary or intermediate component in biomaterials because it possesses excellent biocompatibility along with suitable physicochemical properties. Calcite blocks have already been used during surgical procedures as a bone substitute for reconstructing bone defects formed by diseases and injury. When producing CaCO 3 biomaterials and bioceramics, in particular, in vivo control of the size and polymorphic nature of CaCO 3 is required. In this study, we investigated the effects of PO 4 on calcite formation during the phase conversion of calcium sulfate anhydrate (CaSO 4 , CSA), which is sometimes used as a starting material for bone substitutes because of its suitable setting ability. CSA powder was immersed in 2 mol/L Na 2 CO 3 solution containing a range of PO 4 concentrations (0–60 mmol/L) at 40 °C for 3 days. The treated samples were investigated by X‑ray diffraction, Fourier-transform infrared spectroscopy, X‑ray fluorescence spectroscopy, and thermal analysis. In addition, the fine structures of the treated samples were observed by field-emission scanning electron microscopy, and the specific surface area was measured. We found that PO 4 , which is universally present in vivo, can modulate the calcite crystal size during calcite formation. A fluorescence study and calcite crystal growth experiments indicated that PO 4 adsorbs tightly onto the surface of calcite, inhibiting crystal growth. In the presence of high PO 4 concentrations, vaterite is formed along with calcite, and the appearance and stability of the CaCO 3 polymorphs can be controlled by adjusting the PO 4 concentration. These findings have implications for medical science and pharmacology, along with mineralogy and geochemistry.

Bastnäsite-(Ce), a rare earth element (REE) bearing carbonate (Ce,La,Y,Nd,Pr)CO 3 F, is one of the most common REE-bearing minerals and has importance from both economic and geologic perspectives due to its large REE concentration. It also provides an example of the structural interplay between carbonate groups and fluorine ions, as well as the complex bonding properties of rare earth elements. We report Raman vibrational and Nd 3+ luminescence (4F3/2→4I9/2,4F3/2→4I11/2,$\left( ^{4}{{\text{F}}_{3/2}}{{\to }^{4}}{{\text{I}}_{9/2}}{{,}^{4}}{{\text{F}}_{3/2}}{{\to }^{4}}{{\text{I}}_{11/2}}, \right.$and 4F5/2+2H9/2→4I9/2)$\left. ^{4}{{\text{F}}_{5/2}}{{+}^{2}}{{\text{H}}_{9/2}}{{\to }^{4}}{{\text{I}}_{9/2}} \right)$spectra of natural bastnäsite-(Ce) to 50 GPa at 300 K. Two phase transitions are observed under compression. Bastnäsite-I remains the stable phase up to 25 GPa, where it undergoes a subtle phase transition to bastnäsite-II. This is likely produced by a change in symmetry of the carbonate ion. Bastnäsite-II transforms to bastnäsite-III at ~38 GPa, as demonstrated by changes in the luminescence spectra. This second transition is particularly evident within the 4F3/2→4I9/2$^{4}{{\text{F}}_{3/2}}{{\to }^{4}}{{\text{I}}_{9/2}}$luminescent transitions, and it appears that a new rare earth element site is generated at this phase change. This transition is also accompanied by modest changes in both the Raman spectra and two sets of luminescent transitions. Despite these transformations, the carbonate unit remains a stable, threefold-coordinated unit throughout this pressure range, with a possible increase in its distortion. Correspondingly, the rare-earth element site(s) appears to persist in quasi-ninefold coordination as well, implying that the general bonding configuration in bastnäsite is at least metastable over a ~30% compression range. All pressure-induced transitions are reversible, with some hysteresis, reverting to its ambient pressure phase on decompression.

Low-pressure metastable nanoscale crystals of α-cristobalite have been observed epitaxially ex-solved in cores of UHP clinopyroxene from the Bohemian Massif, Czech Republic. SAED patterns and HRTEM images detail the close structural relationship between host clinopyroxene and α-cristobalite precipitate: [001] Di ║[010]a, (010) Di ~ ║(101)a. TEM results indicate that α-cristobalite exsolved from host clinopyroxene. Non-crystalline Al-bearing silicate phases, also exsolved from UHP clinopyroxene, possesses Al/Si ratios close to eutectic compositions in the system NaAlSi 3 O 8 -SiO 2 -H 2 O system. The presence of glass exsolution suggests a high-temperature formation environment and presence of water. The α-cristobalite formed in a localized low-pressure, micro-environment formed through exsolution of vacancies and excess silica from the host pyroxene lattice. This micro-environment may be a result of negative density changes due to excess lower density silica exsolving from higher density pyroxene during an exsolution process that involved no localized volume change. Interface-controlled exsolution via lattice matching at the diopside/cristobalite interface, and stability changes and melting point depression due to nanoscale size effects contributed to the formation and persistence of this metastable phase. Amphibole in association with α-cristobalite and some non-crystalline silicate phases may be a clue to localized water quantities; silica exsolution with amphibole may have formed below the eutectic temperature and at a later stage than non-crystalline silicate phases without amphibole. Silica rods in Nové Dvory clinopyroxenes were previously thought to be quartz; however, our investigation reveals various low-pressure, high-temperature, and/or metastable phases greatly affected by the presence of vacancy and OH in clinopyroxenes. The results will help us better understand OH in the UHP pyroxene and even water release in the mantle.

Low-pressure polymorphs of AlOOH and FeOOH are common natural oxyhydroxides at the Earth’s surface, which may transport hydrogen to the deep mantle via subduction. At elevated pressures, the low-pressure polymorphs transform into δ-AlOOH and ε-FeOOH with CaCl 2 -type structure, which form a solid solution above 18 GPa. Nevertheless, few studies have examined the solid solution behavior of this binary system in detail. In this study, we ascertain the phase relations in the AlOOH–FeOOH binary system at 15–25 GPa and 700–1200 °C. X‑ray diffraction (XRD) measurements of quenched samples show that δ-AlOOH and ε-FeOOH partly form solid solutions over wide pressure and temperature ranges. Our results demonstrate that a binary eutectic diagram is formed without dehydration or melting below 1200 °C at 20 GPa. We also observe that the maximum solubilities of Al and Fe in the solid solutions are more strongly influenced by temperature than pressure. Our results suggest that the CaCl 2 -type hydroxides subducted into the deep mantle form a solid solution over a wide composition range. As AlOOH and FeOOH are present in hydrous crust, these phases may be subducted into the deep interior, transporting a significant amount of hydrogen to deeper regions. Therefore, a better understanding of this binary system may help elucidate the model geodynamic processes associated with the deep water cycling in the Earth.

Previous inter-laboratory experiments on confined fission-track length measurements in apatite have consistently reported variation substantially in excess of statistical expectation. There are two primary causes for this variation: (1) differences in laboratory procedures and instrumentation, and (2) personal differences in perception and assessment between analysts. In this study, we narrow these elements down to two categories, etching procedure and analyst bias. We assembled a set of eight samples with induced tracks from four apatite varieties, initially irradiated between 2 and 43 years prior to etching. Two mounts were made containing aliquots of each sample to ensure identical etching conditions for all apatites on a mount. We employed two widely used etching protocols, 5.0 M HNO 3 at 20 °C for 20 s and 5.5 M HNO 3 at 21 °C for 20 s. Sets of track images were then captured by an automated system and exchanged between two analysts, so that measurements could be carried out on the same tracks and etch figures, in the same image data, allowing us to isolate and examine the effects of analyst bias. An additional 5 s of etching was then used to evaluate etching behavior at track tips. In total, 8391 confined fission-track length measurements were performed; along with 1480 etch figure length measurements. When the analysts evaluated each other’s track selections within the same images for suitability for measurement, the average rejection rate was ∼14%. For tracks judged as suitable by both analysts, measurements of 2D and 3D length, dip, and c -axis angle were in excellent agreement, with slightly less dispersion when using the 5.5 M etch. Lengths were shorter in the 5.0 M etched mount than the 5.5 M etched one, which we interpret to be caused by more prevalent under-etching in the former, at least for some apatite compositions. After an additional 5 s of etching, 5.0 M tracks saw greater lengthening and more reduction in dispersion than 5.5 M tracks, additional evidence that they were more likely to be under-etched after the initial etching step. Systematic differences between analysts were minimal, with the main exception being likelihood of observing tracks near perpendicular to the crystallographic c axis, which may reflect different use of transmitted vs. reflected light when scanning for tracks. Etch figure measurements were more consistent between analysts for the 5.5 M etch, though one apatite variety showed high dispersion for both. Within a given etching protocol, each sample reflected a decrease of mean track length with time since irradiation, giving evidence of 0.2–0.3 μm of annealing over year to decade timescales.

Interstratified clay minerals reflect the weathering degree and record climatic conditions and the pedogenic processes in the soil. It is hard to distinguish a few layers of interstratified clay minerals from the chlorite matrix, due to their similar two-dimensional tetrahedral-octahedral-tetrahedral (TOT) structure and electron-beam sensitive nature during transmission electron microscopy (TEM) imaging. Here, we used multiple advanced TEM techniques including low-dose high-resolution TEM (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging combined with energy-dispersive spectroscopic (EDS) mapping to study interstratified layers in a chlorite sample from Changping, Beijing, China. We demonstrated an interstratified mica or pyrophyllite monolayer could be well distinguished from the chlorite matrix by projected atomic structures, lattice spacings, and chemical compositions with advanced TEM techniques. Further investigation showed two different transformation mechanisms from mica or pyrophyllite to chlorite: either a 4 Å increase or decrease in the lattice spacing. This characterization approach can be extended to the studies of other electron-beam sensitive minerals.

The diffusive mass exchange of eight major elements (Si, Ti, Al, Fe, Mg, Ca, Na, and K) between natural, nominally dry shoshonitic and rhyolitic melts was studied at atmospheric pressure and temperatures between 1230 and 1413 °C using the diffusion couple method. For six elements, effective binary diffusion coefficients were calculated by means of a concentration-dependent method to obtain an internally consistent data set. Among these components, the range in diffusivities is restricted, pointing to a coupling of their diffusive fluxes. We find that the calculated diffusivities fit well into the Arrhenius relation, with activation energies ( E a ) ranging from 258 to 399 kJ/mol in rhyolitic (70 wt% SiO 2 ) melt and from 294 to 426 kJ/mol in the latitic melt (58 wt% SiO 2 ). Ti shows the lowest E a , while Si, Fe, Mg, Ca, and K have a similar value. A strong linear correlation is observed between logD 0 and E a , confirming the validity of the compensation law for this system. Uphill diffusion is observed in Al in the form of a concentration minimum in the rhyolitic side of the couple, (at ca. 69 wt% SiO 2 ), and in Na indicated by a maximum in the shoshonitic side (ca. 59 wt% SiO 2 ). Fe shows weak signs of uphill diffusion, possibly due to the contribution of ferric iron. The data presented here extend the database of previously published diffusivities in the shoshonite-rhyolite system (González-García et al. 2017) toward the water-free end and allows us to better constrain the water-dependence of major element diffusion at very low water concentrations. Combining both data sets, we find that log D is proportional to the square root of water concentration for a range between 0 and 2 wt% H 2 O. These results are of particular interest in the study of mass transfer phenomena in alkaline volcanic systems.

The grossular-andradite solid solutions in garnet from skarn deposits in relation to hydrothermal processes and physicochemical conditions of ore formation remain controversial. Here we investigate garnet occurring in association with calcic and magnesian skarn rocks in the Cuihongshan polymetallic skarn deposit of NE China. The calcic skarn rocks contain three types of garnets. (1) Prograde type I Al-rich anisotropic garnets display polysynthetic twinning and a compositional range of Grs 18–80 Adr 10–75 . This type of garnet shows markedly low rare earth element (REE) contents (3.27–78.26 ppm) and is strongly depleted in light rare earth elements (LREE, 0.57–44.65 ppm) relative to heavy rare earth elements (HREE, 2.31–59.19 ppm). They also display a significantly negative Eu anomaly (Eu/Eu* of 0.03–0.90). (2) Fe-rich retrograde type II garnets are anisotropic with oscillatory zoning and own wide compositional variations (Grs 1–47 Adr 30–95 ) with flat REE (13.73–377.08 ppm) patterns. (3) Fe-rich retrograde type III isotropic garnets display oscillatory zoning and morphological transition from planar dodecahedral {110} crystal faces to {211} crystal faces in the margin. Types III garnets exhibit relatively narrow compositional variations of Grs 0.1–12 Adr 85–97 with LREE-enrichment (0.80–51.87 ppm), flat HREE patterns (0.15–2.46 ppm) and strong positive Eu anomalies (Eu/Eu* of 0.93–27.07 with almost all >1). The magnesian skarn rocks contain euhedral isotropic type IV Mn-rich garnet veins with a composition of Grs 10–23 Sps 48–62 Alm 14–29 . All calcic garnets contain considerable Sn and W contents. Type II garnet containing intermediate compositions of andradite and grossular shows the highest Sn contents (64.36–2778.92 ppm), albeit the lowest W range (1.11–468.44 ppm). Birefringence of garnet is probably caused by strain from lattice mismatch at a twinning boundary or ion substitution near intermediate compositions of grossular-andradite. The fine-scale, sharp, and straight garnet zones are probably caused by self-organization, but the compositional variations of zones from core to rim are probably caused by external factors. The zoning is likely driven by external factors such as composition of the hydrothermal fluid. REE concentrations are probably influenced by the relative proportion and temperature of the system. Moreover, the LREE-HREE fractionation of garnet can be attributed to relative compositions of grossular-andradite system. The W and Sn concentrations in garnet can be used as indicators for the exploration of W-Sn skarn deposits.

Gasparite-(La), La(AsO 4 ), is a new mineral (IMA 2018-079) from Mn ores of the Ushkatyn-III deposit, Central Kazakhstan (type locality) and from alpine fissures in metamorphic rocks of the Wanni glacier, Binn Valley, Switzerland (co-type locality). Gasparite-(La) is named for its dominant lanthanide, according to current nomenclature of rare-earth minerals. The occurrences and parageneses in both localities are distinct: minute isometric grains up to 15 μm in size, associated with friedelite, jacobsite, pennantite, manganhumite series minerals (alleghanyite, sonolite), sarkinite, tilasite, and retzian-(La) are typically embedded into calcite-rhodochrosite veinlets (Ushkatyn-III deposit) vs. elongated crystals up to 2 mm in size in classical alpine fissures in two-mica gneiss without indicative associated minerals (Wanni glacier). Their chemical compositions have been studied by EDX and WDX; crystal-chemical formulas of gasparite-(La) from the Ushkatyn-III deposit (holotype specimen) and Wanni glacier (co-type specimen) are (La 0.65 Ce 0.17 Nd 0.07 Ca 0.06 Mn 0.05 Pr 0.02 ) 1.02 [(As 0.70 V 0.28 P 0.02 ) 1.00 O 4 ] and (La 0.59 Ce 0.37 Nd 0.02 Ca 0.02 Th 0.01 ) 1.01 [(As 0.81 P 0.16 Si 0.02 S 0.02 ) 1.01 O 4 ], respectively. In polished sections, crystals are yellow and translucent with bright submetallic luster. Selected reflectance values R 1 / R 2 (λ, nm) for the holotype specimen in air are: 11.19/9.05 (400), 11.45/9.44 (500), 10.85/8.81 (600), 11.23/9.08 (700). The structural characteristics of gasparite-(La) were studied by means of EBSD (holotype specimen), XRD, and SREF (co-type specimen). Gasparite-(La) has a monoclinic structure with the space group P 21/ n . Our studies revealed that gasparite-(La) from the Ushkatyn-III deposit and Wanni glacier have different origins. La/Ce and As/P/V ratios in gasparite-(La) may be used as an indicator of formation conditions.

Annealing is commonly used in the recrystallization of metamict minerals in an attempt to reconstruct the original structure. Annealing at 750 °C of Nb-rich chevkinite-(Ce) from the Biraya rare-metal deposit, Russia, resulted in the structural transformation C 2/ m → P 2 1 / a , which defines chevkinite stability in different environments. This transformation seems to be a rapid version of a naturally occurring process that possibly involves twinning of the crystals. Nb-rich chevkinite-(Ce) occurs naturally as two polymorphs, one with the C 2/ m space group and the other with P 2 1 / a . The latter is the stable form under ambient conditions. There are some distinct differences in the values of the structural parameters, such as the average M -O distances or site scattering values of particular sites for both space groups, which can be associated with the redistribution of some lighter cations, mainly Mg 2+ , within the crystal lattice. The use of complementary experimental techniques (electron probe microanalysis, X‑ray diffraction, and photoelectron spectroscopy) has delivered information on the structure and transformation of a very complex, highly zoned and partially metamict solid solution. It should be useful in determining the structure of any mineral where cation disorder is present.

Although REE (lanthanides + Sc + Y) mineralization in alkaline silicate systems is commonly accompanied with Zr mineralization worldwide, our understanding of the relationship between Zr and REE mineralization is still incomplete. The Baerzhe deposit in Northeastern China is a reservoir of REE, Nb, Zr, and Be linked to the formation of an Early Cretaceous, silica-saturated, alkaline intrusive complex. In this study, we use in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of zircon and monazite crystals to constrain the relationship between Zr and REE mineralization at Baerzhe. Three groups of zircon are identified and are differentiated based upon textural observations and compositional characteristics. Type Ia zircons display well-developed oscillatory zoning. Type Ib zircons are darker in cathodoluminescence images and have more irregular zoning and resorption features than type Ia zircons. In addition, type Ib zircons can locally occur as overgrowths on type Ia zircons. Type II zircons contain irregular but translucent cores and rims with oscillatory zoning that are murky brown in color and occur in aggregates. Textural features and compositional data suggest that types Ia and Ib zircon crystallized at the magmatic stage, with type Ia being least-altered and type Ib being strongly altered. Type II zircons, on the other hand, precipitated during the magmatic to magmatic-hydrothermal transition. Whereas the magnitude of the Eu anomaly is moderate in the barren alkaline granite, both magmatic and deuteric zircon exhibit pronounced negative anomalies. Such features are difficult to explain exclusively by feldspar fractionation and could indicate the presence of fluid induced modification of the rocks. Monazite crystals occur mostly through replacement of zircon and sodic amphibole; monazite clusters are also present. Textural and compositional evidence suggests that monazite at Baerzhe is hydrothermal. Types Ia and Ib magmatic zircon yield 207 Pb-corrected 206 Pb/ 238 U ages of 127.2 ± 1.3 and 125.4 ± 0.7 Ma, respectively. Type II deuteric zircon precipitated at 124.9 ± 0.6 Ma. The chronological data suggest that the magmatic stage of the highly evolved Baerzhe alkaline granite lasted less than two million years. Hydrothermal monazite records a REE mineralization event at 122.8 ± 0.6 Ma, approximately 1 or 2 million years after Zr mineralization. We therefore propose a model in which parental magmas of the Baerzhe pluton underwent extensive magmatic differentiation while residual melts interacted with aqueous hydrothermal fluids. Deuteric zircon precipitated from a hydrosilicate liquid, and subsequent REE mineralization, exemplified by hydrothermal monazite, correlates with hydrothermal metasomatic alteration that postdated the hydrosilicate liquid event. Such interplay between magmatic and hydrothermal processes resulted in the formation of discrete Zr and REE mineralization at Baerzhe.

A series of tourmaline reference materials are developed for in situ oxygen isotope analysis by secondary ion mass spectrometry (SIMS), which allow study of the tourmaline compositions found in most igneous and metamorphic rocks. The new reference material was applied to measure oxygen isotope composition of tourmaline from metagranite, meta-leucogranite, and whiteschist from the Monte Rosa nappe (Western Alps). The protolith and genesis of whiteschist are highly debated in the literature. Whiteschists occur as 10 to 50 m tube-like bodies within the Permian Monte Rosa granite. They consist of chloritoid, talc, phengite, and quartz, with local kyanite, garnet, tourmaline, and carbonates. Whiteschist tourmaline is characterized by an igneous core and a dravitic overgrowth ( X Mg > 0.9). The core reveals similar chemical composition and zonation as meta-leucogranitic tourmaline ( X Mg = 0.25, δ 18 O = 11.3–11.5‰), proving their common origin. Dravitic overgrowths in whiteschists have lower oxygen isotope compositions (8.9–9.5‰). Tourmaline in metagranite is an intermediate schorl-dravite with X Mg of 0.50. Oxygen isotope data reveal homogeneous composition for metagranite and meta-leucogranite tourmalines of 10.4–11.3‰ and 11.0–11.9‰, respectively. Quartz inclusions in both meta-igneous rocks show the same oxygen isotopic composition as the quartz in the matrix (13.6–13.9‰). In whiteschist the oxygen isotope composition of quartz included in tourmaline cores lost their igneous signature, having the same values as quartz in the matrix (11.4–11.7‰). A network of small fractures filled with dravitic tourmaline can be observed in the igneous core and suggested to serve as a connection between included quartz and matrix, and lead to recrystallization of the inclusion. In contrast, the igneous core of the whiteschist tourmaline fully retained its magmatic oxygen isotope signature, indicating oxygen diffusion is extremely slow in tourmaline. Tourmaline included in high-pressure chloritoid shows the characteristic dravitic overgrowth, demonstrating that chloritoid grew after the metasomatism responsible for the whiteschist formation, but continued to grow during the Alpine metamorphism. Our data on tourmaline and quartz show that tourmaline-bearing whiteschists originated from the related meta-leucogranites, which were locally altered by late magmatic hydrothermal fluids prior to Alpine high-pressure metamorphism.

Chenmingite (FeCr 2 O 4 ; IMA 2017-036) is a high-pressure mineral, occurring as micrometer- to submicrometer-sized lamellae within precursor chromite grains along with xieite and Fe,Cr-rich ulvöspinel next to shock-induced melt pockets, from the Tissint martian meteorite. The composition of type chenmingite by electron probe analysis shows an empirical formula of (Fe0.752+Mg0.23Mn0.02)$\left( \text{Fe}_{0.75}^{2+}\text{M}{{\text{g}}_{0.23}}\text{M}{{\text{n}}_{0.02}} \right)$(Cr1.60Al0.29Fe0.063+Fe0.042+Ti0.02)Σ2.01O4.${{\left( \text{C}{{\text{r}}_{1.60}}\text{A}{{\text{l}}_{0.29}}\text{Fe}_{0.06}^{3+}\text{Fe}_{0.04}^{2+}\text{T}{{\text{i}}_{0.02}} \right)}_{\Sigma 2.01}}{{\text{O}}_{4}}.$The general and end-member formulas are (Fe,Mg)(Cr,Al) 2 O 4 and FeCr 2 O 4 . Synchrotron X‑ray diffraction reveals that chenmingite has an orthorhombic Pnma CaFe 2 O 4 -type (CF) structure with unit-cell dimensions: a = 9.715(6) Å, b = 2.87(1) Å, c = 9.49(7) Å, V = 264.6(4) Å 3 , and Z = 4. Both chenmingite and xieite formed by solid-state transformation of precursor chromite under high pressure and high temperature during the Tissint impact event on Mars. The xieite regions are always in contact with melt pockets, whereas chenmingite lamellae only occur within chromite, a few micrometers away from the melt pockets. This arrangement suggests that chenmingite formed under similar pressures as xieite but at lower temperatures, in agreement with experimental studies.

The elastic properties of Mg 2.12(2) Fe 0.21(2) Ni 0.01 Si 1.15(1) O 6 H 2.67(8) phase E single crystals with Fe 3+ /ΣFe = 0.25(3) have been determined by Brillouin spectroscopy at ambient conditions. We find that that the elasticity of iron-bearing phase E is described by the six independent stiffness tensor components (all in units of GPa): C 11 = 192.2(6), C 12 = 56.4(8), C 13 = 43.5(8), C 14 = –4.3(3), C 33 = 192.1(7), C 44 = 46.4(3). The Voigt-Reuss-Hill averages of bulk and shear moduli are 95.9(4) and 59.6(2) GPa, respectively. The aggregate velocities of iron-bearing phase E are v P = 7.60(2) and v S = 4.43(1) km/s, markedly lower than those of major mantle minerals at ambient conditions. Modeling based on our results suggests that the presence of iron-bearing phase E may reduce the sound wave velocities in upper mantle and transition zone rocks, making it a possible target for future seismological investigations aiming to map hydration in subducting slabs.