Volume 104, Issue 9

The distribution of heat-producing elements (HPE) potassium (K), uranium (U), and thorium (Th) within planetary interiors has major implications for the thermal evolution of the terrestrial planets and for the inventory of volatile elements in the inner solar system. To investigate the abundances of HPE in Mercury’s interior, we conducted experiments at high pressure and temperature (up to 5 GPa and 1900 °C) and reduced conditions (IW-1.8 to IW-6.5) to determine U, Th, and K partitioning between metal, silicate, and sulfide ( D met/sil and D sulf/sil ). Our experimental data combined with those from the literature show that partitioning into sulfide is more efficient than into metal and that partitioning is enhanced with decreasing FeO and increasing O contents of the silicate and sulfide melts, respectively. Also, at low oxygen fugacity (log f O2 < IW-5), U and Th are more efficiently partitioned into liquid iron metal and sulfide than K. D met/sil for U, Th, and K increases with decreasing oxygen fugacity, while D Umet/sil and D Kmet/sil increase when the metal is enriched and depleted in O or Si, respectively. We also used available data from the literature to constrain the concentrations of light elements (Si, S, O, and C) in Fe metal and sulfide. We calculated chemical compositions of Mercury’s core after core segregation, for a range of f O2 conditions during its differentiation. For example, if Mercury differentiated at IW-5.5, its core would contain 49 wt% Si, 0.02 wt% S, and negligible C. Also if core-mantle separation happened at a f O2 lower than IW-4, the bulk Mercury Fe/Si ratio is likely to be chondritic. We calculated concentrations of U, Th, and K in the Fe-rich core and possible sulfide layer of Mercury. Bulk Mercury K/U and K/Th were calculated taking all U, Th, and K reservoirs into account. Without any sulfide layer, or if Mercury’s core segregated at a higher f O2 than IW-4, bulk K/U and K/Th would be similar to those measured on the surface, confirming more elevated volatile K concentration than previously expected for Mercury. However, Mercury could fall on an overall volatile depletion trend where K/U increases with the heliocentric distance if core segregation occurred near IW-5.5 or more reduced conditions, and with a sulfide layer of at least 130 km thickness. At these conditions, the bulk Mercury K/Th ratio is close to Venus’s and Earth’s values. Since U and Th become more chalcophile with decreasing oxygen fugacity, to a higher extent than K, it is likely that at an f O2 close to, or lower than, IW-6 both K/U and K/Th become lower than values of the other terrestrial planets. Therefore, our results suggest that the elevated K/U and K/Th ratios of Mercury’s surface should not be exclusively interpreted as the result of a volatile enrichment in Mercury, but could also indicate a sequestration of more U and Th than K in a hidden iron sulfide reservoir, possibly a layer present between the mantle and core. Hence, Mercury could be more depleted in volatiles than Mars with a K concentration similar to or lower than the Earth’s and Venus’s, suggesting volatile depletion in the inner solar system. In addition, we show that the presence of a sulfide layer formed between IW-4 and IW-5.5 decreases the total radioactive heat production of Mercury by up to 30%.

Valleyite, Ca 4 (Fe,Al) 6 O 13 , is a new sodalite-type mineral discovered in late Pleistocene basaltic scoria from the Menan Volcanic Complex near Rexburg, Idaho, U.S.A. It is an oxidation product of basaltic glass during the early stage of the scoria formation and is associated with hematite (α-Fe 2 O 3 ), maghemite (γ-Fe 2 O 3 ), luogufengite (ε-Fe 2 O 3 ), and quartz on the surface of vesicles. The measured crystal size of valleyite ranges from ~250 to ~500 nm. The empirical chemical formula of valleyite is (Ca 3.61 Mg 0.39 )(Fe 3.97 Al 1.91 Ti 0.09 )O 13 . The mineral has a space group of I 4̅3 m . The (Fe,Al)-O bond distance and unit-cell edge are slightly larger than those reported for synthetic Ca 4 Al 6 O 13 , presumably due to the presence of the larger Fe 3+ cations, compared with Al 3+ , in the structure. Density functional theory calculations predict that valleyite may be a metastable phase at low temperatures. Measured Curie temperatures for valleyite and luogufengite are 645 and 519 K, respectively. Their magnetization hysteresis loop indicates the magnetic exchange coupling between valleyite (soft magnet) and luogufengite (hard magnet) that aids in the understanding of magnetic properties and paleo-magnetism of basaltic rocks. This new mineral, valleyite, with the sodalite-type cage structure is potentially a functional magnetic material.

The low-temperature magnetic properties and Néel temperature, T N , behavior of four silicate substitutional solid solutions containing paramagnetic ions are analyzed. The four systems are: fayalite-forsterite olivine [Fe 2 2+ SiO 4 -Mg 2 SiO 4 ], and the garnet series, grossular-andradite [Ca 3 (Al x ,Fe 3 1 + –x ) 2 Si 3 O 12 ], grossular-spessartine [(Ca x ,Mn 2 1 + –x ) 3 Al 2 Si 3 O 12 ], and almandine-spessartine [(Fe x 2+ ,Mn 2 1 + –x ) 3 Al 2 Si 3 O 12 ]. Local magnetic behavior of the transition-metal-bearing end-members is taken from published neutron diffraction results and computational studies. T N values are from calorimetric heat capacity, C P , and magnetic susceptibility measurements. These end-members, along with more transition-metal-rich solid solutions, show a paramagnetic to antiferromagnetic phase transition. It is marked by a C P l-anomaly that decreases in temperature and magnitude with increasing substitution of the diamagnetic component. For olivines, T N varies between 65 and 18 K and T N for the various garnets is less than 12 K. Local magnetic behavior can involve one or more superexchange interactions mediated through oxygen atoms. T N behavior shows a quasi-plateau-like effect for the systems fayalite-forsterite, grossular-andradite, and grossular-spessartine. More transition-metal-rich crystals show a stronger T N dependence compared to transition-metal-poor ones. The latter may possibly show superparamagnetic behavior. (Fe x 2+ ,Mn 2 1 + –x ) 3 Al 2 Si 3 O 12 garnets show fundamentally different magnetic behavior. End-member almandine and spessartine have different and complex interacting local superexchange mechanisms and intermediate compositions show a double-exchange magnetic mechanism. For the latter, T N values show negative deviations from linear interpolated T N values between the end-members. Double exchange seldom occurs in oxides, and this may be the first documentation of this magnetic mechanism in a silicate. T N behavior may possibly be used to better understand the nature of macroscopic thermodynamic functions, C P and S °, of both end-member and substitutional solid-solution phases.

Redox-sensitive trace elements and sulfur isotope compositions obtained via in situ analyses of sedimentary pyrites from marine black shales are used to track atmosphere-ocean redox conditions between ∼1730 and ∼1360 Ma in the McArthur Basin, northern Australia. Three black shale formations within the basin (Wollogorang Formation 1730 ± 3 Ma, Barney Creek Formation 1640 ± 3 Ma, and Upper Velkerri Formation 1361 ± 21 Ma) display systematic stratigraphic variations in pyrite trace-element compositions obtained using LA-ICP-MS. The concentrations of several trace elements and their ratios, such as Se, Zn, Se/Co, Ni/Co, Zn/Co, Mo/Co, Se/Bi, Zn/Bi, Ni/Bi, increase from the stratigraphically lower Wollogorang Formation to the Upper Velkerri Formation. Cobalt, Bi, Mo, Cu, and Tl show a consistent decrease in abundance while Ni, As, and Pb show no obvious trends. We interpret these trace element trends as a response to the gradual increase of oxygen in the atmosphere-ocean system from ∼1730 to 1360 Ma. Elements more mobile during erosion under rising atmospheric oxygen show an increase up stratigraphy (e.g., Zn, Se), whereas elements that are less mobile show a decrease (e.g., Co, Bi). We also propose the increase of elemental ratios (Se/Co, Ni/Co, Zn/Co, Mo/Co, Ni/Bi, and Zn/Bi) up stratigraphy are strong indicators of atmospheric oxygenation. Sulfur isotopic compositions of marine pyrite (δ 34 S pyrite ) from these formations, obtained using SHRIMP-SI, are highly variable, with the Wollogorang Formation exhibiting less variation (δ 34 S = –29.4 to +9.5‰; mean –5.03‰) in comparison to the Barney Creek (δ 34 S = –13.8 to +41.8‰; mean +19.88‰) and Velkerri Formations (δ 34 S = –14.2 to +52.8‰; mean +26.9‰). We propose that the shift in mean δ 34 S to heavier values up-section corresponds to increasing deep water oxygenation from ∼1730 to 1360 Ma. Incursion of oxygenated waters possibly caused a decrease in the areal extent of anoxic areas, at the same time, creating a possibly efficient reducing system. A stronger reducing system caused the δ 34 S of the sedimentary pyrites to become progressively heavier. Interestingly, heavy δ 34 S in pyrites overlaps with the increase in the concentration of certain trace elements (and their ratios) in sedimentary pyrites (Se, Zn, Se/Co, Ni/Co, Zn/Co, Mo/Co, Ni/Bi, and Zn/Bi). This study concludes that there was a gradual increase of atmospheric oxygen accompanied by ocean oxygenation through the first ∼400 million years of the Boring Billion (1800–1400 Ma) in the McArthur Basin.

Hydrogen transport from the surface to the deep interior and distribution in the mantle are important in the evolution and dynamics of the Earth. An aluminum oxy-hydroxide, δ-AlOOH, might influence hydrogen transport in the deep mantle because of its high stability extending to lower mantle conditions. The compressional behavior and spin states of δ-(Al,Fe 3+ )OOH phases were investigated with synchrotron X‑ray diffraction and Mössbauer spectroscopy under high pressure and room temperature. Pressure-volume ( P - V ) profiles of the δ-(Al 0.908(9) 57 Fe 0.045(1) )OOH 1.14(3) [Fe/(Al+Fe) = 0.047(10), δ-Fe5] and the δ-(Al 0.832(5) 57 Fe 0.117(1) )OOH 1.15(3) [Fe/(Al+Fe) = 0.123(2), δ-Fe12] show that these hydrous phases undergo two distinct structural transitions involving changes in hydrogen bonding environments and a high- to low-spin crossover in Fe 3+ . A change of axial compressibility accompanied by a transition from an ordered ( P 2 1nm ) to disordered hydrogen bond ( Pnnm ) occurs near 10 GPa for both δ-Fe5 and δ-Fe12 samples. Through this transition, the crystallographic a and b axes become stiffer, whereas the c axis does not show such a change, as observed in pure δ-AlOOH. A volume collapse due to a transition from high- to low-spin states in the Fe 3+ ions is complete below 32–40 GPa in δ-Fe5 and δ-Fe12, which is ~10 GPa lower than that reported for pure e-FeOOH. Evaluation of the Mössbauer spectra of δ-(Al 0.824(10) 57 Fe 0.126(4) )OOH 1.15(4) [Fe/(Al+Fe) = 0.133(3), δ-Fe13] also indicate a spin transition between 32–45 GPa. Phases in the δ-(Al,Fe)OOH solid solution with similar iron concentrations as those studied here could cause an anomalously high r/ v F ratio (bulk sound velocity, defined as K/ρ)$\sqrt{\left. K/\rho \right)}$at depths corresponding to the spin crossover region (~900 to ~1000 km depth), whereas outside the spin crossover region a low r/ v F anomaly would be expected. These results suggest that the δ-(Al,Fe)OOH solid solution may play an important role in understanding the heterogeneous structure of the deep Earth.

Accurate estimation of the depths of spinel peridotite xenoliths for which reliable geobarometers are not available is imperative to be able to reconstruct the precise structures of the lithosphereasthenosphere boundary (LAB). The LAB can be defined based on thermal, chemical, rheological, and petrological contrasts, and knowing its depth is crucial to understanding mantle dynamics. We attack this problem by examining spinel peridotite xenoliths from Ichinomegata maar in the back-arc side of Northeast Japan Arc. Extensive mineral compositions of nine xenolith samples revealed various patterns of chemical zoning in pyroxenes, suggesting diverse thermal histories. We examined the timescales of development of each zoning pattern and identified minerals, grain portions, and components closely approached equilibrium just before xenolith extraction as orthopyroxene and clinopyroxene, the outermost rims, and Ca-Mg-Fe components, respectively. Applying the best pair of geothermobarometers to the chosen analyses, plausible derivation depths of eight samples were obtained. They range from 0.72–1.6 GPa in pressure and from 830–1080 °C in temperature, which defines a high thermal gradient of 10 K/km or 290 K/GPa. There is an intimate correlation between the zoning patterns of pyroxenes and the depth estimates: pyroxenes in the deeper samples have zoning indicating cooling followed by heating just before xenolith extraction, and those of the shallower samples have zoning indicating monotonic cooling. Depth variations of rock microstructures, grain size of olivine, chemical compositions of minerals, and phase assemblage, including the presence or absence of glass or fluid phase, show that the mantle beneath Ichinomegata consists of two distinct layers. The shallower (28–32 km) layer is granular, less oxidized, amphibole- and plagioclase-bearing, and subsolidus, whereas the deeper (41–55 km) layer is porphyroclastic, amphibole- and plagioclase-free, oxidized, and partially molten. The contrasts between the two layers suggest that the upper layer represents a lithospheric mantle and the lower layer a LAB zone. These layers are similar to those reported from the bottom of subcontinental lithospheric mantle in various aspects, but the LAB beneath Ichinomegata is much shallower (40–60 km) and cooler (~1100 °C). The coincidence of (1) the depth of a rheological transition, marked granular to porphyroclastic textures, and (2) the depth of a phase transition, from subsolidus hydrous peridotite to a hydrous mantle with melt in localized pockets, is the remarkable feature of the LAB beneath Ichinomegata. This suggests that a rheological boundary zone in arc settings is governed by melting of the hydrous mantle and that the underlying asthenosphere is partially molten. The depth-dependent thermal history shown by chemical zoning in pyroxenes and the presence of melt as pockets suggest that the LAB beneath Ichinomegata was in a transient state that was affected by thermal and material transport.

Phase stability and elasticity of ammonia hydrate have been studied using Raman spectroscopy and Brillion scattering in diamond-anvil cells up to 53 GPa at 300 K. Here we have established the high-pressure phase diagram of ammonia hydrate in three different compositions, including ammonia monohydrate (AMH, NH 3 ·H 2 O), dihydrate (ADH, NH 3 ·2H 2 O), and trihydrate (ATH, NH 3 ·3H 2 O). In contrast to previous experimental results, our Raman and Brillouin measurements at 300 K have shown that all three ammonia hydrates start to dehydrate at 2.1–2.2 GPa. Dehydration of the ammonia hydrate leads to the formation of single-crystal ice-VII and an increase in the concentration of NH 3 in the residual liquid. The residual liquid finally turns into solid ammonia hemihydrate phase II (AHH-II) at 4–4.6 GPa, leading to a 28% jump in the compressional-wave velocity ( V P ). Considering a 10–15 vol% NH 3 in the mantle of ice giants, AHH should thus be the dominant form of NH 3 coexisting with H 2 O-ice in the ice giants. Further Brillouin measurements provide crucial constraints on the V P of AHH and the single-crystal elasticity of ice-VII at high pressures and 300 K. V P of AHH increases smoothly with pressure. No anomalous change in V P of AHH was identified up to 39 GPa, although a solid to solid phase transition was noted to occur at ~18 GPa by Raman measurements. In addition, the elasticity of single-crystal ice-VII, which was the dehydration product of ammonia hydrate, has been determined up to 53 GPa at 300 K. The deviation of C 12 from C 44 observed at 11.4 and 14.6 GPa could be caused by the hydrogen bond symmetrizations or the ordering of dipole of single-crystal ice-VII. An abnormal softening in the elastic moduli C 11 , C 12 , and the adiabatic moduli K S together with stiffening in C 44 was observed between 42 and 53 GPa, which should be caused by the transition from ice-VII to its pre-transitional state. Of particular interest is the dramatic increase in the anisotropy of ice-VII with increasing pressure. Combining the sound velocity of AHH and ice-VII, we have modeled the V P of ice giants with a volume ratio of 20% AHH and 80% ice-VII in the mantle. The obtained high-pressure phase diagram and elastic properties of ammonia hydrate could contribute to understanding the structure of the mantle in the ice giants and satellites.

The crystal structure and crystal chemistry of kurnakovite from Kramer Deposit (Kern County, California), ideally MgB 3 O 3 (OH) 5 ·5H 2 O, were investigated by single-crystal neutron diffraction (data collected at 293 and 20 K) and by a series of analytical techniques aimed to determine its chemical composition. The concentration of more than 50 elements was measured. The empirical formula of the sample used in this study is Mg 0.99 (Si 0.01 B 3.00 ) Σ3.01 O 3.00 (OH) 5 ·4.98H 2 O. The fraction of rare earth elements (REE) and other minor elements are, overall, insignificant. Even the content of fluorine, as a potential OH-group substituent, is insignificant (i.e., ~0.008 wt%). The neutron structure model obtained in this study, based on intensity data collected at 293 and 20 K, shows that the structure of kurnakovite contains: [BO 2 (OH)]-groups in planar-triangular coordination (with the B-ions in sp 2 electronic configuration), [BO 2 (OH) 2 ]-groups in tetrahedral coordination (with the B-ions in sp 3 electronic configuration), and Mg(OH) 2 (H 2 O) 4 -octahedra, connected into (neutral) Mg(H 2 O) 4 B 3 O 3 (OH) 5 units forming infinite chains running along [001]. Chains are mutually connected to give the tri-dimensional structure only via hydrogen bonding, and extra-chains “zeolitic” H 2 O molecules are also involved as “bridging molecules.” All the oxygen sites in the structure of kurnakovite are involved in hydrogen bonding, as donors or as acceptors . The principal implications of these results are: (1) kurnakovite does not act as a geochemical trap of industrially relevant elements (e.g., Li, Be, or REE), (2) the almost ideal composition makes kurnakovite a potentially good B-rich aggregate in concretes (for example, used for the production of radiation-shielding materials for the elevated ability of 10 B to absorb thermal neutrons), which avoids the risk to release undesirable elements, for example sodium, that could promote deleterious reactions for the durability of cements.

The occurrence of minor elements in the structure of biogenic diatomaceous opal-A is an important issue because it is closely related to biogeochemical processes driven by the precipitation, sedimentation, and storage of diatoms, as well as to the properties and applications of diatomite, which is the sedimentary rock composed of diatomaceous opal-A. However, to date, there is no direct microscopic evidence for the existence of minor elements, such as Al, Fe, and Mg, in the structure of diatomaceous opal-A, because such evidence requires observation of the internal structure of frustules to exclude the disturbance of impurity minerals, which is technically challenging using conventional techniques. In this work, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) combined with energy-dispersive X‑ray spectroscopy (EDS) mapping analysis were performed on diatomaceous opal-A from three typical diatomite specimens that were pretreated using focused ion beam (FIB) thinning. This technique produces a slice of a diatom frustule for direct TEM observation of the internal structure of the diatomaceous opal-A. The results of this work clearly indicate that minor elements, such as Al, Fe, Ca, and Mg, conclusively exist within the siliceous framework of diatomaceous opal-A. The contents of these minor elements are at atomic ratio levels of 1 (minor element)/ 10 000 (Si) – 1/100, regardless of the genus of the diatoms. The occurrence of minor elements in the internal structure is likely through biological uptake during biosynthesis by living diatoms. Moreover, surface coatings composed of aluminosilicates on diatom frustules are common, and the contents of elements such as Al and Fe are tens or hundreds of times higher in the coatings than in the internal siliceous structure of diatomaceous opal-A. The discovery of the incorporation of the above-mentioned minor elements in the diatomaceous opal-A structure, both in the internal Si-O framework and on the surface, updates the knowledge about the properties of diatomite.

Nixonite (IMA 2018-133), ideally Na 2 Ti 6 O 13 , is a new mineral found within a heavily metasomatized pyroxenite xenolith from the Darby kimberlite field, beneath the west-central Rae Craton, Canada. It occurs as microcrystalline aggregates, 15 to 40 mm in length. Nixonite is isostructural with jeppeite, K 2 Ti 6 O 13 , with a structure consisting of edge- and corner-shared titanium-centered octahedra that enclose alkali-metal ions. The Mohs hardness is estimated to be between 5 and 6 by comparison to jeppeite, and the calculated density is 3.51(1) g/cm 3 . Electron microprobe wavelength-dispersive spectroscopic analysis (average of 6 points) yielded: Na 2 O 6.87, K 2 O 5.67, CaO 0.57, TiO 2 84.99, V 2 O 3 0.31, Cr 2 O 3 0.04, MnO 0.01, Fe 2 O 3 0.26, SrO 0.07, total 98.79 wt%. The empirical formula, based on 13 O atoms, is: (Na 1.24 K 0.67 Ca 0.06 ) S1.97 (Ti 5.96 V 0.023 Fe 0.018 ) S6.00 O 13 with minor amounts of Cr and Mn. Nixonite is monoclinic, space group C 2/ m , with unit-cell parameters a = 15.3632(26) Å, b = 3.7782(7) Å, c = 9.1266(15) Å, b = 99.35(15)°, and V = 522.72(1) Å 3 , Z = 2. Based on the average of seven integrated multi-grain diffraction images, the strongest diffraction lines are [ d obs in Å ( I in %) ( hkl )]: 3.02 (100) (310), 3.66 (75) (110), 7.57 (73) (200), 6.31 (68) (201), 2.96 (63) (311), 2.96 (63) (203), and 2.71 (62) (402). The five main Raman peaks of nixonite, in order of decreasing intensity, are at 863, 280, 664, 135, and 113 cm –1 . Nixonite is named after Peter H. Nixon, a renowned scientist in the field of kimberlites and mantle xenoliths. Nixonite occurs within a pyroxenite xenolith in a kimberlite, in association with rutile, priderite, perovskite, freudenbergite, and ilmenite. This complex Na-K-Ti-rich metasomatic mineral assemblage may have been produced by a fractionated Na-rich kimberlitic melt that infiltrated a mantle-derived garnet pyroxenite and reacted with rutile during kimberlite crystallization.

Goldschmidtite is a new perovskite-group mineral (IMA No. 2018-034) with the ideal formula (K,REE,Sr)(Nb,Cr)O 3 . A single grain of goldschmidtite with a maximum dimension of ∼100 μm was found as an inclusion in a diamond from the Koffiefontein pipe in South Africa. In addition to the dark green and opaque goldschmidtite, the diamond contained a Cr-rich augite (websteritic paragenesis) and an intergrowth of chromite, Mg-silicate, and unidentified K-Sr-REE-Nb-oxide. Geothermobarometry of the augite indicates that the depth of formation was ∼170 km. The chemical composition of gold-schmidtite determined by electron microprobe analysis ( n = 11, WDS, wt%) is: Nb 2 O 5 44.82, TiO 2 0.44, ThO 2 0.10, Al 2 O 3 0.35, Cr 2 O 3 7.07, La 2 O 3 11.85, Ce 2 O 3 6.18, Fe 2 O 3 1.96, MgO 0.70, CaO 0.04, SrO 6.67, BaO 6.82, K 2 O 11.53, total 98.53. The empirical formula (expressed to two decimal places) is (K 0.50 La 0.15 Sr 0.13 Ba 0.09 Ce 0.08 ) Σ0.95 (Nb 0.70 Cr 0.19 Fe 0.05 Al 0.01 Mg 0.04 Ti 0.01 ) Σ1.00 O 3 . Goldschmidtite is cubic, space group Pm 3 m , with unit-cell parameters: a = 3.9876(1) Å, V = 63.404(6) Å 3 , Z = 1, resulting in a calculated density of 5.32(3) g/cm 3 . Goldschmidtite is the K-analog of isolueshite, (Na,La)NbO 3 . Raman spectra of goldschmidtite exhibit many second-order broad bands at 100 to 700 cm –1 as well as a pronounced peak at 815 cm –1 , which is possibly a result of local ordering of Nb and Cr at the B site. The name goldschmidtite is in honor of the eminent geochemist Victor Moritz Goldschmidt (1888–1947), who formalized perovskite crystal chemistry and identified KNbO 3 as a perovskite-structured compound.

Edscottite (IMA 2018-086a), Fe 5 C 2 , is a new iron carbide mineral that occurs with low-Ni iron (kamacite), taenite, nickelphosphide (Ni-dominant schreibersite), and minor cohenite in the Wedderburn iron meteorite, a Ni-rich member of the group IAB complex. The mean chemical composition of edscottite determined by electron probe microanalysis, is (wt%) Fe 87.01, Ni 4.37, Co 0.82, C 7.90, total 100.10, yielding an empirical formula of (Fe 4.73 Ni 0.23 Co 0.04 )C 2.00 . The end-member formula is Fe 5 C 2 . Electron backscatter diffraction shows that edscottite has the C 2 /c Pd 5 B 2 -type structure of the synthetic phase called Hägg-carbide, c-Fe 5 C 2 , which has a = 11.57 Å, b = 4.57 Å, c = 5.06 Å, b = 97.7 °, V = 265.1 Å 3 , and Z = 4. The calculated density using the measured composition is 7.62 g/cm 3 . Like the other two carbides found in iron meteorites, cohenite (Fe 3 C) and haxonite (Fe 23 C 6 ), edscottite forms in kamacite, but unlike these two carbides, it forms laths, possibly due to very rapid growth after supersaturation of carbon. Haxonite (which typically forms in carbide-bearing, Ni-rich members of the IAB complex) has not been observed in Wedderburn. Formation of edscottite rather than haxonite may have resulted from a lower C concentration in Wedderburn and hence a lower growth temperature. The new mineral is named in honor of Edward (Ed) R.D. Scott, a pioneering cosmochemist at the University of Hawai‘i at Manoa, for his seminal contributions to research on meteorites.

The oxygen fugacity in the interior of the Earth is largely controlled by iron-bearing minerals. Recent studies have reported various iron oxides with chemical compositions between FeO and Fe 3 O 4 above ~10 GPa. However, the stabilities of these high-pressure iron oxides remain mostly uninvestigated. In this study, we performed in situ X-ray diffraction (XRD) measurements in a laser-heated diamond-anvil cell (DAC) to determine the phase relations in both Fe 5 O 6 and Fe 4 O 5 bulk compositions to 61 GPa and to 2720 K. The results show that Fe 5 O 6 is a high-temperature phase stable above 1600 K and ~10 GPa, while FeO + Fe 4 O 5 are formed at relatively low temperatures. We observed the decomposition of Fe 5 O 6 into 2FeO + Fe 3 O 4 above 38 GPa and the decomposition of Fe 4 O 5 into FeO + h-Fe 3 O 4 at a similar pressure range. The coexistence of FeO and Fe 3 O 4 indicates that none of the recently discovered compounds between FeO and Fe 3 O 4 (i.e., Fe 5 O 6 , Fe 9 O 11 , Fe 4 O 5 , and Fe 7 O 9 ) are formed beyond ~40 GPa at 1800 K, corresponding to conditions in the shallow lower mantle. Additionally, as some superdeep diamonds have genetic links with these high-pressure iron oxides, our results give constraints on pressure and temperature conditions of their formation.

In this issue This New Mineral Names has entries for 9 new minerals, including argentotetrahedrite-(Fe), bytízite, calamaite, fluorlamprophyllite, honzaite, katerinopoulosite, meitnerite, melcherite, and rozhdestvenskayaite-(Zn).