Volume 105, Issue 3

Volatile elements (e.g., H, C, N) have a strong influence on the physical and chemical evolution of planets and are essential for the development of habitable conditions. Measurement of the volatile and incompatible heavy halogens, Cl, Br, and I, can provide insight into volatile distribution and transport processes, due to their hydrophilic nature. However, information on the bulk halogen composition of martian meteorites is limited, particularly for Br and I, largely due to the difficulty in measuring ppb-level Br and I abundances in small samples. In this study, we address this challenge by using the neutron irradiation noble gas mass spectrometry (NI-NGMS) method to measure the heavy halogen composition of five olivine-phyric shergottite meteorites, including the enriched (Larkman Nunatak LAR 06319 and LAR 12011) and depleted (LAR 12095, LAR 12240, and Tissint) compositional end-members. Distinct differences in the absolute abundances and halogen ratios exist between enriched (74 to136 ppm Cl, 1303 to 3061 ppb Br, and 4 to 1423 ppb I) and depleted (10 to 26 ppm Cl, 46 to 136 ppb Br, and 3 to 329 ppb I) samples. All halogen measurements are within the ranges previously reported for martian shergottite, nakhlite, and chassignite (SNC) meteorites. Enriched shergottites show variable and generally high Br and I absolute abundances. Halogen ratios (Br/Cl and I/Cl) are in proportions that exceed those of both carbonaceous chondrites and the martian surface. This may be linked to a volatile-rich martian mantle source, be related to shock processes or could represent a small degree of heavy halogen contamination (a feature of some Antarctic meteorites, for example). The differences observed in halogen abundances and ratios between enriched and depleted compositions, however, are consistent with previous suggestions of a heterogeneous distribution of volatiles in the martian mantle. Depleted shergottites have lower halogen abundances and Br and Cl in similar proportions to bulk silicate Earth and carbonaceous chondrites. Tissint in particular, as an uncontaminated fall, allows an estimate of the depleted shergottite mantle source composition to be made: 1.2 ppm Cl, 7.0 ppb Br, and 0.2 ppb I. The resultant bulk silicate Mars (BSM) estimate (22 ppm Cl, 74 ppb Br, and 6 ppb I), including the martian crust and depleted shergottite mantle, is similar to estimates of the bulk silicate earth (BSE) halogen composition.

We present in situ secondary ion mass spectrometry (SIMS) and electron microprobe analyses of coexisting garnet, omphacite, phengite, amphibole, and apatite, combined with pyrohydrolysis bulk-rock analyses to constrain the distribution, abundance, and behavior of halogens (F and Cl) in six MORB-like eclogites from the Raspas Complex (Southern Ecuador). In all cases concerning lattice-hosted halogens, F compatibility decreases from apatite (1.47–3.25 wt%), to amphibole (563–4727 μg/g), phengite (610–1822 μg/g), omphacite (6.5–54.1 μg/g), and garnet (1.7–8.9 μg/g). The relative compatibility of Cl in the assemblage is greatest for apatite (192–515 μg/g), followed by amphibole (0.64–82.7 μg/g), phengite (1.2–2.1 μg/g), omphacite (<0.05–1.0 μg/g), and garnet (<0.05 μg/g). Congruence between SIMS-reconstructed F bulk abundances and yield-corrected bulk pyrohydrolysis analyses indicates that F is primarily hosted within the crystal lattice of eclogitic minerals. However, SIMS-reconstructed Cl abundances are a factor of five lower, on average, than pyrohydrolysis-derived bulk concentrations. This discrepancy results from the contribution of fluid inclusions, which may host at least 80% of the bulk rock Cl. The combination of SIMS and pyrohydrolysis is highly complementary. Whereas SIMS is well suited to determine bulk F abundances, pyrohydrolysis better quantifies bulk Cl concentrations, which include the contribution of fluid inclusion-hosted Cl. Raspas eclogites contain 145–258 μg/g F and at least 7–11 μg/g Cl. We estimate that ~95% of F is retained in the slab through eclogitization and returned to the upper mantle during subduction, whereas at least 95% of subducted Cl is removed from the rock by the time the slab equilibrates at eclogite facies conditions. Our calculations provide further evidence for the fractionation of F from Cl during high-pressure metamorphism in subduction zones. Although the HIMU (high U/Pb) mantle source (dehydrated oceanic crust) is often associated with enrichments in Cl/K and F/Nd, Raspas eclogites show relatively low halogen ratios identical within uncertainty to depleted MORB mantle (DMM). Thus, the observed halogen enrichments in HIMU ocean island basalts require either further fractionation during mantle processing or recycling of a halogen-enriched carrier lithology such as serpentinite into the mantle.

The pressure dependence of Si diffusion in γ-Fe was investigated at pressures of 5–15 GPa and temperatures of 1473–1673 K using the Kawai-type multi-anvil apparatus to estimate the rate of mass transportation for the chemical homogenization of the Earth’s inner core and those of small terrestrial planets and large satellites. The obtained diffusion coefficients D were fitted to the equation D = D 0 exp[–( E * + PV *)/(R T )], where D 0 is a constant, E * is the activation energy, P is the pressure, V * is the activation volume, R is the gas constant, and T is the absolute temperature. The least-squares analysis yielded D 0 = 10 -1.17±0.54 m 2 /s, E * = 336 ± 16 kJ/mol, and V * = 4.3 ± 0.2 cm 3 /mol. Moreover, the pressure and temperature dependences of diffusion coefficients of Si in γ-Fe can also be expressed well using homologous temperature scaling, which is expressed as D = D 0 exp{–g[ T m ( P )]/ T }, where g is a constant, T m ( P ) is the melting temperature at pressure P , and D 0 and g are 10 -1.0±0.3 m 2 /s and 22.0 ± 0.7, respectively. The present study indicates that even for 1 billion years, the maximum diffusion length of Si under conditions in planetary and satellite cores is less than ~1.2 km. Additionally, the estimated strain of plastic deformation in the Earth’s inner core, caused by the Harper–Dorn creep, reaches more than 10 3 at a stress level of 10 3 –10 4 Pa, although the inner core might be slightly deformed by other mechanisms. The chemical heterogeneity of the inner core can be reduced only via plastic deformation by the Harper–Dorn creep.

Carbonates play an important role in the transport and storage of carbon in the Earth’s mantle. However, the abundance of carbon and carbonates in subduction zones is still an unknown quantity. To determine the most abundant accessory phases and how they influence the dynamical processes that operate within the Earth, investigations on the vibrational, elastic, and thermodynamic properties of these phases are crucial for interpreting seismological observations. Recently, the nuclear inelastic scattering (NIS) method has proved to be a useful tool to access information on the lattice dynamics, as well as to determine Debye sound velocities of Fe-bearing materials. Here we derive the acoustic velocities from two carbonate compositions in the FeCO 3 -MgCO 3 binary system up to ~70 GPa using the NIS method. We conclude that more Mg-rich samples, in this case (Fe 0.26 Mg 0.74 )CO 3 , have ~19% higher sound velocities than the pure end-member Fe composition. In addition, we observed a significant velocity increase after the Fe 2+ spin transition was complete. After laser heating of FeCO 3 at lower mantle conditions, we observed a dramatic velocity drop, which is probably associated with thermal decomposition to another phase. Parallel to our NIS experiments, we conducted a single-crystal X‑ray diffraction (SCXRD) study to derive the equation of states of FeCO 3 and (Fe 0.26 Mg 0.74 )CO 3 . The combined information from NIS (i.e., Debye velocities) and SCXRD (i.e., densities and bulk moduli) experiments enabled us to derive the primary and shear wave velocities of our samples. Our results are consistent with results obtained by other methods in previous studies, including Brillouin spectroscopy, inelastic X‑ray scattering, and DFT calculations, supporting NIS as a reliable alternative method for studying the elastic properties of Fe-bearing systems at high pressures and temperatures. Finally, we discuss the seismic detectability of carbonates. We determine that nearly 22 wt% CO 2 must be present in the subduction slab to detect a 1% shear wave velocity decrease compared to non-carbonated lithologies at the transition zone to lower mantle boundary depths.

Experimental studies and measurements of inclusions in diamonds show that ferric iron components are increasingly stabilized with depth in the mantle. To determine the thermodynamic stability of such components, their concentration needs to be measured at known oxygen fugacities. The metal-oxide pair Ru and RuO 2 are ideal as an internal oxygen fugacity buffer in high-pressure experiments. Both phases remain solid to high temperatures and react minimally with silicates, only exchanging oxygen. To calculate oxygen fugacities at high pressure and temperature, however, requires information on the phase relations and equation of state properties of the solid phases. We have made in situ synchrotron X‑ray diffraction measurements in a multi-anvil press on mixtures of Ru and RuO 2 to 19.4 GPa and 1473 K with which we have determined phase relations of the RuO 2 phases and derived thermal equations of state (EoS) parameters for both Ru and RuO 2 . Rutile-structured RuO 2 was found to undergo two phase transformations, first at ~7 GPa to an orthorhombic structure and then above 12 GPa to a cubic structure. The phase boundary of the cubic phase was constrained for the first time at high pressure and temperature. We have derived a continuous Gibbs free energy expression for the tetragonal and orthorhombic phases of RuO 2 by fitting the second-order phase transition boundary and P - V - T data for both phases, using a model based on Landau theory. The transition between the orthorhombic and cubic phases was then used along with EoS terms derived for both phases to determine a Gibbs free energy expression for the cubic phase. We have used these data to calculate the oxygen fugacity of the Ru + O 2 = RuO 2 equilibrium, which we have parameterized as a single polynomial across the stability fields of all three phases of RuO 2 . The expression is log 10 f O2 (Ru – RuO 2 ) = (7.782 – 0.00996 P + 0.001932 P 2 – 3.76 × 10 –5 P 3 ) + (–13 763 + 592 P – 3.955 P 2 )/T + (–1.05 × 10 6 – 4622P)/T 2 , which should be valid from room pressure up to 25 GPa and 773–2500 K, with an estimated uncertainty of 0.2 log units. Our calculated f O2 is shown to be up to 1 log unit lower than estimates that use previous expressions or ignore EoS terms.

The stability relations of Pt and Pd antimonides and bismuthinides in the Sb- and Bi-bearing Fe-Ni-Cu sulfide systems have been experimentally determined at temperatures between 1100 and 700 °C in evacuated silica tubes. Both PtSb and PdSb are stable as immiscible liquids at temperatures above 1100 and 1000 °C, respectively. The Fe-Ni-Cu-sulfide melt that coexists with the immiscible antimonide melt can dissolve up to 3.8 wt% Sb at 1100 °C, whereas monosulfide solid solution (mss) dissolves very low amounts of Sb over the entire 1100–700 °C temperature range. The liquidus of Pt-antimonides and Pd-antimonides are above 980 and 750 °C, respectively. Bismuth does not form immiscible melt at 1100 °C but may partially partition into a vapor phase at 1050 °C. The Pt- and Pd-bismuthinides crystallize directly from immiscible bismuthinide melt only after crystallization of the sulfide melt into intermediate solid solution (iss). Insizwaite (PtBi 2 ) and froodite (PdBi 2 ) are stable at 780 and 700 °C, respectively. At the last stage of evolution of Sb-bearing magmatic Fe-Ni-Cu sulfide melts, Sb will form immiscible antimonide melt that will extract Pt and Pd from the sulfide melt. During cooling, Pt and Pd-antimonides will crystallize directly from the immiscible antimonide melt, and Pt-phases will form at higher temperatures relative to Pd-phases. Bismuth will partition into vapor phase and concentrate into a low-temperature melt in hydrothermal and porphyry systems that scavenge precious metals. The Sb and Bi (like Te) will be highly incompatible at moderate degrees of mantle partial melting.

The primary aim of this study was the accurate determination of unit-cell parameters and description of disorder in chlorites with semi-random stacking using common X‑ray diffraction (XRD) data for bulk powder samples. In the case of ordered chlorite structures, comprehensive crystallographic information can be obtained based on powder XRD data. Problems arise for samples with semi-random stacking, where due to strong broadening of hkl peaks with k ≠ 3n , the determination of unit-cell parameters is demanding. In this study a complete set of information about the stacking sequences in chlorite structures was determined based on XRD pattern simulation, which included determining a fraction of layers shifted by ±1/3 b , interstratification with different polytypes and 2:1 layer rotations. A carefully selected series of pure Mg-Fe tri-trioctahedral chlorites with iron content in the range from 0.1 to 3.9 atoms per half formula unit cell was used in the study. In addition, powder XRD patterns were carefully investigated for the broadening of the odd-number basal reflections to determine interstratification of 14 and 7 Å layers. These type of interstratifications were finally not found in any of the samples. This result was also confirmed by the XRD pattern simulations, assuming interstratification with R 0 ordering. Based on h 0 l XRD reflections, all the studied chlorites were found to be the IIbb polytype with a monoclinic-shaped unit cell (β ≈ 97°). For three samples, the hkl reflections with k ≠ 3n were partially resolvable; therefore, a conventional indexing procedure was applied. Two of the chlorites were found to have a monoclinic cell (with α, γ = 90°). Nevertheless, among all the samples, the more general triclinic (pseudomonoclinic) crystal system with symmetry C 1 was assumed, to calculate unit-cell parameters using Le Bail fitting. A detailed study of semi-random stacking sequences shows that simple consideration of the proportion of IIb-2 and IIb-4/6 polytypes, assuming equal content of IIb-4 and IIb-6, is not sufficient to fully model the stacking structure in chlorites. Several, more general, possible models were therefore considered. In the first approach, a parameter describing a shift into one of the ±1/3 b directions (thus, the proportion of IIb-4 and IIb-6 polytypes) was refined. In the second approach, for samples with slightly distinguishable hkl reflections with k ≠ 3n , some kind of segregation of individual polytypes (IIb-2/4/6) was considered. In the third approach, a model with rotations of 2:1 layers about 0°, 120°, 240° was shown to have the lowest number of parameters to be optimized and therefore give the most reliable fits. In all of the studied samples, interstratification of different polytypes was revealed with the fraction of polytypes being different than IIbb ranging from 5 to 19%, as confirmed by fitting of h 0 l XRD reflections.

Diopside is one of the most important end-members of clinopyroxene, which is an abundant mineral in upper-mantle petrologic models. The amount of clinopyroxene in upper-mantle pyrolite can be ∼15 vol%, while pyroxenite can contain as high as ∼60 vol% clinopyroxene. Knowing the elastic properties of the upper-mantle diopside at high pressure-temperature conditions is essential for constraining the chemical composition and interpreting seismic observations of region. Here we have measured the single-crystal elasticity of Fe-enriched diopside (Di 80 Hd 20 , Di-diopside, and Hd-hedenbergite; also called Fe-enriched clinopyroxene) at high-pressure conditions up to 18.5 GPa by using in situ Brillouin light-scattering spectroscopy (BLS) and synchrotron X-ray diffraction in a diamond-anvil cell. Our experimental results were used in evaluating the effects of pressure and Fe substitution on the full single-crystal elastic moduli across the Di-Hd solid-solution series to better understand the seismic velocity profiles of the upper mantle. Using the third- or fourth-order Eulerian finite-strain equations of state to model the elasticity data, the derived aggregate adiabatic bulk and shear moduli ( K S 0, G 0 ) at ambient conditions were determined to be 117(2) and 70(1) GPa, respectively. The first- and second-pressure derivatives of bulk and shear moduli at 300 K were (∂ K S / ∂ P ) T = 5.0(2), (∂ 2 K S / ∂ P 2 ) T = –0.12(4) GPa− 1 and (∂ G/ ∂ P ) T = 1.72(9), (∂ 2 G/ ∂ P 2 ) T = –0.05(2) GPa −1 , respectively. A comparison of our results with previous studies on end-member diopside and hedenbergite in the literatures shows systematic linear correlations between the Fe composition and single-crystal elastic moduli. An addition of 20 mol% Fe in diopside increases K S0 by ∼1.7% (∼2 GPa) and reduces G 0 by ∼4.1% (∼3 GPa), but has a negligible effect on the pressure derivatives of the bulk and shear moduli within experimental uncertainties. In addition, our modeling results show that substitution of 20 mol% Fe in diopside can reduce V P and V S by ∼1.8% and ∼3.5%, respectively, along both an expected normal mantle geotherm and a representative cold subducted slab geotherm. Furthermore, the modeling results show that the V P and V S profiles of Fe-enriched pyroxenite along the cold subducted slab geotherm are ∼3.2% and ∼2.5% lower than AK135 model at 400 km depth, respectively. Finally, we propose that the presence of Fe-enriched pyroxenite (including Fe-enriched clinopyroxene, Fe-enriched orthopyroxene, and Feenriched olivine), can be an effective mechanism to cause low-velocity anomalies in the upper mantle regions atop the 410 km discontinuity at cold subudcted slab conditions.

X‑ray absorption near-edge structure (XANES) spectroscopy is a powerful technique to quan‑ titatively investigate sulfur speciation in geologically complex materials such as minerals, glasses, soils, organic compounds, industrial slags, and extraterrestrial materials. This technique allows non-destructive investigation of the coordination chemistry and oxidation state of sulfur species ranging from sulfide (2– oxidation state) to sulfate (6+ oxidation state). Each sulfur species has a unique spectral shape with a characteristic K -edge representing the s → p and d hybridization photoelectron transitions. As such, sulfur speciation is used to measure the oxidation state of samples by comparing the overall XANES spectra to that of reference compounds. Although many S XANES spectral standards exist for terrestrial applications under oxidized conditions, new sulfide standards are needed to investigate reduced (oxygen fugacity, f O2 , below IW) silicate systems relevant for studies of extraterrestrial ma‑ terials and systems. Sulfides found in certain meteorites (e.g., enstatite chondrites and aubrites) and predicted to exist on Mercury, such as CaS (oldhamite), MgS (niningerite), and FeCr 2 S 4 (daubréelite), are stable at f O2 below IW-3 but rapidly oxidize to sulfate and/or produce sulfurous gases under terrestrial surface conditions. XANES spectra of these compounds collected to date have been of variable quality, possibly due to the unstable nature of certain sulfides under typical (e.g., oxidizing) laboratory conditions. A new set of compounds was prepared for this study and their XANES spectra are analyzed for comparison with potential extraterrestrial analogs. S K -edge XANES spectra were collected at Argonne National Lab for FeS (troilite), MnS (alabandite), CaS (oldhamite), MgS (niningerite), Ni 1–x S, NiS 2 , CaSO 4 (anhydrite), MgSO 4 , FeSO 4 , Fe 2 (SO 4 ) 3 , FeCr 2 S 4 (daubréelite), Na 2 S, Al 2 S 3 , Ni 7 S 6 , and Ni 3 S 2 ; the latter five were analyzed for the first time using XANES. These standards expand upon the existing S XANES end-member libraries at a higher spectral resolution (0.25 eV steps) near the S K -edge. Processed spectra, those that have been normalized and “flattened,” are compared to quantify uncertainties due to data processing methods. Future investigations that require well-characterized sulfide standards, such as the ones presented here, may have important implications for understanding sulfur speciation in reduced silicate glasses and minerals with applications for the early Earth, Moon, Mercury, and enstatite chondrites.

Northeastern China is an important Mo resource region in China, with more than 80 Mo deposits and occurrences. The Huojihe deposit located in the Lesser Xing’an Range represents one of the many Mesozoic porphyry Mo deposits in NE China and has been selected for investigation attempting to clarify the possible mechanisms controlling Mo mineralization. In this study, accessory minerals, including zircon and apatite from the causative intrusions (biotite monzogranite and granodiorite), have been analyzed to reveal their chemical and isotopic compositions, which provide insights into the nature of the source magmas and a better understanding of the factors affecting their mineralization potential. Zircon U-Pb dating shows that the biotite monzogranite from the Huojihe deposit formed at 181.6 ± 0.6 Ma, which is identical to the previously reported molybdenite Re-Os age (~181 Ma), indicating that the Mo mineralization is probably genetically related to the intrusion. The intrusion samples share homogeneous geochemical and Sr-Nd isotopic compositions, with initial 87 Sr/ 86 Sr ratios of 0.7072–0.7075 and slightly negative ε Nd (t) values from –2.3 to –1.4, reflecting a uniform magma source. The least-altered apatites show similar (or slightly enriched) initial 87 Sr/ 86 Sr ratios (0.7080–0.7108) and ε Nd (t) values (–4.0 to –1.8), whereas the hydrothermally altered apatites are characterized by significantly higher initial 87 Sr/ 86 Sr ratios (0.7091–0.7119) and more negative ε Nd (t) values (–4.9 to –4.4), probably due to the interaction between the hydrothermal fluids and wall rocks. The zircon ε Hf (t) values vary from –0.9 to 1.7, corresponding to a restricted range of T DM2 ages from 1279 to 1120 Ma. The Sr-Nd-Hf isotope results suggest that the primary magmas associated with the Mo mineralization could be generated from a dominantly Mesoproterozoic lower crust source, with rare contributions from the depleted mantle. The low Ga and Ce and high Eu contents in the magmatic apatite demonstrate that the original magmas have a relatively high oxygen fugacity, which is also supported by the high zircon Ce N /Ce N * (22–568) and Eu N /Eu N * (0.38–0.71) values. Estimates of absolute sulfur concentrations in the mineralization-related melt using available partitioning models for apatite return relatively low magmatic sulfur concentrations in Huojihe (20–100 ppm), indistinguishable from those of larger or smaller deposits or even barren magmatic bodies. Using the sulfur concentration data, a minimum volume of 10–50 km 3 magma has been suggested to be necessary to produce the Huojihe Mo deposit based on mass balance modeling. Besides, the Mo concentration in the original magma has also been roughly estimated based on the magma size (10–50 km 3 ) and the contained Mo in Huojihe (0.275 Mt). The magmatic Mo concentrations (2–10 ppm) are similar to many other porphyry Mo systems (e.g., the Climax-type porphyry Mo deposits), and are also comparable to subeconomic to barren magma systems. This study suggests that pre-degassing enrichments of Mo and S in the original magma is not necessarily important in the formation of the Huojihe Mo deposit; rather, factors other than melt composition may be more critical in forming a porphyry Mo deposit. This understanding might also apply to other porphyry Mo mineralized systems worldwide.

Magnetite is a common mineral in many ore deposits and their host rocks. It contains a wide range of trace elements that can be used to fingerprint deposit types and hydrothermal processes. In this study, we present detailed textural and compositional data on magnetite of the Mina Justa deposit in southern Perú to constrain the formation of iron oxides in the iron oxide Cu-Au (IOCG) deposit type. Two types of magnetite, i.e., mushketovite (T M 1) and granular (T M 2) magnetite, are identified based on their morphology. Mushketovite shows three different zones (central bright, dark, and outer bright) in backscattered electron (BSE) images. The central bright part (T M1 -1), characterized by abundant porosity and inclusions, was intensively replaced by dark magnetite of the median rim (T M1 -2). The outer rim (T M1 -3) is also bright but lacks porosity and inclusions. Granular magnetite (T M 2) is anhedral and shows two different brightness levels (dark and bright) in BSE images. The dark (T M2 -1) and bright (T M2 -2) domains are intergrown, with irregular boundaries. In general, the dark zones of both magnetite types (T M1 -2 and T M2 -1) are characterized by higher Si, Ca, Al, and lower Fe contents than the bright zones. Additionally, the lattice parameters of the two types of magnetite are similar and slightly lower than that of pure magnetite, indicating that some cations (e.g., Si 4+ , Al 3+ ) whose ionic radii are smaller than Fe 2+ or Fe 3+ may have entered into the magnetite structure by simple or coupled substitutions. Our study shows that oxygen fugacity and temperature change are the dominant mechanisms leading to the formation of different types of magnetite at Mina Justa. Primary hematite, identified by Raman spectroscopy, was transformed into magnetite (T M1 -1) due to a sharp decline of f O2 and then replaced by T M1 -2 magnetite during temperature increase, followed by the formation of T M1 -3 due to decreasing temperature, eventually forming the mushketovite with different zones. The granular magnetite may have originally precipitated from hydrothermal fluid that crystallized T M2 -1 and also T M1 -2 magnetite and was then modified by changing temperature and f O2 to form T M2 -2. Even though the iron oxides in IOCG deposits may have formed in the same alteration stage, they could undergo a very complicate evolution process. Therefore, it is important to combine texture and mineral chemistry to investigate the origin and evolution history of iron oxides.

A new mineral, siwaqaite, ideally Ca 6 Al 2 (CrO 4 ) 3 (OH) 12 ·26H 2 O [ P 31 c , Z = 2, a = 11.3640(2) Å, c = 21.4485(2) Å, V = 2398.78(9) Å 3 ], a member of the ettringite group, was discovered in thin veins and small cavities within the spurrite marble at the North Siwaqa complex, Lisdan-Siwaqa Fault, Hashem region, Jordan. This complex belongs to the widespread pyrometamorphic rock of the Hatrurim Complex. The spurrite marble is mainly composed of calcite, fluorapatite, and brownmillerite. Siwaqaite occurs with calcite and minerals of the baryte-hashemite series. It forms hexagonal prismatic crystals up to 250 μm in size, but most common are grain aggregates. Siwaqaite exhibits a canary yellow color and a yellowish-gray streak. The mineral is transparent and has a vitreous luster. It shows perfect cleavage on (1010). Parting or twinning is not observed. The calculated density of siwaqaite is 1.819 g/cm 3 . Siwaqaite is optically uniaxial (–) with ω = 1.512(2), ε = 1.502(2) (589 nm), and non-pleochroic. The empirical formula of the holotype siwaqaite calculated on the basis of 8 framework cations and 26 water molecules is Ca 6.01 (Al 1.87 Si 0.12 ) S1.99 [(CrO 4 ) 1.71 (SO 4 ) 1.13 (SeO 4 ) 0.40 ] S3.24 (OH) 11.63 ·26H 2 O. X‑ray diffraction (XRD), Raman, and infrared spectroscopy confirm the presence of OH - groups and H 2 O molecules and absence of (CO 3 ) 2– groups. The crystal structure of this Cr 6+ -analog of ettringite was solved by direct methods using single-crystal synchrotron XRD data. The structure was refined to an agreement index R 1 = 4.54%. The crystal structure of siwaqaite consists of {Ca 6 [Al(OH) 6 ] 2 ·24H 2 O} 6+ columns with the inter-column space (channels) occupied by (CrO 4 ) 2– , (SO 4 ) 2– , (SeO 4 ) 2– , and (SO 3 ) 2– groups and H 2 O molecules. The tetrahedrally coordinated site occupied by different anion groups is subjected to disordering and rotation of these tetrahedra within the structure. The temperature of siwaqaite formation is not higher than ~70–80 °C, as is evident from the mineral association and as inferred from the formation conditions of the natural and synthetic members of the ettringite group minerals, which are stable at conditions of T < 120 °C and pH = 9.5–13. The name siwaqaite is derived from the name of the holotype locality—Siwaqa area, where the mineral was found.

Negevite, ideally NiP 2 , is a new phosphide mineral from pyrometamorphic complex of the Hatrurim Formation (the Mottled Zone), Southern Levant. It is found in phosphide assemblages of the Hatrurim Basin, south Negev Desert, Israel, and Daba-Siwaqa complex, Jordan. The mineral occurs as tiny isometric grains reaching 15 μm in size and forms intimate intergrowths with other phosphides related to the Fe-Ni-P system. In reflected light, negevite is white with yellowish tint and isotropic. Reflectance values for COM recommended wavelengths [ R (%), λ (nm)] are as follows: 54.6 (470), 55.0 (546), 55.3 (589), 55.6 (650). Chemical composition of the holotype specimen (electron micro-probe, wt%): Ni 42.57, Co 3.40, Fe 2.87, P 42.93, S 8.33, total 100.10, corresponding to the empirical formula (Ni 0.88 Co 0.07 Fe 0.06 ) S1.01 (P 1.68 S 0.31 ) S1.99 . The crystal structure of negevite was solved and refined to R 1 = 1.73% based on 52 independent observed [ I >2σ( I )] reflections. The mineral is cubic, space group Pa 3̅, a = 5.4816(5) Å, V = 164.71(3) Å 3 , and Z = 4. D x = 4.881(1) g/cm 3 calculated on the basis of the empirical formula. Negevite is a first natural phosphide belonging to the pyrite structure type. It is a chemical and structural analog of vaesite, NiS 2 , krutovite, NiAs 2 , and penroseite, NiSe 2 . The well-explored catalytic and photocatalytic properties of a synthetic counterpart of negevite could provide new insights into the possible role of higher phosphides as a source of low-valent phosphorus in prebiotic phosphorylation processes.

This paper is a first detailed report of natural hexagonal solid solutions along the join Fe 2 P–Ni 2 P. Transjordanite, Ni 2 P, a Ni-dominant counterpart of barringerite (a low-pressure polymorph of Fe 2 P), is a new mineral. It was discovered in the pyrometamorphic phosphide assemblages of the Hatrurim Formation (the Dead Sea area, Southern Levant) and was named for the occurrence on the Transjordan Plateau, West Jordan. Later on, the mineral was confirmed in the Cambria meteorite (iron ungrouped, fine octahedrite), and it likely occurs in CM2 carbonaceous chondrites (Mighei group). Under reflected light, transjordanite is white with a beige tint. It is non-pleochroic and weakly anisotropic. Reflectance values for four COM recommended wavelengths are [ R max /R min , % (λ, nm)]: 45.1/44.2 (470), 49.9/48.5 (546), 52.1/50.3 (589), 54.3/52.1 (650). Transjordanite is hexagonal, space group P 62 m ; unit-cell parameters for the holotype specimen, (Ni 1.72 Fe 0.27 ) 1.99 P 1.02 , are: a = 5.8897(3), c = 3.3547(2) Å, V = 100.78(1) Å 3 , Z = 3. D calc = 7.30 g/cm 3 . The crystal structure of holotype transjordanite was solved and refined to R 1 = 0.013 based on 190 independent observed [ I > 2σ( I )] reflections. The crystal structure represents a framework composed of two types of infinite rods propagated along the c -axis: (1) edge-sharing tetrahedra [ M (1)P 4 ] and (2) edge-sharing [ M (2)P 5 ] square pyramids. Determination of unit-cell parameters for 12 members of the Fe 2 P–Ni 2 P solid-solution series demonstrates that substitution of Ni for Fe in transjordanite and vice versa in barringerite does not obey Vegard’s law, indicative of preferential incorporation of minor substituent into M (1) position. Terrestrial transjordanite may contain up to 3 wt% Mo, whereas meteoritic mineral bears up to 0.2 wt% S.