Volume 107, Issue 10

The conditions under which halogens partition in favor of an exsolved fluid relative to the coexisting melt are key for understanding many magmatic processes, including volcanic degassing, evolution of crustal melt bodies, and ore formation. We report new F, Cl, and Br fluid/melt partition coefficients for intermediate to silicic melts, for which F and Br data are particularly lacking; and for varying CO 2 - H 2 O contents to assess the efects of changing fluid composition (X H2O ) on Br fluid/melt partitioning for the first time. The experiments were conducted at pressures 50–120 MPa, temperatures 800–1100 °C, and volatile compositions [molar X H2O = H 2 O/(H 2 O +CO 2 )] of 0.55 to 1, with redox conditions around the Nickel-Nickel Oxygen bufer (ƒ O2 ≈ NNO). Experiments were not doped with Cl, Br, or F and were conducted on natural crystal-bearing volcanic products at conditions close to their respective pre-eruptive state. The experiments therefore provide realistic constraints on halogen partitioning at naturally occurring, brine-undersaturated conditions. Measurements of Br, Cl, and F were made by Secondary Ion Mass Spectrometry (SIMS) on 13 experimental glass products spanning andesite to rhyolitic compositions, together with their natural starting materials from Kelud volcano, Indonesia, and Quizapu volcano, Chile. Fluid compositions were constrained by mass balance. Average bulk halogen fluid/melt partition coefficients and standard deviations are: D Cl fluid/melt = 3.4 (±3.7 1 s.d.), D F fluid/melt = 1.7 (±1.7), and D Br fluid/melt = 7.1 (±6.4) for the Kelud starting material (bulk basaltic andesite), and D Cl fluid/melt = 11.1 (±3.5), D F fluid/melt = 0.8 (±0.8), and D Br fluid/melt = 31.3 (±20.9) for Quizapu starting material (bulk dacite). The large range in average partition coeficients is a product of changing X H2O , pressure and temperature. In agreement with studies on synthetic melts, our data show an exponential increase of halogen D fluid/melt with increasing ionic radius, with partitioning behavior controlled by melt composition according to the nature of the complexes forming in the melt (e.g., SiF 4 , NaCl, KBr). The fundamental chemistry of the diferent halogens (differing ionic size and electronegativities) controls the way in which partitioning responds to changes in melt composition and other variables. Experimental results confirm that more Cl partitions into the fluid at higher bulk Cl contents, higher melt Na, higher fluid X H2O ratios, and lower temperatures. Bromine shows similar behavior, though it seems to be more sensitive to temperature and less sensitive to Na content and X H2O . In contrast, F partitioning into the fluid increases as the melt silica content decreases (from 72 to 56 wt% SiO 2 ), which we attribute to the lower abundance of Si available to form F complexes in the melt. These new data provide more insights into the conditions and processes that control halogen degassing from magmas and may help to inform the collection and interpretation of melt inclusions and volcano gas data.

The incorporation of sulfate ( S O 4 2 − $ \mathrm{SO}_4^{2-} $) into the scorodite (FeAsO 4 ·2H 2 O) lattice is an important mechanism during arsenic (As) fixation in natural and engineered settings. However, spectroscopic evidence of S O 4 2 − $ \mathrm{SO}_4^{2-} $ speciation and local structure in scorodite lattice is still lacking. In this study, X‑ray difraction (XRD), Fourier transform infrared spectroscopy (FTIR), sulfur K -edge X‑ray absorption near edge structure (XANES), and extended X‑ray absorption fine structure (EXAFS) spectroscopic analyses in combination with density functional theory (DFT) calculations were used to determine the local coordination environment of S O 4 2 − $ \mathrm{SO}_4^{2-} $ in the naturally and hydrothermally synthesized scorodite. The S O 4 2 − $ \mathrm{SO}_4^{2-} $ retention in natural scorodite and the effect of pH value and initial Na + concentration on the incorporation of S O 4 2 − $ \mathrm{SO}_4^{2-} $ in synthetic scorodite were investigated. The results showed that trace amounts of S O 4 2 − $ \mathrm{SO}_4^{2-} $ were incorporated in natural scorodite samples. Scanning electron microscopy (SEM) results revealed that S O 4 2 − $ \mathrm{SO}_4^{2-} $ was homogeneously distributed inside the natural and synthetic scorodite particles, and its content in the synthetic scorodite increased slightly with the initial Na + concentration at pH of 1.2 and 1.8. The FTIR features and XANES results indicated that the coordination number (CN) of FeO 6 octahedra around S O 4 2 − $ \mathrm{SO}_4^{2-} $ in scorodite lattice is four. The DFT calculation optimized interatomic distances of S-O were 1.45, 1.46, 1.48, and 1.48 Å with an average of ~1.47 Å, and the interatomic distances of S-Fe were 3.29, 3.29, 3.33, and 3.41 Å with an average of ~3.33 Å. EXAFS analysis gave an average S-O bond length of 1.47(1) and S-Fe bond length of 3.33(1) Å with a CN S-Fe = 4 for S O 4 2 − $ \mathrm{SO}_4^{2-} $ in the scorodite structure, in good agreement with the DFT optimized structure. The results conclusively showed that S O 4 2 − $ \mathrm{SO}_4^{2-} $ in the scorodite lattice may be in the form of a Fe 2 (SO 4 ) 3 -like local structure. The present study is significant for understanding the formation mechanism of scorodite in natural environments and hydrometallurgical unit operations for waste sulfuric acid treatment.

Plate subduction links the Earth’s surface and interior and may change the redox state of the Earth’s mantle. Mantle wedges above subduction zones have high oxygen fugacity compared with other mantle reservoirs, but the cause is debated. Here we analyze high-pressure metamorphic rocks derived from ferromanganese pelagic sediments in the Qilian subduction complex, northwest (NW) China. We show that progressive metamorphism is a process of reducing reactions, in which Mn 4+ is reduced to Mn 2+ . On the global scale, such reactions would release significant amounts of oxygen (~1.27 × 10 12 g year −1 ), estimated from the global flux of MnO in sediments passing into subduction zones. This budget is sufficient to raise the oxygen fugacity of the mantle wedge, hence arc magmas, to a higher level than other mantle reservoirs. In contrast, ferric iron (Fe 3+ ) enters hematite, aegirine, and garnet, without valence change and plays little role in the oxidation of the mantle wedge. Fe 3+ remains stable to depths of >100 km but will transfer to the deeper mantle along with the subducting slab. The manganese reduction process provides a new explanation for high oxygen fugacity in the mantle wedge.

δ-AlOOH is regarded as a potential water carrier that is stable in the Earth’s lower mantle down to the core-mantle boundary along the cold slab geotherm; thus, knowledge of its structural evolution under high pressure is very important for understanding water transport in the Earth’s interior. In this work, we conducted Raman scattering and luminescence spectroscopic experiments on δ-AlOOH at pressures up to 34.6 and 22.1 GPa, respectively. From the collected Raman spectra, significant changes in the pressure dependence of the frequencies of Raman-active modes were observed at ~8 GPa, with several modes displaying softening behavior. In particular, the soft A 1 mode, which corresponds to a lattice vibration of the AlO 6 octahedron correlated to OH stretching vibrations, decreases rapidly with increasing pressure and shows a trend of approaching 0 cm −1 at ~9 GPa according to a quadratic polynomial extrapolation. These results provide clear Raman-scattering spectroscopic evidence for the P 2 1 nm -to- Pnnm structural transition. Similarly, the phase transition was also observed in the luminescence spectra of Cr 3+ in both powder and single-crystal δ-AlOOH samples, characterized by abrupt changes in the pressure dependences of the wavelength of the R-lines and sidebands across the P 2 1 nm -to- Pnnm transition. The continuous decrease in R 2 -R 1 splitting with pressure indicated that the distortion of the AlO 6 octahedron was suppressed under compression. No abnormal features were clearly observed in our Raman or luminescence spectra at ~18 GPa, where the ordered symmetrization or fully centered state with hydrogen located at the midpoint of the hydrogen bond was observed by a previous neutron difraction study. However, some subtle changes in Raman and luminescence spectra indicated that the ordered symmetrization state might form at around 16 GPa.

Jadeite is frequently reported in shocked meteorites, displaying a variety of textures and grain sizes that suggest formation by either solid-state transformation or by crystallization from a melt. Sometimes, jadeite has been identified solely on the basis of Raman spectra. Here we argue that additional characterization is needed to confidently identify jadeite and distinguish it from related species. Based on chemical and spectral analysis of three new occurrences, complemented by first-principles calculations, we show that related pyroxenes in the chemical space (Na) M2 (Al) M1 (Si 2 ) T O 6–(Ca) M2 (Al) M1 (AlSi) TO 6–(⎕) M2 (Si) M1 (Si 2 ) T O6 with up to 2.25 atoms Si per formula unit have spectral features similar to jadeite. However, their distinct stability fields (if any) and synthesis pathways, considered together with textural constraints, have different implications for precursor phases and estimates of impactor size, encounter velocity, and crater diameter. A reassessment of reported jadeite occurrences casts a new light on many previous conclusions about the shock histories preserved in particular meteorites.

The phase stability of orthorhombic Fe 2 S was explored to 194 GPa and 2500 K using powder and multigrain synchrotron X‑ray diffraction techniques. Between 30 and 120 GPa, a C 23-like (Co 2 P, Pnma , Z = 4) Fe 2 S structure is observed and determined to exhibit a highly compressible a axis. A softening of the a axis occurs between 120 and 150 GPa and a relative stiffening of the b and c axes accompanies this compressibility change. Above 150 GPa, the a axis stifens as the b and c axes soften, and a C 37-like (Co 2 Si, Pnma , Z = 4) Fe 2 S unit cell is measured. On the basis of these changes in unit cell geometry, a pressure-induced C 23– C 37 Fe 2 S phase transition is inferred between 120–150 GPa. The C 23 and C 37 ( Pnma , Z = 4) structures are closely related and share the same site symmetries. Forming the C 37 structure from the C 23 structure requires a shortening of the a axis and lengthening of the b and c axes accompanied by a four- to fivefold coordination change. The softening of the a axis above 120 GPa may therefore indicate the onset of a coordination change, and the final compressibility change above 150 GPa may mark the completion of this phase transition. The presented pressure-temperature ( P-T ) stabilities of C 23 and C 37 structures of Fe 2 S are in agreement with and resolve the differing observations of two previous studies (Tateno et al. 2019; Zurkowski et al. 2022). As C 37 Fe 2 S is observed to core-mantle boundary pressures and high temperatures, the C 37 Fe 2 S density profile through Earth’s outer core was determined by fitting the C 23 Fe 2 S equation of state (<120 GPa) and applying a 1.6% volume reduction based on the C 37 Fe 2 S volume residuals to this fit. Comparing the density of liquid C 37 Fe 2 S with that of liquid hcp-Fe (Dewaele et al. 2006) and the seismologically determined density deficit of Earth’s core (Irving et al. 2018), 13.9 ± 1.5 wt% and 8.6 ± 0.8 wt% sulfur are required to match the densities at the CMB and ICB, respectively, for a purely Fe-S core.

Clinopyroxene ferric iron content is an important consideration for garnet-clinopyroxene geothermometry and estimations of water storage in the Earth’s interior but remains difficult and expensive to measure. Here, we develop seven classic algorithms and machine learning methods to estimate Fe 3+ /ΣFe in clinopyroxene using major element data from electron microprobe analyses. The models were first trained using a large data set of clinopyroxene Fe 3+ /ΣFe values determined by Mössbauer spectroscopy and spanning a wide compositional range, with major uncertainties ranging from 0.25 to 0.3 and root-mean-square errors on the test data set ranging from 0.071 to 0.089. After dividing the entire data set into three compositional sub-data sets, the machine learning models were trained and compared for each sub-data set. Our results suggest that ensemble learning algorithms (random forest and Extra-Trees) perform better than principal component analysis-based elastic net polynomial, artificial neural network, artificial neural network ensemble, decision trees, and linear regressions. Using a sub-data set excluding clinopyroxene in spinel peridotite and omphacite in eclogite, the new models achieved uncertainties of 0.15 to 0.2 and root-mean-square errors on the test data set ranging from 0.051 to 0.078, decreasing prediction errors by 30–40%. By incorporating compositional data on coexisting spinel, new models for clinopyroxene in spinel peridotite show improved performance, indicating the interaction between spinel and clinopyroxene in spinel peridotite. Feature importance analysis shows Na + , Ca 2+ , and Mg 2+ to be the most important for predicting Fe 3+ content, supporting the coupled substitution between Ca 2+ -M 2+ and Na + -M 3+ in natural clinopyroxenes. The application of our models to garnet-clinopyroxene geothermometry greatly improves temperature estimates, achieving uncertainties of ±50 °C, compared with uncertainties of ±250 °C using previous models assuming all Fe as Fe 2+ or calculating Fe 3+ by charge conservation. Differences in the ferric iron contents, as calculated using the machine learning models, of clinopyroxenes that did or did not experience hydrogen difusion during their crystallization from basaltic magma support a redox-driven mechanism for hydrogen diffusion in clinopyroxene.

Although everything seemed clear about the Ag-Sb-S compounds belonging to one of the more deeply studied experimental systems, nature allowed us to discover a new polymorph of Ag 3 SbS 3 , which could represent a compound for assessing new technological potentialities. The new mineral species pyradoketosite, Ag 3 SbS 3 (IMA 2019-132), was discovered in the pyrite + baryte + iron oxide ore deposit of the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. It occurs as brittle orange acicular crystals, up to 200 μm in length and 25 μm in thickness, with adamantine luster. Under reflected light, pyradoketosite is slightly bluish-gray, with abundant orange internal reflections. Bireflectance is weak, and anisotropism was not observed, being masked by abundant internal reflections. Minimum and maximum reflectance data for the wavelengths recommended by the Commission on Ore Mineralogy [ R min / R max (%) (λ, nm)] are 32.8/32.9 (470), 30.2/30.7 (546), 29.0/29.6 (589), and 27.5/28.4 (650). Electron microprobe analysis gave (mean of 6 spot analyses, in wt%): Ag 59.81, Sb 22.63, S 17.78, total 100.22. On the basis of (Ag+Sb) = 4 atoms per formula unit, the empirical formula of pyradoketosite is Ag 2.996(11) Sb 1.004(11) S 2.996(15) . Pyradoketosite is monoclinic, space group P 2 1 / n , with a = 13.7510(15), b = 6.9350(6), c = 19.555(2) Å, β = 94.807(4)°, V = 1858.3(3) Å 3 , Z = 12. The crystal structure was solved and refined to R 1 = 0.063 on the basis of 2682 unique reflections with F o > 4σ( F o ) and 191 refined parameters. The structure of pyradoketosite can be described as formed by the alternation of {101} layers: an Sb-rich layer, Sb 3 AgS 3 , and two distinct Ag 8 S 6 layers. This layered organization allows identifying structural relationships with the wittichenite-skinnerite pair. Pyradoketosite is associated with pyrargyrite, tetrahedrite-(Hg), valentinite, and probable pyrostilpnite in baryte + dolomite + quartz veins embedded in metadolostone. Its name derives from the old Greek words “πυρ” (fire) and “άδόκητος” (unforeseen), because of the unexpected occurrence of this third polymorph of the compound Ag 3 SbS 3 .

Trace element geochemistry of pyrite is widely used to monitor ore-forming processes of various types of deposits, but its application to skarn mineral systems is not well constrained due to the multi-stage nature and complex associated mineral assemblages for skarn-type pyrite. The Jiguanzui skarn Au-Cu deposit in the Middle-Lower Yangtze River Valley Metallogenic Belt (Eastern China) is characterized by abundant pyrite that formed in the main-ore (Py1), late-ore (Py2), and post-ore (Py3) stages, which makes it ideal for unraveling skarn ore-fluid evolution. Specifically, Py1 is composed of quartz–pyrite (Py1a), quartz–calcite–pyrite (Py1b), quartz–sericite–pyrite (Py1c), quartz–chlorite±epidote–pyrite (Py1d), and quartz–K-feldspar–pyrite (Py1e), among which Py1a is the most widespread. Py2 comprises calcite–pyrite (Py2a) and calcite–K-feldspar–pyrite (Py2b), and Py3 comprise bird’s-eye pyrite (Py3a) and fingerprint-like pyrite (Py3b). The varying Co/Ni ratios (mostly >2) and coexistence with hydrothermal minerals (quartz, calcite, K-feldspar, chlorite, and epidote) reveal the hydrothermal origin of Py1 and Py2. The Co/Ni (0.97–7.30), Cu/Ni (8.94–186), and As/Ni (0.80–11.7) ratios, and the high trace-element contents indicate that Py3a may have been genetically linked to the waning magmatic-hydrothermal system and increasing meteoric fluid influx. Py1 generally has higher Co-Ni-Se but lower Zn-As-Mo contents than Py2. Py1 in the orebodies also has higher Cu-Au contents than Py2, consistent with the formation of Py1 during the main Au-Cu ore stage. During the ore-fluid evolution, meteoric water input and abundant galena formation in the late-ore calcite-sulfide stage may have controlled the decreasing Se-Co-Ni contents from Py1 to Py2, while the fluid cooling and pH rise (caused by acidic fluid-carbonate rock reactions) may have increased the As-Zn-Mo contents from Py1 to Py2. Py1a in the orebodies has higher As-Ag-Te, but lower Co-Ni-Se contents than Py1a in the wallrocks. The decompression and phase separation (fluid boiling) by extensive hydraulic fracturing may have caused the higher temperature, pH, and f O2 for the Py1a-forming fluids in the orebodies (than those in the wallrocks). Such fluid physicochemical differences may have been the main controlling factor on trace element spatial variations of Py1a. More importantly, the spatial variation of these trace elements in Py1a provides insights for using pyrite trace element geochemistry in skarn mineral exploration.

Clay minerals are widely distributed on the surface of Earth, Mars, and Ceres in the solar systems. Among numerous clay minerals, smectites can record the history of the environment through the exchange of interlayer cations with those in water or through redox reactions with the atmosphere. Therefore, characterization of chemical compositions and crystal structures of smectites is crucial for revealing the paleoenvironment. For instance, the crystal structure within octahedral sheets of iron-bearing smectites changes to trioctahedral sheets under reduced or dioctahedral sheets under oxidizing conditions. Orbital infrared and X‑ray diffraction (XRD) analyses by Mars orbiters/rovers revealed the presence of (Fe,Mg)-smectites on the surface of Mars; however, it has been difficult to characterize the properties of these (Fe,Mg)-smectites, which are rare on the surface of Earth. In this study, we synthesized ferrian (ferric ion-rich) and ferrous (ferrous ion-rich) (Fe,Mg)-saponite and revealed the effect of valence states and iron contents on the crystal structures. These saponites were synthesized using a hydrothermal method under reduced conditions. The crystal structures and valence states of iron were analyzed by XRD, Fourier-transform infrared spectroscopy, transmission electron microscopy, Mössbauer spectroscopy, and X‑ray absorption near edge measurements. The synthesized clays were trioctahedral swelling clays and were identified as saponites. The valence state of iron in these synthesized saponites is altered by oxygen and a reducing agent in water; however, the trioctahedral structures are maintained under both oxidizing and reduced conditions, following a reversible reaction. This mechanism can be interpreted by the desorption and adsorption of hydrogen in the hydroxyls of the octahedral sheets of the smectite layers. The maximum basal spacing of the (02 l ) lattice plane in the octahedral sheets was defined by compiling various smectite data. When the basal spacing of (02 l ) is larger than the maximum in dioctahedral smectites, smectite can be identified as trioctahedral smectite. The redox state of iron in the octahedral sheet cannot be determined from the basal spacing of (02 l ). We revealed that the iron content in the trioctahedral sheet has a linear relationship with lattice parameter b . This provides a method to estimate the iron content in saponite from XRD data. The XRD profiles of smectites found at the Yellowknife Bay on Mars can be explained only by trioctahedral smectites, and the iron content in the octahedral sheet is roughly estimated to be 0.5–1.7 in a half-unit cell. These results indicate that the presence of (Fe,Mg)-saponite implies a reduced environment during the formation and that this iron-bearing saponite has both oxidation and reduction capabilities depending on the environment.

Perovskite, CaTiO 3 , originally described as a cubic mineral, is known to have a distorted (orthorhombic) crystal structure. We herein report on the discovery of natural cubic perovskite. This was identified in gehlenite-bearing rocks occurring in a pyrometamorphic complex of the Hatrurim Formation (the Mottled Zone), in the vicinity of the Dead Sea, Negev Desert, Israel. The mineral is associated with native α-(Fe,Ni) metal, schreibersite (Fe 3 P), and Si-rich fluorapatite. The crystals of this perovskite reach 50 μm in size and contain many micrometer-sized inclusions of melilitic glass. The mineral contains significant amounts of Si substituting for Ti (up to 9.6 wt% SiO 2 ), corresponding to 21 mol% of the davemaoite component (cubic perovskite-type CaSiO 3 ), in addition to up to 6.6 wt% Cr 2 O 3 . Incorporation of trivalent elements results in the occurrence of oxygen vacancies in the crystal structure; this is the first example of natural oxygen-vacant AB O 3 perovskite with the chemical formula Ca(Ti,Si,Cr)O 3–δ (δ ~0.1). Stabilization of cubic symmetry (space group Pm 3̅ m ) is achieved via the mechanism not reported so far for CaTiO 3 , namely displacement of an O atom from its ideal structural position (site splitting). The mineral is stable at atmospheric pressure to 1250 ± 50 °C; above this temperature, its crystals fuse with the embedded melilitic glass, yielding a mixture of titanite and anorthite upon melt solidification. The mineral is stable upon compression to at least 50 GPa. The a lattice parameter exhibits continuous contraction from 3.808(1) Å at atmospheric pressure to 3.551(6) Å at 50 GPa. The second-order truncation of the Birch-Murnaghan equation of state gives the initial volume V 0 equal to 55.5(2) Å 3 and room temperature isothermal bulk modulus K 0 of 153(11) GPa. The discovery of oxygen-deficient single perovskite suggests previously unaccounted ways for incorporation of almost any element into the perovskite framework up to pressures corresponding to those of the Earth’s mantle.

Nazarovite, Ni 12 P 5 , is a new natural phosphide discovered on Earth and in meteorites. Terrestrial nazarovite originates from phosphide assemblages confined to pyrometamorphic suite of the Hatrurim Formation (the Mottled Zone), the Dead Sea basin, Negev desert, Israel. Meteoritic nazarovite was identified among Ni-rich phosphide precipitates extracted from the Marjalahti meteorite (main group pallasite). Terrestrial mineral occurs as micrometer-sized lamella intergrown with transjordanite (Ni 2 P). Meteoritic nazarovite forms chisel-like crystals up to 8 μm long. The mineral is tetragonal, space group I 4/ m . The unit-cell parameters of terrestrial and meteoritic material, respectively: a 8.640(1) and 8.6543(3), c 5.071(3), and 5.0665(2) Å, V 378.5(2), and 379.47(3) Å 3 , Z = 2. The crystal structure of terrestrial nazarovite was solved and refined on the basis of X‑ray single-crystal data ( R 1 = 0.0516), whereas the structure of meteoritic mineral was refined by the Rietveld method using an X‑ray powder diffraction profile ( R B = 0.22%). The mineral is structurally similar to phosphides of schreibersite–nickelphosphide join, Fe 3 P-Ni 3 P. Chemical composition of nazarovite (terrestrial/meteoritic, electron microprobe, wt%): Ni 81.87/78.59, Fe <0.2/4.10; Co <0.2/0.07, P 18.16/17.91, total 100.03/100.67, leading to the empirical formula Ni 11.97 P 5.03 and (Ni 11.43 Fe 0.63 Co 0.01 ) 12.07 P 4.94 , based on 17 atoms per formula unit. Nazarovite formation in nature, both on Earth and in meteorites, is related to the processes of Fe/Ni fractionation in solid state, at temperatures below 1100 °C.

Zinconigerite-2 N 1 S ZnSn 2 Al 12 O 22 (OH) 2 and zinconigerite-6 N 6 S Zn 3 Sn 2 Al 16 O 30 (OH) 2 are two new minerals with different numbers and ratios of nolanite ( N ) and spinel ( S ) modules. Both phases have been discovered in the Xianghualing skarn, Hunan Province, China. Zinconigerite-2 N 1 S (zn-2 N 1 S ) and zinconigerite-6 N 6 S (zn-6 N 6 S ) are named for their chemical composition, number, and ratios of N - S modules, according to the nomenclature of the nolanite-spinel polysomatic series of Armbruster (2002). Both phases occur as aggregates, sub-to-euhedral crystals, with maximal dimensions up to 100 μm, within fluorite aggregates, and they are closely associated with phlogopite, chrysoberyl, magnetite, cassiterite, margarite, and nigerite-taaffeite group minerals. They do not show fluorescence in long- or short-wave ultraviolet light. The calculated densities are 4.456 g/cm 3 for zn-2 N 1 S and 4.438 g/cm 3 for zn-6 N 6 S . Optically, zn-2 N 1 S is uniaxial (+) with ω = 1.83(1), ε = 1.84(2); zn-6 N 6 S is uniaxial (+) with ω = 1.85(1), ε = 1.87(2) (λ = 589 nm). Their chemical compositions by electron-microprobe analyses give the empirical formulas (Zn 0.734 Mn 0.204 Na 0.122 Ca 0.063 Mg 0.044 ) Σ1.166 (Sn 1.941 Zn 0.053 Ti 0.007 ) Σ2 A l 11.018 F e 0.690 3 + Z n 0.200 S i 0.092 12 O 22 ( O H ) 2 $\left(\mathrm{Al}_{11.018} \mathrm{Fe}_{0.690}^{3+} \mathrm{Zn}_{0.200} \mathrm{Si}_{0.092}\right)_{12} \mathrm{O}_{22}(\mathrm{OH})_2$for zn-2 N 1 S and Z n 1.689 M n 0.576 M g 0.328 F e 0.407 3 + Σ 3 S n 1.882 Z n 0.047 $\left(\mathrm{Zn}_{1.689} \mathrm{Mn}_{0.576} \mathrm{Mg}_{0.328} \mathrm{Fe}_{0.407}^{3+}\right)_{\Sigma 3}\left(\mathrm{Sn}_{1.882} \mathrm{Zn}_{0.047}\right.$ T i 0.071 Σ 22 A l 14.675 F e 1.088 3 + N a 0.13 C a 0.086 S i 0.017 Σ 15.996 O 30 ( O H ) 2 $ \left.\mathrm{Ti}_{0.071}\right)_{\Sigma 22}\left(\mathrm{Al}_{14.675} \mathrm{Fe}_{1.088}^{3+} \mathrm{Na}_{0.13} \mathrm{Ca}_{0.086} \mathrm{Si}_{0.017}\right)_{\Sigma 15.996} \mathrm{O}_{30}(\mathrm{OH})_2$for zn-6 N 6 S . Both phases have trigonal symmetry; the unit-cell parameters of zn-2 N 1 S ( P 3 m 1) and zn-6 N 6 S ( R 3 m ), refined from single-crystal X‑ray diffraction data, are, a = 5.7191(2) and 5.7241(2) Å, c = 13.8380(6) and 55.5393(16) Å, V = 391.98(3) and 1575.96(12) Å 3 , and Z = 1 and 3, respectively. The structure of zn-2 N 1 S is characterized by the alternating O-T 1 -O-T 2 -O-T 1 layers stacked along the c -axis, showing the connectivity of N - S - N . The polyhedral stacking sequence of zn-6 N 6 S is 3 × (O-T 1 -O-T 2 -O-T 2 -O-T 1 ), reflecting a N - S - S - N - N - S - S - N - N - S - S - N connectivity of the polysomatic structure. By contrast, the structure of zn-2 N 1 S shows the elemental replacements of Al → Sn and Al → Zn, suggesting the substitution mechanism of 2Al → Zn + Sn. The complex substitution of Zn by multiple elements (Al, Fe 3+ , Mn, Mg) in the structure of zn-6 N 6 S , is coupled with the low occupancy of Al5-octahedra. Fe 3+ → Al substitution occurs in Al1-tetrahedra of both zn-2 N 1 S and zn-6 N 6 S . The new polysomes, zn-2 N 1 S and zn-6 N 6 S , likely crystallized under F-rich conditions during the late stages of the Xianghualing skarn formation. The discovery of zn-2 N 1 S and zn-6 N 6 S provides new insights into the crystal chemistry of the N - S polysomatic series and its origin.

Ikaite is a calcium carbonate hexahydrate that forms at temperatures close to the freezing point of water; thus, its occurrence is associated with cryogenic conditions. This mineral is metastable and quickly transforms to calcite at temperatures above 5 °C. Pseudomorphs of calcite after ikaite are known as glendonite. The nanostructure of 25 000–43 000 year old glendonite from Victoria cave (Southern Ural, Russia) was investigated in search of structural features indicative of the ikaite-to-calcite transformation. Scanning electron microscope images display several micrometer- to submicrometer-size pores and indicate high intergranular porosity among the loosely aggregated grains. Transmission electron microscopy (TEM) data show evidence of 10–20 nm nanotwins [twin law (101̅4)] and 10–40 nm overlapping nanograins. Scanning TEM images reveal that the individual grains contain 5–10 nm long and 2–4 nm wide mesopores (sizes between 2 and 50 nm), which are aligned parallel to [1010] of calcite and might be associated with a crystallographically oriented dehydration of the precursor ikaite. Fourier transform infrared spectroscopy reveals no evidence of structural water but absorption bands related to molecular water trapped in fluid inclusions are present. Nitrogen absorption/desorption measurements show that the specific surface area of 5.78 m 2 /g and the pore volume of ~0.07 cm 3 /g for calcite, the constituent of glendonite, are comparable to those of a common natural calcite. We suggest that the aligned mesopores, frequently occurring twins, small grain size, presence of aqueous inclusions and the high micrometer- to submicrometer-size intergranular porosity arise from the ikaite-to-calcite transformation and thus may be used as criteria for the former presence of ikaite and hence for cold paleotemperatures. However, since similar features might also be common in biogenic carbonates, the diagnostic macroscopic pseudomorphs after ikaite are equally important for identifying glendonites and inferring cryogenic conditions.

In this study, we use Cr-spinel as an efficient indicator to evaluate the oxygen fugacity evolution of the Xiarihamu Ni-Cu deposit and the Shitoukengde non-mineralized intrusion. Oxygen fugacity is calculated using an olivine-spinel oxybarometer, with spinel Fe 3+ /ΣFe ratios determined by a secondary standard calibration method using an electron microprobe. Cr-spinel Fe 3+ /ΣFe ratios of the Xiarihamu Ni-Cu deposit vary from 0.32 ± 0.09 to 0.12 ± 0.01, corresponding to magma f O2 values ranging from ΔQFM+2.2 ± 1.0 to ΔQFM-0.6 ± 0.2. By contrast, those of the Shitoukengde mafic-ultramafic intrusion increase from 0.07 ± 0.02 to 0.23 ± 0.04, corresponding to magma f O2 varying from ΔQFM-1.3 ± 0.3 to ΔQFM+1.0 ± 0.5. A positive correlation between f O2 and Cr-spinel Fe 3+ /ΣFe ratios suggests that the Cr-spinel Fe 3+ /ΣFe ratios can be used as an indicator for magma f O2 . The high f O2 (QFM+2.2) of the harzburgite in the Xiarihamu Ni-Cu deposit suggests that the most primitive magma was characterized by relatively oxidized conditions, and then became reduced during magmatic evolution, causing S saturation and sulfide segregation to form the Xiarihamu Ni-Cu deposit. The evolution trend of the magma f O2 can be reasonably explained by metasomatism in mantle source by subduction-related fluid and addition of external reduced sulfur from country gneisses (1.08–1.14 wt% S) during crustal processes. Conversely, the primitive magma of the Shitoukengde intrusion was reduced and gradually became oxidized (from QFM-1.3 to QFM+1.0) during crystallization. Fractional crystallization of large amounts of Cr-spinel can reasonably explain the increasing magma f O2 during magmatic evolution, which would hamper sulfide precipitation in the Shitoukengde intrusion. We propose that the temporal evolution of oxygen fugacity of the mantle-derived magma can be used as one of the indicators for evaluating metallogenic potential of Ni-Cu sulfide deposits and the reduction processes from mantle source to shallow crust play an important role in the genesis of magmatic Ni-Cu sulfide deposits.

In this issue of New Mineral Names, a thematic approach is used to help provide context for advances and discoveries in mineralogy. Changes in nomenclature and the definition of minerals have led to new mineral descriptions. Here we look into minerals, which might not have been classified as such 50 years ago, and their associated new classification schemes that open the door to new mineral descriptions: goldhillite, radvaniceite, zvěstovite-(Zn), oberwolfachite, cesiokenopyrochlore, orishchinite, kahlenbergite, zoisite-(Pb), and dendoraite-(NH 4 ).