Volume 106, Issue 4

Trace/minor element variation in pyrite is a feature that has proved invaluable for reconstructing a wide range of geological processes. Routine reflectance observations commonly fail to constrain this variation due to the typically subtle and barely perceptible change in reflectance brought about by deviation from ideal stoichiometry. Such differences may be difficult or impossible to observe in conventional polished sections using standard optical microscopes, at least without oil immersion. Chemical etching and staining, although widely used, are destructive, hazardous, or both, and the etching process is not completely reproducible. Here we use the γ correction method to enhance optical digital signal differences obtained in reflected light to constrain compositional heterogeneity in pyrite from a representative hydrothermal ore deposit in eastern China. The γ-enhanced images show significant reflectance variation caused by compositional heterogeneity, confirmed by quantitative electron microprobe analysis and qualitative imaging. Higher reflectance domains in γ-enhanced images correspond to increases in the effective number of free electrons, whereas darker domains are attributed to the decrease of these free electrons by trace/minor element substitution in pyrite (e.g., As). Gamma correction provides a rapid, effective, non-destructive method to constrain compositional heterogeneity of pyrite through enhancement of reflectance variation. Used alone, this method is unable to determine the chemical composition due to simultaneous substitutions, causing a disparate increase or decrease of reflectance, in most ore minerals. Nevertheless, γ correction may be sufficient to predict the substitution of trace/minor elements under the optical microscope prior to scanning electron microscope imaging and quantitative investigation of mineral composition and may help constrain links between textures and compositions of pyrite in evolving ore systems, which could also be applied to other ore minerals with negligible bireflectance.

Parent body thermal metamorphism is an important process that alters the structure of organic matter in the parent asteroid of meteorites. Increasing and progressing thermal metamorphism results in carbonization and graphitization of carbonaceous matter in the parent body. Such modifications in the carbon structures can be studied by Raman microspectroscopy, thanks to its high sensitivity to structure and bonding within carbonaceous molecules. We have characterized polyaromatic carbonaceous matter in a total of 24 Antarctic CV3 and CO3 chondrites using micro-Raman imaging spectroscopy in an effort to better understand parent body thermal metamorphism and assess its effects on the carbon structures. Raman spectral parameters of the first-order carbon peaks (D and G) were extracted from at least 200 spectra for each meteorite and were compared to deduce relationships that yield information regarding the thermal metamorphism conditions. We also show, for the first time, spectral trends and relations of the second-order carbon peaks (2D and D+G) within the 2500–3200 cm –1 with thermal metamorphic history. The second-order peaks appear to contain information that is lacking in the first-order peaks. Based on the second-order carbon peak parameters, we tentatively classify four CV3 chondrites into subtypes, and reclassify another. Peak metamorphic temperatures of the investigated meteorites have been estimated based on the width of the D band as well as the calculated Raman spectral curvature. Estimated temperatures appear to correlate well with the assigned petrologic types. We have calculated higher peak metamorphic temperatures for the CV3 chondrites than for the considered CO3 chondrites and further showed that the peak metamorphic temperatures of CV3 oxA chondrites are higher than those of CV3 oxB , indicating possibly different metamorphic conditions for the two oxidized subtypes. We observe that there is a relatively larger temperature increase going from CO3.2 to CO3.4 (150 °C increase) compared to CO3.4–CO3.6 (20 °C), which may indicate that the graphitization and structural ordering of carbon reach a critical temperature regime around petrologic type CO3.3.

We measured the apatite etch rate v R in 5.5 M HNO 3 at 21 °C as a function of orientation. Results for Durango apatite evidence that v R varies by a factor >5 with angle to the c -axis. Our measurements also provided track etch rates v T and surface etch rates v S . However, these cannot be combined for calculating track etching or counting efficiencies. By inserting the measured etch rates in a recent model, we calculate the geometries and dimensions of surface tracks in different apatite faces. The proposed model must be recalibrated for different etching protocols and adapted for other minerals. We submit that the new model justifies reviewing track counting efficiencies based on the existing ( v B - v T ) etch model. We anticipate that this will have an effect on practical aspects of fission track dating. Single-track step-etch data show that the confined track lengths increase with etch time at a decreasing average rate v L that differs from the track etch rate v T and the apatite etch rate v R . Both v T and v L exhibit large track-to-track differences that produce irreducible length variation related to the latent-track structure resulting from formation and annealing. Step etching and track width measurements are effective for reducing or eliminating procedure-related artifacts from track length data, and so for accessing more fundamental track properties.

Nanophase materials including silicates, aluminosilicates, and iron oxides are widespread on Mars. These minerals are important because they likely represent a solid-phase record of ancient climatic conditions on the martian surface. Identification and characterization of nanophase compounds is technically challenging due to the small size and poorly ordered nature of these materials, particularly because their chemical compositions can vary widely. This study presents spectra of several synthetic allophane and imogolite samples with a range of chemical compositions that are typical of the natural variability of allophanic materials. These samples were formed under controlled conditions and have been thoroughly characterized in terms of chemical composition and short-range structure. Analyses confirmed that the synthetic materials were allophane and imogolite and were structurally similar to previously studied natural and synthetic examples of these phases. NMR and XAFS data indicated that high-Al proto-imogolite allophanes were similar in structure to imogolite but were less well ordered, and supported the proposed nanoball structures based on rolled octahedral Al sheets. Increasing Si content in allophane produced increasing tetrahedral Al substitution as well as polymerized Si chain structures at Al-Si mole ratios of 1:1, and sheets and possible framework structures at Al-Si mole ratios of 1:2. Fe in allophanes and imogolites substituted exclusively for octahedral Al. Reflectance spectra of the synthetic allophanes and imogolites were comparable to previously analyzed samples. Variations in Fe content of allophane and imogolite resulted in some observable changes in visible/near-infrared (VNIR) reflectance spectra, but these changes were not detectable in emission spectra. Emission spectra of the samples suggest that variations in Al-Si ratio of allophanes should be detectable using remotely sensed data. Because allophanes with different Al-Si ratios typically form in very different environments, this could be significant for interpretation of formation conditions on Mars, with high-Al compositions suggesting possible tephra weathering and high-Si compositions indicating possible formation from thermal waters.

In this paper, we discuss information on crystal structure and morphology available from nuclear magnetic resonance (NMR) spectroscopy of 207 Pb for the mineral family [Pb(4 f )] 2 [Pb(6 h )] 3 (AO 4 ) 3 Cl with A = V (vanadinite), P (pyromorphite), and As (mimetite). The isotropic chemical shift of the 207 Pb atoms at Wyckoff positions 4 f and 6 h was (re-)determined from either static single-crystal or magic angle spinning NMR experiments. This isotropic shift can be linearly correlated to the unit-cell volume within the mineral family, and in the wider context of lead-bearing minerals, to the shortest Pb-O distance for position 4 f , in which 207 Pb is solely coordinated by oxygen. By evaluating the number of resonances and their respective line widths in the 207 Pb-NMR spectra of these three naturally grown minerals, it could be established that vanadinite forms single-domain macroscopic crystals with very small mosaicity, whereas pyromorphite crystals show NMR characteristics, which can be interpreted as being caused by significant mosaicity. In some instances, this mosaic spread could be quantitatively approximated by a Gaussian distribution with a standard deviation angle of σ = 5°. In contrast, our mimetite specimen was composed of multiple sub-crystals with a very high variability of orientations, going beyond mere mosaicity effects. By extending the NMR methodology presented here to other minerals, it may be possible to gain new insights about structure-property relationships and the morphology of natural grown minerals.

The high-pressure CaCO 3 phase diagram has been the most extensively studied within the carbonates group. However, both the diverse mineralogy of carbonates and the abundance of solid solutions in natural samples require the investigation of multi-component systems at high pressures ( P ) and temperatures ( T ). Here we studied a member of the CaCO 3 –SrCO 3 solid-solution series and revealed the effect of cationic substitution on the pressure-induced phase transitions in calcium carbonate. A synthetic solid solution Ca 0.82 Sr 0.18 CO 3 was studied in situ by Raman spectroscopy in a diamond-anvil cell (DAC) up to 55 GPa and 800 K. The results of this work show significant differences in the high-pressure structural and vibrational behavior of the (Ca,Sr)CO 3 solid solution compared to that of pure CaCO 3 . The monoclinic CaCO 3 -II-type structure (Sr-calcite-II) was observed already at ambient conditions instead of the “expected” rhombohedral calcite. The stress-induced phase transition to a new high-pressure modification, termed here as Sr-calcite-IIIc, was detected at 7 GPa. Sr-calcite-VII formed already at 16 GPa and room T , which is 14 GPa lower compared to CaCO 3 -VII. Finally, crystallization of Sr-aragonite was detected at 540 K and 9 GPa, at 200 K lower T than pure aragonite. Our results indicate that substitution of Ca 2+ by bigger cations, such as Sr 2+ , in CaCO 3 structures can stabilize phases with larger cation coordination sites (e.g., aragonite, CaCO 3 -VII, and post-aragonite) at lower P-T conditions compared to pure CaCO 3 . The present study shows that the role of cationic composition in the phase behavior of carbonates at high pressures should be carefully considered when modeling the deep carbon cycle and mantle processes involving carbonates, such as metasomatism, deep mantle melting, and diamond formation.

Controversies on the origin of zircon, corundum, titanomagnetite, and quartz megacrysts in alkali basalts mostly reflect the lack of direct evidence of a “melt reservoir” required for their formation. Various mineral megacrysts are carried up by Cenozoic (mostly younger than 25 Ma) alkali basalts that extend more than 4000 km along eastern China. Here we report unusual inclusions in corundum megacrysts from Changle, and we attribute their origin to the existence of a FeO*-SiO 2 -Al 2 O 3 - ZrO 2 -rich melt. The inclusions, analyzed using electron microprobe and Raman microscopy, may be divided into two types. Type I inclusions are dominated by glassy materials, may exhibit a dark part in backscattered eletron (BSE) images composed of quartz, corundum, and an amorphous substance (AS-1), and a bright part in BSE images composed of baddeleyite and a second distinct amorphous substance (AS-2). Compared with AS-1, AS-2 has higher concentrations of ZrO 2 and FeO* but lower concentrations of Al 2 O 3 and SiO 2 . We argue that the formation temperature of Type I inclusions is ~1200 °C, and the generation of their bright and dark parts in BSE images may be attributed to the coexistence of immiscible melts. Type II inclusions are composed of zircon, quartz, and an amorphous substance (AS-3). Both types of inclusions might be derived from a similar parent melt, which is FeO*-SiO 2 -Al 2 O 3 -ZrO 2 -rich. New secondary ion mass spectroscopy (SIMS) in situ U-Pb ages of 18 Ma and 13–14 Ma for zircon inclusions suggest that the corundum megacrysts, occurring in basaltic host rocks distributed along the middle segment of the north and south-trending Tanlu fault zone, formed from precursor residual magmas related to underplating basalts stalled at the crust-mantle boundary, and were brought to the surface by entrainment in later basalts. This study provides new insights into the genesis of the corundum-related megacryst suite.

The study of nominally anhydrous minerals with vibrational spectroscopy, despite its sensitivity, tends to produce large uncertainties (in absorbance or intensity) if the observed dispersion of the values arising from the anisotropy of interaction with light in non-cubic minerals is not assessed. In this study, we focused on Raman spectroscopy, which allows the measurement of crystals down to a few micrometers in size in backscattered geometry, and with any water content, down to 200 ppm by weight of water. Using synthetic hydrous single-crystals of olivine and wadsleyite, we demonstrate that under ideal conditions of measurement and sampling, the data dispersion reaches ±30% of the average (at 1σ) for olivine and ±32% for wadsleyite, mostly because of their natural anisotropy. As this anisotropy is linked to physical properties of the mineral, it should not be completely considered as an error without treatment. By simulating a large number of measurements with a 3D model of the OH/Si spectral intensity ratio for olivine and wadsleyite as a function of orientation, we observe that although dispersion increases when increasing the number of measured points in the sample, analytical error decreases, and the contribution of anisotropy to this error decreases. With a sufficient number of points (five to ten, depending on the measurement method), the greatest contribution to the error on the measured intensities is related to the instrument’s biases and reaches 12 to 15% in ideal cases, indicating that laser and power drift corrections have to be carefully performed. We finally applied this knowledge on error sources (to translate data dispersion into analytical error) on olivine and wadsleyite standards with known water contents to build calibration lines for each mineral to convert the intensity ratio of the water bands over the structural bands (OH/Si) to water content. The conversion factors from OH/Si to parts per million by weight of water (H 2 O) are 93 108 ± 24 005 for olivine, 250 868 ± 53 827 for iron-bearing wadsleyite, and 57 862 ± 12 487 for iron-free wadsleyite, showing the strong effect of iron on the spectral intensities.

Carbonate minerals play a dominant role in the deep carbon cycle. Determining the high-pressure and high-temperature vibrational properties of carbonates is essential to understand their anharmonicity and their thermodynamic properties under crustal and upper mantle conditions. Building on our previous study on aragonite, calcite (both CaCO 3 polymorphs), dolomite [CaMg(CO 3 ) 2 ], magnesite (MgCO 3 ), rhodochrosite (MnCO 3 ), and siderite (FeCO 3 ) (Farsang et al. 2018), we have measured the pressure- and temperature-induced frequency shifts of Raman-active vibrational modes up to 6 GPa and 500 °C for all naturally occurring aragonite- and calcite-group carbonate minerals, including cerussite (PbCO 3 ), strontianite (SrCO 3 ), witherite (BaCO 3 ), gaspeite (NiCO 3 ), otavite (CdCO 3 ), smithsonite (ZnCO 3 ), and spherocobaltite (CoCO 3 ). Our Raman and XRD measurements show that cerussite decomposes to a mixture of Pb 2 O 3 and tetragonal PbO between 225 and 250 °C, smithsonite breaks down to hexagonal ZnO between 325 and 400 °C, and gaspeite to NiO between 375 and 400 °C. Spherocobaltite breaks down between 425 and 450 °C and otavite between 375 and 400 °C. Due to their thermal stability, carbonates may serve as potential reservoirs for several metals (e.g., Co, Ni, Zn, Cd) in a range of crustal and upper mantle environments (e.g., subduction zones). We have determined the isobaric and isothermal equivalents of the mode Grüneisen parameter and the anharmonic parameter for each Raman mode and compare trends in vibrational properties as a function of pressure, temperature, and chemical composition with concomitant changes in structural properties. Finally, we use the anharmonic parameter to calculate the thermal contribution to the internal energy and entropy, as well as the isochoric and isobaric heat capacity of certain carbonates.

High concentrations of vanadium cause very unusual coloration in hibonite (purple) and grossite (light violet) crystals in an exotic mineral assemblage from Sierra de Comechingones (Argentina). In the hibonite (CaAl 12 O 19 ) structure vanadium ions, in various valence states (divalent, trivalent, and tetravalent), may be distributed over five crystallographic sites with coordinations corresponding to different polyhedra, namely, three unequal octahedra [ M 1 (D 3 d ), M 4 (C 3 v ), and M 5 (C s )], one M 3 tetrahedron (C 3 v ), and one unusual fivefold-coordinated trigonal bipyramid M 2 (D 3 h ). Possible locations of vanadium ions in grossite (CaAl 4 O 7 ) are limited to two crystallographically distinct sites ( T 1 and T 2, both C 1 ) in tetrahedral coordination. The combination of single-crystal X‑ray diffraction and absorption spectroscopy techniques aided by chemical analyses has yielded details on the nature of the vanadium-induced color in both hibonite and grossite crystals. In hibonite, both M 4 face-sharing octahedral and M 2 trigonal bipyramid sites of the R -block are partially occupied by V 3+ . Strongly polarized bands recorded at relatively low energies in optical absorption spectra indicate that V 2+ is located at the M 4 octahedral site of the hibonite R -block. Chemical analyses coupled with an accurate determination of the electron densities at structural sites in hibonite suggest that the vanadium ions occupy about 10 and 5% of the M 4 and M 2 sites, respectively. For grossite, polarized optical absorption spectra reveal no indications of V 2+ ; all observed absorption bands can be assigned to V 3+ in tetrahedral coordination. Although not evident by the observed electron densities at the T sites of grossite (due to the low-V content), longer bond distances, and a higher degree of polyhedral distortion suggest that V 3+ is located at the T 2 site.

Rutile is a common mineral in many types of ore deposits and can carry chemical or isotopic information about the ore formation. For closer understanding of this information, the mechanisms of incorporation of minor elements should be known. In this work, we have investigated natural rutile crystals with elevated concentrations of WO 3 (up to 17.7 wt%), Cr 2 O 3,tot (7.5), V 2 O 3,tot (4.1), FeO tot (7.3), and other metals. X‑ray absorption spectroscopy (XAS) of rutile at the Fe K , Cr K , V K , and W L 1 and L 3 edges shows that all cations are coordinated octahedrally. The average oxidation state of V is +3.8, and that of Cr is near +4. Shell-by-shell fitting of the W L 3 EXAFS data shows that W resides in the rutile structure. Raman spectroscopy excludes the possibility of hydrogen as a charge-compensating species. High-resolution TEM and electron diffraction confirm this conclusion as the entire inspected area consists of rutile single crystal with variable amounts of metals other than Ti. Our results show that rutile or its precursors can be efficient vehicles for tungsten in sedimentary rocks, leading to their enrichment in W and possibly later fertility with respect to igneous ore deposits. Leucoxene, a nanocrystalline mixture of Ti and Fe oxides, is an especially suitable candidate for such a vehicle.

Many rhyolites contain quartz crystals with relatively Ti-rich rims and Ti-poor cores, with a sharp interface between zones, attributed to partial dissolution followed by overgrowth following a heating event due to mafic recharge of the system. Quartz crystals in the compositionally zoned, high-silica rhyolite Tshirege Member of the Bandelier Tuf erupted at 1.26 Ma from the Valles caldera, New Mexico, show a range in zoning styles with Ti-rich rims becoming more abundant upward in the ignimbrite sheet among progressively less evolved magma compositions. Here we compare times between quartz overgrowth and eruption obtained by applying Ti diffusion coefficients to Ti concentration profiles in Tshirege Member quartz crystals with those from cathodoluminescence (CL) brightness profiles and show that panchromatic CL provides only a crude proxy for Ti in quartz in this unit. Titanium concentrations are measured to detection limits of ~1.2 ppm with small analytical errors (<5%) using MAN backgrounds, blank corrections, and oblique corrected transects to resolve diffusion-relaxed zone boundaries as thin as ~10 μm. Timescales derived from Ti profiles using the widely applied Ti-in-quartz diffusion coefficients of Cherniak et al. (2007) range from 60 to 10 000 years, suggesting heating and mobilization events at different times prior to the eruption. However, the use of the newer Ti diffusivities reported by Jollands et al. (2020) yields timescales up to three orders of magnitude longer, including results that are geologically unreasonable for the Bandelier system. We suggest that assumptions commonly made in diffusion modeling, specifically about the form of the Ti zoning profile prior to diffusive relaxation, may be invalid. Melt inclusions in the Ti-poor cores of late-erupted quartz are chemically akin to early erupted melt compositions, while adhering and groundmass glasses more closely reflect the composition of the host pumice. Heating and mobilization events identified from quartz Ti zoning are thus linked to overall compositional zoning of the tuff, which may have been produced by repeated episodes of melting of a crystal cumulate cognate to the early-erupted, evolved rhyolite. Quartz-hosted melt inclusion faceting suggests the development of a crystal mush over a minimum time frame of 1000–10 000 years prior to the recharge events that produced the erupted Tshirege magma at 1.26 Ma.

The new mineral vasilseverginite, ideally Cu 9 O 4 (AsO 4 ) 2 (SO 4 ) 2 , was found in the Arsenatnaya fumarole at the second scoria cone of the Northern Breakthrough of the Great Tolbachik Fissure Eruption, Tolbachik volcano, Kamchatka, Russia. It is associated with tenorite, lammerite, stranskiite, lammerite-β, langbeinite, dolerophanite, sanidine, hematite, and gahnite. Vasilseverginite occurs as prismatic crystals up to 0.02 × 0.02 × 0.06 mm 3 combined in groups or interrupted crusts up to 1 × 2 cm 2 in area and up to 0.1 mm thick. It is transparent, bright green, with vitreous luster. D calc is 4.41 g⋅cm –3 . Vasilseverginite is optically biaxial (–), α 1.816(5), β 1.870(5), γ 1.897(5), estimated 2 V is 30(15)°. Chemical composition (wt%, electron-microprobe) is: CuO 64.03, ZnO 0.79, Fe 2 O 3 0.25, P 2 O 5 0.05, As 2 O 5 20.83, SO 3 14.92, total 100.87. The empirical formula calculated on O = 20 apfu is Cu8.78Zn0.11Fe0.033+Σ 8.92As1.98P0.01 S2.03O20 $\left(\mathrm{Cu}_{8.78} \mathrm{Zn}_{0.11} \mathrm{Fe}_{0.03}^{3+}\right)_{\Sigma 8.92} \mathrm{As}_{1.98} \mathrm{P}_{0.01} \mathrm{~S}_{2.03} \mathrm{O}_{20}$. Vasilseverginite is monoclinic, P 21/ n , a = 8.1131(4), b = 9.9182(4), c = 11.0225(5) Å, β = 110.855(2)°, V = 828.84(6) Å 3 , and Z = 2. The strongest reflections in the powder XRD pattern [ d ,Å( I )( hkl )] are: 7.13(41)(101̅), 5.99(70)(110, 111̅), 5.260(100)(101), 4.642(46)(111), 3.140(31)(031̅), 2.821(35)(023̅), 2.784(38)(132̅, 032̅), 2.597(35)(204̅), and 2.556(50) (231̅, 212). The crystal structure, solved using single-crystal X‑ray diffraction data, R 1 = 0.025, is based upon complex [O 4 Cu 9 ] 10+ layers parallel to (101̅) that are composed of edge- and corner-sharing (OCu 4 ) tetrahedra. The topology is unprecedented in inorganic structural chemistry. The crystal structure can be considered a hybrid of the structures of popovite Cu 5 O 2 (AsO 4 ) 2 and dolerophanite Cu 2 O(SO 4 ) according to the scheme Cu 9 O 4 (AsO 4 ) 2 (SO 4 ) 2 = Cu 5 O 2 (AsO 4 ) 2 + 2Cu 2 O(SO 4 ). The chemical hybridization does not result in a significant increase in chemical complexity of vasilseverginite compared to the sum of those of popovite and dolerophanite, whereas the structural hybridization leads to the doubling of structural information per unit cell. The mineral is named in memory of the outstanding Russian mineralogist, geologist, and chemist Vasiliy Mikhailovich Severgin (1765–1826).

Priscillagrewite-(Y), ideally (Ca 2 Y)Zr 2 Al 3 O 12 ( Ia 3̅̅ d , a = 12.50 Å, V = 1953.13 Å 3 , Z = 8), a new member of the garnet supergroup and bitikleite group, was discovered in a fluorapatite layer (meta-phosphorite) hosted by varicolored spurrite marble in the Daba- Siwaqa area of the Transjordan plateau south of Amman, central Jordan. The Daba-Siwaqa area is the largest field of the Hatrurim Complex pyrometamorphic rocks distributed along the rift of the Dead Sea. Priscillagrewite-(Y) and other accessory minerals (such as members of the brownmillerite- srebrodolskite series, fluormayenite, lakargiite, baghdadite, hematite, sphalerite, zincite, garnet of the andradite-grossular series, tululite, vapnikite, minerals of the lime-monteponite series and members of the magnesiochromite-zincochromite series, cuprite, and Y-bearing and Y-free perovskite) are distributed irregularly in varicolored spurrite marble. The empirical formula of priscillagrewite-(Y), based on 12 O atoms, is Ca2.19Y0.65Ce0.033+Nd0.033+Gdd0.023+Dy0.023+$\left(\mathrm{Ca}_{2.19} \mathrm{Y}_{0.65} \mathrm{Ce}_{0.03}^{3+} \mathrm{Nd}_{0.03}^{3+} \mathrm{Gdd}_{0.02}^{3+} \mathrm{Dy}_{0.02}^{3+}\right.$Er0.023+Yb0.023+La0.013+Sm0.013++Σ 3.00Zr1.79Ti0.134+Sb0.075+U0.016+Σ 2.00Al1.70Fe1.213+Si0.04P0.04S+Σ 2.99O12.$\left.\mathrm{Er}_{0.02}^{3+} \mathrm{Y} \mathrm{b}_{0.02}^{3+} \mathrm{L} \mathrm{a}_{0.01}^{3+} \mathrm{Sm}_{0.01}^{3++}\right)_{\Sigma 3.00}\left(\mathrm{Zr}_{1.79} \mathrm{Ti}_{0.13}^{4+} \mathrm{Sb}_{0.07}^{5+} \mathrm{U}_{0.01}^{6+}\right)_{\Sigma 2.00}\left(\mathrm{Al}_{1.70} \mathrm{Fe}_{1.21}^{3+} \mathrm{S} \mathrm{i}_{0.04} \mathrm{P}_{0.04}^{\mathrm{S}+}\right)_{\Sigma 2.99} \mathrm{O}_{12}.$ A good match was obtained for electron backscatter diffraction (EBSD) patterns with a garnet model having a = 12.50 Å. The new garnet forms idiomorphic, isometric crystals up to 15 μm in size. It is transparent and has pale yellowish tinge, and its luster is vitreous. Priscillagrewite-(Y) is isotropic: n = 1.96 based on the Gladstone-Dale calculation using a = 12.50 Å and the empirical formula. The Mohs hardness is about 7–7.5. Density calculated from the empirical formula is 4.48 g/cm 3 . Raman spectrum of priscillagrewite-(Y) is similar to those of other minerals of the bitikleite group and contains the following bands (cm –1 ): 150, 163, 240, 269, 289, 328, 496, 508, 726, and 785. The strongest lines of the calculated powder diffraction data are as follows [( hkl ) d hkl ( I )]: (422) 2.552 (100), (642) 1.670 (96), (420) 2.795 (84), (400) 3.125 (72), (200) 4.419 (35), (640) 1.733 (32), and (1042) 1.141 (25). Priscillagrewite-(Y) is interpreted to be a relic of the high-temperature association formed in the progressive stage at the peak pyrometamorphism conditions when temperature could have reached close to 1000 °C.

We present the crystal structure, composition, and occurrence of stöfflerite, the naturally occurring Ca-aluminosilicate with hollandite-type structure. Stöfflerite is a high-pressure polymorph of anorthite and an approved mineral. The type material was found in the shergottitic martian meteorite Northwest Africa 856. Type stöflerite (Sto 60 Lin 40 ) assumes space group I 4/ m with unit-cell dimensions a = 9.255(1) Å, c = 2.742(3) Å, V = 235.1(2) Å 3 , and Z = 2.

An intensive study of the geochemical characteristics (including the volatile elements Cl and S) of apatite associated with porphyry deposits was undertaken to address the debate about the crust- or mantle-derivation of their copper and gold and to better understand the controls on the transport of metals in magmatic fluids in post-subduction settings. New geochemical data on apatite reveal parameters to discriminate mineralized porphyry systems across Iran and western China (Tibet and Yunnan), from coeval barren localities across this post-subduction metallogenic belt. Apatites in fertile porphyries have higher Cl and S concentrations (reflecting water-rich crystallization conditions) than those from coeval barren ones. Our new isotopic data also indicate these volatiles are likely derived from pre-enriched sub-continental lithospheric mantle, metasomatized by previous oceanic subduction. This study demonstrates that refertilization of suprasubduction lithospheric mantle during previous collision events is a prerequisite for forming post-subduction fertile porphyries, providing an evidence-based alternative to current ore-enrichment models.

The behavior of nitrogen during magmatic degassing and the potential kinetic fractionation between N and other volatile species (H, C, O, noble gases) are poorly known due to the paucity of N diffusion data in silicate melts. To better constrain N mobility during magmatic processes, we investigated N diffusion in silicate melts under reducing conditions. We developed uniaxial diffusion experiments at 1 atm, 1425 °C, and under nominally anhydrous reducing conditions fO2 $\left(f_{\mathrm{O}_{2}}\right.$≤ IW-5.1, where IW is oxygen fugacity, f O2 , reported in log units relative to the iron-wüstite buffer), in which N was chemically dissolved in silicate melts as nitride (N 3– ). Although several experimental designs were tested (platinum, amorphous graphite, and compacted graphite crucibles), only N diffusion experiments at IW-8 in compacted graphite crucibles for simplified basaltic andesite melts were successful. Measured N difusivity ( D N ) is on the order of 5.3 ± 1.5 × 10 –12 m 2 s –1 , two orders of magnitude lower than N chemical diffusion in soda-lime silicate melts (Frischat et al. 1978). This difference suggests that nitride difusivity increases with an increasing degree of melt depolymerization. The dependence of N 3– diffusion on melt composition is greater than that of Ar. Furthermore, N 3– diffusion in basaltic-andesitic melts is significantly slower than that of Ar in similarly polymerized andesitic-tholeiitic melts at magmatic temperatures (1400–1450 °C; Nowak et al. 2004). This implies that N/Ar ratios can be fractionated during reducing magmatic processes, such as during early Earth’s magma ocean stages.