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<jats:title>Abstract</jats:title> <jats:p>The solubility, speciation, and local atomic environment of chlorine have been determined for aluminoborosilicate glasses equilibrated with various sources of chlorine (NaCl and PdCl2) at high pressure (0.5–1.5 GPa) and high temperature (1350–1400 °C). The Cl solubility reaches up to 11 mol% in borosilicate glass and appears to be strongly influenced by the concentration of network-modifying cations (Ca and Na) and increases with increasing CaO + Na2O content. The Cl solubility is enhanced in Ca-bearing rather than Na-bearing borosilicate glass, suggesting a higher affinity of chlorine for alkaline-earth cations. Cl K-edge XANES and Cl 2p XPS spectra reveal that chlorine dissolves in glasses only as chloride species (Cl–) and no evidence of oxidized species is observed. Using PdCl2 as a chlorine source leads to a pre-edge signal for PdCl2 in the XANES spectra. The EXAFS simulations show that the Cl– local environment is charge compensated by Na+ or Ca2+ at a distance to first neighbor on the order of 2.7 Å, which is comparable to the observed distances in crystalline chloride compounds. The coordination to charge compensating cation is lower in the case of Ca2+ (~1.1) than Na+ (~4.3).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Apatite containing 14 wt% TREO (total rare earth oxide) and coexisting with calciobritholite with 37.2 wt% TREO has been synthesized at 800 °C and 10 kbar from a felsic melt with the addition of NaCl. The analysis of the experimental products with regression analysis of time-resolved (RATR) laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data allowed to estimate the composition of the coexisting phases. The results suggest that equilibrium has been established during the run and both apatite and calciobritholite contained REE in [Si4+REE3+] to [Ca2+P5+] solid solution, whereas the coupled substitution [Na1+REE3+] to [2Ca2+] was insignificant despite crystallization from an alkaline, Na-rich melt. The coexistence of the apatite and calciobritholite and available experimental data allowed the miscibility gap to be constrained between apatite and calciobritholite, and suggest complete miscibility between apatite and britholite above 950 °C. The melt that produced coexisting apatite and calciobritholite was characterized by a significant Cl content of (0.51 wt%) and elevated REE (526 ± 19 ppm Ce) and low-P content (112 ± 49 ppm). The change of the accessory mineral association from monazite to apatite and calciobritholite with the addition of NaCl illustrates the importance of halogens for mineral associations. The partition coefficients of britholite are similar to those of apatite and are distinguished mainly by a higher preference for REE and Th. Henry’s law was not acting for the total REE content in the melt because of the buffered system; however the partition coefficients could still be used for the prediction of the relative REE patterns for melts that generated high-REE apatite and/or calciobritholite. These results have implications for the interpretation of the phosphate associations in alkaline volcanic and plutonic rocks.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The environment model is used to describe the location of carbonate in nine carbonated apatites containing varied percentages of carbonate and Na+, K+, or NH4+ ions. Unlike the traditional model for carbonate substitution, which identifies different locations and orientations of the carbonate ion in the apatite structure, the environment model utilizes the different structural surroundings to describe the different types of carbonate. The A-type carbonate environment is assigned to channels lined only with calcium ions (A-channel configuration = Ca6) or to channels containing one Na+ or a vacancy (A′-channel configuration = Ca5Na or Ca5☐), and the B-type carbonate environment is the surroundings of the replaced phosphate ion. The assignments are made by peak-fitting the carbonate asymmetric stretch region (ν3) of the IR spectrum, following previously published criteria. These assignments lead to the conclusion that the percentage of channel carbonate (A- and A′-environments) is greater than that of B-type for each of these carbonated apatites. In general, the use of triammonium phosphate as the phosphate source in the synthesis produces apatites with larger amounts of channel carbonate (A- and A′-environments), while the use of sodium-containing phosphate reagents produces smaller amounts of channel carbonate.</jats:p> <jats:p>The environment model provides explanations for the differences within IR and NMR spectra obtained for apatites containing a range of total carbonate content. The B-type appearance of the carbonate ν3 region of the IR spectrum is found primarily in apatites containing sodium, which allows increased amounts of carbonation via co-substitution of Na+ with carbonate and creation of A′-environments with populations equal to that of B-type carbonate. The presence of ammonium or alkali metal salts with cations larger than Na+ results in the utilization of a charge-balance mechanism that produces vacancies rather than cation substitution in the channel. The carbonated apatites formed with primary utilization of the vacancy mechanism generally contain greater percentages of carbonate in the A-environment and carbonate IR spectra that contain an obvious high-frequency peak at about 1550 cm–1. The multiple peaks in the solid state 13C NMR spectra previously observed for carbonated apatite are attributed to substitution in the A-, A′-, and B-environments rather than different stereo-chemical orientations of the carbonate ion.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The various intervalence charge transfer (IVCT) mechanisms that can occur in silicate garnet, general crystal-chemical formula {X3}[Y2](Z3)O12, are not fully understood. The single-crystal UV/Vis/NIR absorption spectra of two different almandine-rich, spessartine-rich and grossular-rich garnets, as well as an intermediate almandine-pyrope garnet, were measured. Absorption was observed from roughly 15 000 to 30 000 cm–1. The spectra were deconvoluted and a very broad band with FWHM values ranging from 5000 to 7000 cm–1 (except in the case of one grossular where the FWHM is 8700 cm–1) and having an intensity maximum located between about 20 000 and 22 000 cm–1 in the visible region could be fit. Small weaker features located on this broad band were fit as well. The broad band is strongest in a nearly end-member composition almandine and weakest in a very grossular-rich iron-poor crystal. It is assigned to {Fe2+} + [Fe3+] → {Fe3+} + [Fe2+] IVCT. This is the first recognition of this type of electronic transition mechanism in different aluminosilicate garnet species. Photon-induced electron transfer probably occurs through an overlap of the d orbitals of Fe2+ and Fe3+ in their edge-shared triangular dodecahedral and octahedral coordination polyhedra, respectively. The two Fe cations with different formal charges should have markedly different energy potentials giving rise to asymmetric IVCT behavior. This, together with the relatively long Fe2+-Fe3+ distances (greater than 3.2 Å), could explain the higher energy of the IVCT in garnet compared to Fe2+ + Fe3+ → Fe3+ + Fe2+ IVCT mechanisms observed in other minerals. The latter typically have iron cations in octahedral or quasi-octahedral coordination. The IVCT in aluminosilicate garnet can occur in different species that grew under dissimilar P-T-X conditions. The resulting electronic absorption band affects color markedly, because it is centered at higher energies in the blue visible region. It remains to be determined why IVCT is observed in the spectra of some garnets but not others. The various proposed IVCT mechanisms in Ca-Ti-bearing and aluminosilicate garnets are reviewed and analyzed.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We conducted a petrologic study of apatite within one LL chondrite, six R chondrites, and six CK chondrites. These data were combined with previously published apatite data from a broader range of chondrite meteorites to determine that chondrites host either chlorapatite or hydroxylapatite with ≤33 mol% F in the apatite X-site (unless affected by partial melting by impacts, which can cause F-enrichment of residual apatite). These data indicate that either fluorapatite was not a primary condensate from the solar nebula or that it did not survive lower temperature nebular processes and/or parent body processes. Bulk-rock Cl and F data from chondrites were used to determine that the solar system has a Cl/F ratio of 10.5 ± 1.0 (3σ). The Cl/F ratios of apatite from chondrites are broadly reflective of the solar system Cl/F value, indicating that apatite in chondrites is fluorine poor because the solar system has about an order of magnitude more Cl than F. The Cl/F ratio of the solar system was combined with known apatite-melt partitioning relationships for F and Cl to predict the range of apatite compositions that would form from a melt with a chondritic Cl/F ratio. This range of apatite compositions allowed for the development of a crude model to use apatite X-site compositions from achondrites (and chondrite melt rocks) to determine whether they derive from a volatile-depleted and/or differentiated source, albeit with important caveats that are detailed in the manuscript. This study further highlights the utility of apatite as a mineralogical tool to understand the origin of volatiles (including H2O) and the diversity of their associated geological processes throughout the history of our solar system, including at its nascent stage.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Fluorine behavior during the partial melting of two mica-bearing protoliths has been experimentally investigated at 700 to 930 °C and 0.4 and 0.6 GPa. Muscovite dehydration and H2O-HF fluid-assisted partial-melting experiments were carried out using both a natural and synthetic two-mica schist made of natural micas. The mineral composition of the experiments was assessed by BSE imaging and EDS analyses. The F, Cl, and major elements contents of the glass and micas were determined by EPMA.</jats:p> <jats:p>The muscovite dehydration melting reaction is muscovite + quartz + plagioclase = peraluminous melt + biotite + sillimanite + potassic feldspar ± hercynite. The starting biotite stays largely stable, showing only minor melt + ilmenite and trace magnetite formation in the cleavages. The newly formed biotite shows similar F contents and a slightly higher XSid component when compared to the starting biotite. HF-added experiments yield F-rich newly formed biotite.</jats:p> <jats:p>The experimentally produced melts were of a peraluminous leucogranitic composition with F contents increasing with F-rich protoliths. The bulk partition coefficient DFschist/melt increases from 0.5 to 3.0 when the F content of the protolith rises from 0.05 to 1.2 wt%. The partition coefficient, DFBt/melt, increases from 2.0 to 6.0 where the biotite MgO content increases from 5 to 18 wt%. The natural partition coefficient DFBt/Ms, measured for a set of rocks with a varied lithology from the Seridó Belt, northeastern Brazil, was 2.7 ± 0.5.</jats:p> <jats:p>The F partition coefficients measured in this study, along with published F partition coefficients between biotite and melt, biotite and muscovite, and fluid and melt, allow for the modeling of F behavior during muscovite dehydration and fluid-present melting. F-rich, two-mica protoliths will increase F partitioning in favor of the micaceous anatectic residue compared to the peraluminous melt. Furthermore, the model indicates that the more Fe-rich the schist and its residual biotite are, the higher the F content of the melt and the fluid. Fluorine-rich peraluminous leucogranites and related fluids may be generated by the anatexis of F- and Fe-rich, two-mica protoliths. As F can be a complexing ligand for Li, Be, Cs, Nb, Ta, W, Sn, and U, muscovite dehydration could potentially be associated with metallic occurrences associated with peraluminous melts.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Society annually consumes 250% more Sb relative to the year 1960 and a sustainable supply of antimony depends critically on understanding the precipitation mechanism of stibnite (Sb2S3) that is the globally predominant source of this important technology metal. Previous solubility studies revealed that antimony is transported in mesothermal hydrothermal fluids mainly as the aqueous species thioantimonite (H2Sb2S4, HSb2S4−, Sb2S42−) and hydroxothioantimonite [Sb2S2(OH)2]. Thioantimonite can transform to hydroxothioantimonite with a decline of H2S concentration. However, whether this transition occurs in hydrothermal systems and its role in stibnite precipitation are unknown. In this work, bulk Sb isotope measurements for stibnite from the world’s largest Sb deposit in Xikuangshan China were conducted to address ore fluid evolution and stibnite precipitation mechanisms. The abundance of the stable antimony isotopes 121Sb and 123Sb were measured in stibnite from the Xikuangshan orebodies and reported as δ123Sb. The δ123Sb values show a trend of decreasing first and then increasing from proximal to distal parts of orebodies. This reveals that 123Sb had been preferentially partitioned from the ore fluid into stibnite first, then 123Sb remained preferentially dissolved in the ore fluid. These data indicate that the dominant Sb-complex transforms to Sb2S2(OH)2 from H2Sb2S4 with consumption of H2S. Speciation diagram considerations indicate that stibnite precipitation from the ore fluid was controlled by two telescoped processes: (1) boiling of the ore fluid induced a decrease in H2S that reduced the solubility of H2Sb2S4, and (2) subsequent cooling that induced a decrease in the solubility of Sb2S2(OH)2. This study highlights that understanding the controls of Sb isotope fractionation is critical to constrain fluid evolution and stibnite precipitation mechanisms in Sb-rich mineral systems. In particular, the stable Sb complex in the hydrothermal ore fluid may change during fluid evolution and affect the isotope fractionation mechanism.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Quartz cathodoluminescence (CL) images are commonly combined with trace element concentrations to decipher complex histories of hydrothermal systems. However, the correlations between aluminum content and CL zoning of low-temperature hydrothermal quartz and their genesis remain controversial. In this contribution, a multiparametric study was carried out on CL-aluminum zoning of low-temperature hydrothermal quartz (&amp;lt;350 °C) from the Shihu and Rushan quartz-vein type Au deposits in the North China Craton. The results show that aluminum concentration correlates negatively with CL intensity in quartz from the Shihu Au deposit. CL-dark quartz zoning has significant Al concentrations as well as detectable Al-H bonds. However, in the Rushan Au deposit, the correlation is positive, and aluminum is enriched in the CL-bright quartz zoning. The Al content is positively correlated with K content with r2 = 0.769. Combined with the electron backscatter diffraction (EBSD), X-ray single crystal diffraction (XRD), and transmission electron microscope (TEM) data, we infer that the genesis of CL zoning in the low-temperature hydrothermal quartz is closely related to Al3+-H+ and Al3+-K+ concentrations. The Al3+-K+ may act as the CL-activator, while the Al3+-H+ may act as the CL-dampener. Where Al3+-Si4+ substitution is charge balanced by hydrogen, the intensity of CL response decreases; where Al3+-Si4+ substitution is charge balanced by potassium, the intensity of CL response increases. The correlations between CL intensity and aluminum concentration in the low-temperature hydrothermal quartz reflect pH fluctuations of hydrothermal system.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The transition zone is dominated by polymorphs of olivine, wadsleyite, and ringwoodite, which are to date considered the main water carriers in the Earth’s mantle. Despite considerable studies on water solubility and its impact on physical properties of the two minerals, knowledge of their hydrogen defects and framework behavior at high temperature and high pressure is still lacking. Here, we systematically assess this issue, by in situ high-temperature (20–800 °C) infrared spectroscopic studies, in situ high-temperature (20–800 °C) and high temperature-pressure (14.27 and 18.84 GPa, 20–400 °C) Raman spectroscopic studies on the iron-bearing wadsleyite and ringwoodite. The results show that dehydrogenation in wadsleyite happens at a higher temperature than in ringwoodite. The infrared absorption patterns of hydrogen defects in wadsleyite and ringwoodite are temperature sensitive, resulting from hydrogen defects transfer and site-specific stabilities. As for the framework, it is more sensitive to temperature and pressure for ringwoodite than wadsleyite. These results provide new knowledge about hydrogen defects and framework of wadsleyite and ringwoodite at high temperature and high pressure, which is indispensable for understanding water solubility and its impacts on physical properties of these two minerals.</jats:p>