Volume 109, Issue 2

Zircon-class ternary oxide compounds have an ideal chemical formula of ATO 4 , where A is commonly a lanthanide and an actinide, with T = As, P, Si, or V. Their structure ( I 4 1 / amd ) accommodates a diverse chemistry on both A- and T-sites, giving rise to more than 17 mineral end-members of five different mineral groups, and in excess of 45 synthetic end-members. Because of their diverse chemical and physical properties, the zircon structure-type materials are of interest to a wide variety of fields and may be used as ceramic nuclear waste forms and as aeronautical environmental barrier coatings, to name a couple. To support advancement of their applications, many studies have been dedicated to the understanding of their structural and thermodynamic properties. The emphasis in this review will be on recent advances in the structural and thermodynamic studies of zircon structure-type ceramics, including pure end-members [e.g., zircon (ZrSiO 4 ), xenotime (YPO 4 )] and solid solutions [e.g., Er x Th 1–x (PO 4 ) x (SiO 4 ) 1–x ]. Specifically, we provide an overview on the crystal structure, its variations and transformations in response to non-ambient stimuli (temperature, pressure, and radiation), and its correlation to thermophysical and thermochemical properties.

A natural intermediate member of the scapolite solid solution {Me 47 ; chemical formula: (Na 1.86 Ca 1.86 K 0.23 Fe 0.01 )(Al 4.36 Si 7.64 )O 24 [Cl 0.48 (CO 3 ) 0.48 (SO 4 ) 0.01 ]}, with the unusual I 4/ m space group, has been studied at various temperatures and combined high- T and high- P by means of in situ single-crystal and powder X-ray diffraction, using both conventional and synchrotron X-ray sources. In addition, single-crystal neutron diffraction data were collected at ambient- T and 685 °C. A fit of the experimental V - T data with a thermal equation of state yielded a thermal expansion coeficient at ambient conditions: α V25°C = 1/ V 0 ·(∂ V /∂ T ) P ,25°C = 1.74(3)·10 –5 K –1 . A comparative analysis of the elastic behavior of scapolite based on this study and previous high- T XRD data suggests that a thorough re-investigation of the diferent members of the marialite-meionite solid solution is needed to fully understand the role of crystal chemistry on the thermal behavior of these complex nonbinary solid solutions. The experimental data obtained within the full temperature range of analysis at ambient pressure confirm that the investigated sample always preserves the I 4/ m space group, and possible implications on the metastability of I 4/ m intermediate scapolite are discussed. Neutron difraction data show that no significant Si and Al rearrangement among the T sites occurs between 25 and 685 °C. The combined high- T and high- P data show that at 650 °C and between 10.30(5) and 10.71(5) GPa a phase transition toward a triclinic polymorph occurs, with a positive Clapeyron slope (i.e., d P /d T > 0). A comprehensive description of the atomic-scale structure deformation mechanisms induced by temperature and/or pressure, including those leading to structural instability, is provided based on single-crystal structure refinements.

The structural response to compression of the synthetic high-pressure hydroxide perovskite MgSi(OH) 6 , the so-called “3.65 Å phase,” has been determined to 8.4 GPa at room temperature using single-crystal XRD in the diamond-anvil cell. Two very similar structures have been determined in space groups P 2 1 and P 2 1 / n , for which differences in oxygen donor-acceptor distances indicate that the non-centrosymmetric structure is likely the correct one. This structure has six nonequivalent H sites, of which two are fully occupied and four are half-occupied. Half-occupied sites are associated with a well-defined crankshaft of hydrogen-bonded donor-acceptor oxygens extending parallel to c . Half occupancy of these sites arises from the averaging of two orientations of the crankshaft H atoms (|| ± c ) in equal proportions. The P 2 1 and P 2 1 / n structures are compared. It is shown that the former is likely the correct space group, which is also consistent with recent spectroscopic studies that recognize six nonequivalent O-H. The structure of MgSi(OH) 6 at pressures up to 8.4 GPa was refined in both space groups to see how divergent the two models are. There is a very close correspondence between the responses of the two structures implying that, at least to 8.4 GPa, non-centrosymmetry does not affect compressional behavior. The very different compressional behavior of MgO 6 and SiO 6 octahedra observed in this study suggests that structural phase transformations or discontinuities likely occur in MgSi(OH) 6 above 9 GPa.

Equilibrium Sn isotope fractionation properties between aqueous Sn (2+, 4+) species and Sn-bearing minerals are the key to using tin isotopes to trace the transportation, enrichment, and precipitation of tin in various geological processes. However, the application of Sn isotope geochemistry has been impeded by the absence of equilibrium Sn isotopic fractionation factors between Sn-bearing minerals and fluid and between mineral pairs. In this contribution, we conducted first-principles calculations based on the density functional theory to obtain the equilibrium Sn isotopic fractionation factors between aqueous Sn complexes and minerals. For Sn-bearing complexes in solution, the reduced partition function ratios (β) are determined by taking snapshots from the molecular dynamics trajectories and computing the average β of the snapshots based on the lowest energy atomic coordinates. For Sn-bearing minerals, static first-principles periodic density functional theory methods are performed. The results show that the β factors decrease in the sequence of  malayaite  ( s ) S n 4 + >  cassiterite  ( s ) S n 4 + > S n 4 + C l 4 H 2 O 2 ( a q ) > S n 2 + F 3 ( a q ) − > $ \text { malayaite }_{(\mathrm{s})}\left(\mathrm{Sn}^{4+}\right)>\text { cassiterite }_{(\mathrm{s})}\left(\mathrm{Sn}^{4+}\right)>\mathrm{Sn}^{4+} \mathrm{Cl}_4\left(\mathrm{H}_2 \mathrm{O}\right)_{2(\mathrm{aq})}>\mathrm{Sn}^{2+} \mathrm{F}_{3(\mathrm{aq})}^{-}>$ S n 2 + ( O H ) 2 ( a q ) > S n 2 + C O 3 ( a q ) > stannite ( s ) S n 4 + > S n 2 + C l 3 ( a q ) − . $\mathrm{Sn}^{2+}(\mathrm{OH})_{2(\mathrm{aq})}>\mathrm{Sn}^{2+} \mathrm{CO}_{3(\mathrm{aq})}>\operatorname{stannite}_{(\mathrm{s})}\left(\mathrm{Sn}^{4+}\right)>\mathrm{Sn}^{2+} \mathrm{Cl}_{3(\mathrm{aq})}^{-}.$The predicted Sn isotope fractionation follows several distinct patterns. (1) For minerals, the Sn isotope fractionations (1000lnα minerals-stannite ) of cassiteritestannite and malayaite-stannite mineral pairs are controlled by the properties of elements coordinating with tin, and the equilibrium Sn isotope fractionation factors between mineral pairs are large enough to make them powerful Sn isotope thermometers. (2) For Sn-bearing aqueous species, the β values of tin (4+) complexes are remarkably larger than those of all aqueous Sn 2+ species, indicating that higher valence tin is preferentially enriched heavy tin isotopes. For aqueous Sn 2+ species, the aqueous species with shorter bonds are more-enriched in heavy Sn isotopes than those with longer bonds. When both the valence state and bond length are different, the valence state is the main factor controlling tin isotope fractionation. (3) During the precipitation of various Sn 2+ aqueous complexes into cassiterite or malayaite, heavy Sn isotopes tend to be enriched in minerals, while there are two situations for the precipitation of Sn 2+ complexes into stannite. When Sn is transported in hydrothermal solution as Sn 2+ Cl 3– , stannite precipitation leads to the enrichment of light tin isotopes in the residual solution and late minerals. On the contrary, other S n 2 +  species  S n 2 + F 3 − , S n 2 + ( O H ) 2  and  S n 2 + C O 3 $ \mathrm{Sn}^{2+} \text { species }\left[\mathrm{Sn}^{2+} \mathrm{F}_3^{-}, \mathrm{Sn}^{2+}(\mathrm{OH})_2 \text { and } \mathrm{Sn}^{2+} \mathrm{CO}_3\right]$that precipitate as stannite will result in the enrichment of heavy tin isotopes in the residual solutions . In addition, the direct precipitation of Sn 4+ complexes into cassiterite, malayaite, or stannite also produces considerable tin isotope fractionation. During precipitation, Sn 4+ aqueous complexes form cassiterite or malayaite, and heavy Sn isotopes tend to be enriched in minerals; whereas when aqueous Sn 4+ species are precipitated into stannite, heavy Sn isotopes are enriched in the residual fluid and late minerals. The calculated results are essential for further understanding the mechanisms of Sn isotopic fractionation in various Sn-involved geological processes.

Uranyl sulfates are important constituents of uranium ores and represent a significant fraction of U(VI) minerals discovered in recent years owing to their propensity to form in mine tailings and legacy sites related to uranium exploration. Recently, we surveyed all published Raman spectra for uranium minerals and found significantly less easily accessible data available for uranyl sulfates relative to other groups of uranium minerals (Spano et al. 2023). In that work, we described average spectra for groups of uranyl minerals to understand common vibrational spectroscopic features attributable to similarities in oxyanion chemistry among U(VI) minerals, but only data for three uranyl sulfate minerals were included in the study. The present work reports on Raman spectra collected for 18 additional uranyl sulfate minerals. To better understand underlying structural and chemical features that give rise to spectroscopic observables, we relate differences in structural topology, charge-balancing cations, and locality of origin to features observed in the Raman spectra of selected natural uranyl sulfates.

Modified magnetite and hydrothermal apatite in banded iron formations (BIFs) are ideal minerals for studying hydrothermal and metamorphic processes and are applied to linking with high-grade Fe mineralization and metamorphism in iron deposits hosted by BIFs. In this study, we have investigated the geochemical composition of modified magnetite and hydrothermal apatite and in situ U-Pb geochronology on apatite from the Huogezhuang BIF-hosted Fe deposit in northeastern China. The magnetite in metamorphosed BIF is modified, locally fragmented, and forms millimeter- to micrometer-scale bands. The apatite is present surrounding or intergrowing with magnetite, has corroded surfaces, and contains irregular impurities and fluid inclusions, indicating that it has been partly hydrothermally altered. Original element compositions (e.g., Fe, Al, Ti, K, Mg, and Mn) of magnetite in BIFs have been modified during high-grade Fe mineralization and retrogressive metamorphism with temperature reduction and addition of acids. The hydrothermally altered apatite has been relatively reduced in the contents of Ca, P, F, La, Ce, Nd, δCe, δEu, and total REEs compared to non-altered apatite. The magnetite and apatite in low-grade BIFs are poorer in FeO T than those from the high-grade Fe ores, indicating that Fe is remobilized during the transition from BIFs to high-grade Fe ores. The magnetite and apatite in high-grade Fe ores are overgrown by greenschist-facies minerals formed during retrograde metamorphism, suggesting that the high-grade Fe mineralization may be related to retrogressive metamorphism. In situ U-Pb geochronology of apatite intergrown with magnetite and zircon LA-ICP-MS U-Pb dating at Huogezhuang deposit reveals that the BIF-hosted magnetite was altered and remobilized at ca. 1950–1900 Ma, and deposition of the BIF began during the Late Neoarchean. The changes of elements in the modified magnetite and diferent geochemical compositions of the altered and unaltered apatite confirm that the modified magnetite and hydrothermal apatite can be efective in tracing high-grade Fe mineralization and retrogressive metamorphism in BIFs.

The diverse suite of trace elements incorporated into apatite in ore-forming systems has important applications in petrogenesis studies of mineral deposits. Trace element variations in apatite can be used to distinguish between fertile and barren environments, and thus have potential as mineral exploration tools. Such classification approaches commonly employ two-variable scatterplots of apatite trace element compositional data. While such diagrams offer accessible visualization of compositional trends, they often struggle to efectively distinguish ore deposit types because they do not employ all the high-dimensional (i.e., multi-element) information accessible from high-quality apatite trace element analysis. To address this issue, we use a supervised machine-learning-based approach (eXtreme Gradient Boosting, XGBoost) to correlate apatite compositions with ore deposit type, utilizing such high-dimensional information. We evaluated 8629 apatite trace element data from five ore deposit types (porphyry, skarn, orogenic Au, iron oxide copper gold, and iron oxide-apatite) along with unmineralized magmatic and metamorphic apatite to identify discriminating parameters for the individual deposit types, as well as for mineralized systems. According to feature selection, eight elements (Th, U, Sr, Eu, Dy, Y, Nd, and La) improve the model performance. We show that the XGBoost classifier eficiently and accurately classifies high-dimensional apatite trace element data according to the ore deposit type (overall accuracy: 94% and F1 score: 89%). Interpretation of the model using the SHAPley Additive exPlanations (SHAP) tool shows that Th, U, Eu, and Nd are the most indicative elements for classifying deposit types using apatite trace element chemistry. Our approach has broad implications for the better understanding of the sources, chemistry, and evolution of melts and hydrothermal fluids resulting in ore deposit formation.

The three main serpentine minerals, chrysotile, lizardite, and antigorite, form in various geological settings and have different chemical compositions and rheological properties. The accurate identification of serpentine minerals is thus of fundamental importance to understanding global geochemical cycles and the tectonic evolution of serpentine-bearing rocks. However, it is challenging to distinguish specific serpentine species solely based on geochemical data obtained by traditional analytical techniques. Here, we apply machine learning approaches to classify serpentine minerals based on their chemical compositions alone. Using the Extreme Gradient Boosting (XGBoost) algorithm, we trained a classifier model (overall accuracy of 87.2%) that is capable of distinguishing between low-temperature (chrysotile and lizardite) and high-temperature (antigorite) serpentines mainly based on their SiO 2 , NiO, and Al 2 O 3 contents. We also utilized a k -means model to demonstrate that the tectonic environment in which serpentine minerals form correlates with their chemical compositions. Our results obtained by combining these classification and clustering models imply the increase of Al 2 O 3 and SiO 2 contents and the decrease of NiO content during the transformation from low- to high-temperature serpentine (i.e., lizardite and chrysotile to antigorite) under greenschist–blueschist conditions. These correlations can be used to constrain mass transfer and the surrounding environments during the subduction of hydrated oceanic crust.

The morphologies and size distributions of groundmass crystals record conditions of magma ascent through volcanic conduits. However, morphological information (such as crystal shapes) has not been incorporated into crystal size distributions (CSDs). Here, we focused on the crystal habit, especially the shape variation due to the combination of ( hk 0) faces (hereafter “tracht”) of pyroxene microlites and nano-crystals, and measured CSDs for each crystal habit (tracht) to more comprehensively characterize the crystallization kinetics. We refer to the CSDs measured for each tracht as “tracht-specific CSDs.” Pyroclasts from the 2011 eruption of Shinmoedake (Kirishima volcano group, Japan) were examined by field-emission scanning electron microscopy, electron backscatter diffraction analysis, synchrotron radiation X-ray computed nanotomography, and transmission electron microscopy. The samples contain groundmass pyroxenes of two main trachts: octagonal prisms consisting of {100}, {010}, and {110} faces and hexagonal prism lacking {100} faces. The pumice clasts formed by different eruption styles showed different trends of tracht-specific CSDs. Sub-Plinian pumice clasts were characterized by octagonal microlites (1–10 μm wide) and numerous hexagonal nano-crystals (0.2–2 μm wide), and a Vulcanian pumice clast with the same glass composition showed the same characteristics. In contrast, Vulcanian pumice clasts with more evolved glass compositions contained mostly octagonal pyroxenes. The tracht-specific CSDs and growth zonations indicate a change from octagon-dominant to hexagon-dominant growth conditions during syneruptive ascent. We infer that the hexagonal tracht resulted from a large degree of effective undercooling due to rapid decompression in the shallow conduit. Moreover, the texture of the less-evolved Vulcanian pumice indicates that a portion of the magma erupted on the Vulcanian eruption followed almost the same ascent paths just prior to the fragmentation as those during the sub-Plinian eruptions, and thus the Vulcanian eruption may have involved the rapid ascent of deeper magma. We propose that tracht analyses of groundmass pyroxenes provide detailed information about time-evolution of magma conditions during syneruptive ascent.

Primary phases in iron-rich chemical sedimentary rocks are important archives of seawater geochemistry throughout the Precambrian. The record of seawater chemistry, however, is obscured by post-depositional changes that occur during diagenesis, metamorphism, and modern weathering. Recent studies have identified silica-cemented horizons in some Archean and Paleoproterozoic iron formation that may preserve reduced, texturally early mineral phases, which may inform interpretations of oxygen dynamics preceding atmospheric oxygen accumulation before the ~2.3 Ga Great Oxidation Event (GOE). However, fewer investigations focus on silica-cemented horizons in Paleoproterozoic iron formation deposited after the GOE, a period where oxygen levels are poorly constrained. Here, we present petrographic observations, scanning electron microscopy, electron microprobe analysis, and Raman spectroscopy of iron mineral phases preserved within silica-cemented horizons of the ~1.9 Ga Biwabik Iron Formation (Minnesota, U.S.A.) to constrain texturally early iron formation mineralogy from this crucial post-GOE interval. Based on textural relationships, the iron silicate greenalite is identified as the earliest-forming iron silicate mineral preserved within silica-cemented horizons. The magnesium- and aluminum-rich iron silicates chamosite and stilpnomelane are preserved proximal to fine-grained, non-silicified horizons, suggesting local geochemical exchange during early diagenesis. The presence of well preserved, early-forming silicates containing predominantly ferrous iron may indicate reducing conditions at the sediment-water interface during deposition of the Biwabik Iron Formation. More definitively, future studies using iron silicate mineralogy as seawater geochemistry proxies should consider preservation by silica cementation, in addition to the efects of local geochemical exchange during diagenesis.

Black shale-hosted vanadium (V) deposits account for about 80% vanadium resources in the world, but only <2% vanadium in the black shale can be extracted mainly due to insufficient recognition on the occurrence mode of vanadium. It is commonly agreed that most vanadium in the black shale is hosted in clay minerals and organic matters, but it is not clear how the other parts of vanadium exist and whether there exists a vanadium mineral, which has limited our understanding of metallogenic mechanism of black shale-hosted vanadium deposits. The Jiujiang Basin at the Lower Yangtze Block is a significant black shale-hosted vanadium metallogenic district. In this work, we conducted systematic studies of mineralogy, lithology and geochemistry on the occurrence of vanadium hosted in the black shales. Electron probe microanalysis (EPMA), Raman spectroscopy, and X-ray diffraction (XRD) show that the main vanadium-hosting mineral in the black shale is mannardite, with a structural formula of B a 0.96 ⋅ H 2 O T i 5.87   V 1.87 3 + V 0.11 4 + S i 0.07 C r 0.07 F e 0.02 3 + O 16.00 , $ \left[\mathrm{Ba}_{0.96} \cdot \mathrm{H}_2 \mathrm{O}\right]\left(\mathrm{Ti}_{5.87} \mathrm{~V}_{1.87}^{3+} \mathrm{V}_{0.11}^{4+} \mathrm{Si}_{0.07} \mathrm{Cr}_{0.07} \mathrm{Fe}_{0.02}^{3+}\right) \mathrm{O}_{16.00},$ space group I 4 1 / a , unit-cell parameters a = b = 14.346(7) Å, c = 5.899(1) Å, α = β = γ = 90°, Z = 4. Data from EPMA, TESCAN integrated mineral analyzer (TIMA), and whole-rock geochemistry indicate that 12.32–44.06% (average 24.95%) vanadium exists in mannardite. Most vanadium atoms in mannardite occupy its structural sites as trivalent vanadium (V 3+ ), forming chemical bonds with O atoms as V O 2 − , $\mathrm{VO}_2^{-},$ whereas a minor amount of vanadium atoms replace titanite atoms (Ti 4+ ) as quadrivalent vanadium (V 4+ ) by isomorphism. Mannardite precipitates under a strong reductive condition with sufficient trivalent vanadium species, titanium and biogenic barium (bio-barite). Our first identification of mannardite in black shale-hosted vanadium deposits thus sheds light on the occurrence mode of vanadium and the metallogenic mechanism of black shale-hosted vanadium deposits.

Field, petrochemical, and stable isotope data from peraluminous microgranite dikes and their molybdenite-bearing vugs in the Archean Preissac-Lacorne batholith, Québec, provide evidence for the transport and deposition of molybdenum by an aqueous fluid that exsolved from a felsic magma. A model is proposed in which the microgranite-forming liquid fractionated from a magma that was parental to and intruded less-evolved muscovite monzogranite. Fluid saturation is interpreted to have occurred immediately prior to emplacement, allowing sufficient time for small spherical pockets of fluid to form and rise through the liquid but insufficient time for them to leave the system. These fluid pockets scavenged molybdenum and other components during their migration but were frozen in situ to form molybdenite-bearing vugs as a result of the quenching of the magma. We therefore conclude that the vugs are relicts of the exsolved fluid pockets and that the molybdenite in them reflects the concentration of molybdenum in the fluid, estimated from volumetric relationships to have reached ~7800 ppm.

The stability of CO 2 fluid in the Earth’s mantle is restricted by the carbonation of rock-forming minerals. Among those, the reaction with garnet is of particular interest because it constrains the stability of CO 2 fluid in eclogites, whose minerals have been found in the CO 2 -bearing diamonds. In this work, we determined the equilibrium boundary for the reaction Mg 3 Al 2 Si 3 O 12 (Prp) + 3CO 2 (fluid) = Al 2 SiO 5 (Ky) + 2SiO 2 (Coe/Qz) + 3MgCO 3 (Mgs) over the pressure interval 3–6 GPa using a multi-anvil press. Owing to the slow kinetics, the reaction was studied in both forward (left to right) and reverse (right to left) directions in experiments with durations extending up to 260 h. Our newly determined boundary is situated 3 GPa/950 ± 50 °C, 4.5 GPa/1150 °C, and 6 GPa/1350 ± 50 °C and has the equation P (GPa) = 0.0075 × T (°C) – 4.125. The boundary crosses the graphite-to-diamond transition curve near 4.7 GPa and 1180 °C. Thus, the assemblage garnet + CO 2 fluid is stable in the diamond (Dia) stability field under P-T conditions of the continental geotherm with a heat flow of 41 mW/m 2 .