Coupled Substitution Research Articles

Unravelling the incorporation mechanisms of water in aluminous orthoenstatite: I. First-principles calculations

We performed first-principles calculations on the energy, and NMR and polarized infrared (IR) spectra for anhydrous and hydrous aluminous orthoenstatite (OEn) models to help clarify the incorporation mechanisms of Al and OH defects in OEn. Our calculations revealed that proton pairs in M2 vacancies ((2H)M2) adjacent to a tetrahedral Al (AlIV) are energetically more favorable than those remote from Al, and may contribute to the observed correlated 1H NMR peaks near 3.7 and 8.0 ppm, and IR bands near 3550–3570 and 3066 cm−1 (A4 band) for aluminous OEn. Coupled substitutions of AlVI (octahedral Al) + H for 2Mg were found to adopt multiple configurations, and may contribute to the observed IR bands near 3520, 3475 and 3320 cm−1. Coupled substitution of AlIV + H for 1Si may contribute to the observed IR band near 3380–3400 cm−1. 4H in SiB vacancies ((4H)SiB) adjacent to an AlVI were found to be energetically more favorable than those remote from Al, and may be the origin for an IR band observed near 3600–3620 cm−1. These results allow the incorporation mechanisms of water in synthetic and natural aluminous orthopyroxenes to be deciphered from the available NMR and IR data, and suggest that both (2H)M2 defects associated with Al and simultaneous coupled substitutions of Al + H for 2Mg and 1Si contribute to the observed correlation between Al and water incorporation, and the nearly unity AlIV/AlVI ratio. (4H)SiB defects associated with Al may also be present in some synthetic OEn and mantle-derived orthopyroxene.

Intra-grain variability of hydrogen and trace elements in rutile

Rutile is a commonly used petrogenetic indicator mineral to determine metamorphic temperatures, ages, host−/source lithologies, geochemical reservoirs, and subduction conditions. However, intra grain variabilities of trace elements in rutile are rarely considered. We performed trace element and hydrogen mapping of rutile to assess zoning and diffusion in natural rutile from various lithologies. Trace element and hydrogen show distinct zoning patterns, with mostly regular zoning in rutile from pegmatites and low-T hydrothermal clefts, and typically irregular zoning in rutile from high-P veins and metamorphic rocks. Whereas no clear patterns of trace element correlations can be identified, hydrogen, tri-, tetra- and pentavalent cations can show the same zoning patterns within single rutile grains, despite different substitution mechanisms. This indicates that hydrogen and trace element incorporation is externally controlled by availability and diffusivity of hydrogen and trace elements within the rock matrix, as well as rutile growth rates. H2O mapping reveals that hydrogen is retained in rutile at temperatures of up to ~650 °C. Coupled substitution of hydrogen with divalent and trivalent cations requires coupled diffusion processes for charge balance if hydrogen is diffusively re-equilibrated. Slow diffusion rates and thus relatively high temperatures for diffusive closure in rutile lead to retention of primary hydrogen and trace element zoning. At high-T conditions of >650 °C, diffusive re-equilibration of all trace elements can be observed. Complex zoning patterns of Zr in rutile show that Zr incorporation into rutile is not purely temperature dependent. In this study, Zirconium-undersaturation can lead to inaccurate Zr-in-rutile temperatures of up to ~35 °C difference from peak formation temperatures within single rutile grains and might be a useful tool to evaluate rutile growth conditions. Niobium and Ta are highly zoned in rutile, leading to extremely variable Nb/Ta ratios within single rutile grains that cannot be reconstructed from single spot analyses. Overall, mapping offers a novel and promising tool to understanding trace element behavior in rutile.

Systematic trends in the substitution mechanisms of minor elements in cassiterite

A dataset of 4751 EPMA point analyses on cassiterite crystals from 18 localities spanning a range of different granite-related Sn-bearing mineral systems was investigated and compared with thermodynamic considerations from speciation and lattice strain models. The existence of previously inferred charge balanced coupled substitution mechanisms of Fe3+, Fe2+, Mn2+, Nb5+, Ta5+ and W6+ for Sn4+ are confirmed. The homo­valent substitution of Sn4+ by Ti4+ and Zr4+ is also confirmed, and the first evidence for homovalent substitution of Nb4+ and W4+ in natural cassiterite is presented. The preferred substitution mechanisms for Ti, Fe, Mn, Nb, Ta, W and Zr vary systematically as a function of the mineralising environment. Cassiterite precipitated from silicate liquid dominated systems (such as in pegmatites) primarily display a coupled Fe–(Nb,Ta) substitution mechanism with a 1:2 stoichiometry, whereas cassiterite precipitated in systems dominated by autometasomatic processes (such as greisens) more closely follow a 1:1 stoichiometry, as well as increased levels of Ti. Cassiterite precipitated solely from hydrothermal fluids (such as Sn–(W) quartz vein systems) exhibit the highest Ti concentrations, with lower contents of Fe–(Nb,Ta) coupled substitutions, instead displaying a switch to the incorporation of Fe3+ via FeO.OH stoichiometry. Hydrothermally precipitated cassiterite from skarn systems display the highest Fe3+ contents, and the lowest Nb, Ta and Ti contents. These systematic changes define a continuous trend in a Ti–(Fe + Mn)–(Nb,Ta) ternary diagram, allowing for classification of the primary source of alluvial cassiterite.

Lithium, Phosphorus, and rare earth elements in magmatic garnets from granitic Pegmatites: Coupled substitution and petrogenetic implications

The general chemical formula of garnet can be expressed as X3Y2(ZO4)3 where X, Y, and Z represent dodecahedral, octahedral, and tetrahedral sites, respectively. Lithium, phosphorus, and rare earth elements (including yttrium) in garnets from the Luanzhuang-Guanpo and Ziyugou (ZYG) pegmatite fields in the East Qinling orogen and the Kalu’an (KLA) Li pegmatites in the Chinese Altay orogen have been investigated both at a mineral scale and in the context of pegmatite types to identify the substitution mechanisms for these elements. Our results show that Y + REE or P can enter the garnet lattice through coupled substitution with Li (+Na) that can compensate the charge differences due to Y + REE in the X-site or P in the Z-site of the garnet. Two substitutions, namely [X(Li,Na)+1X(Y,REE)3+1(R2+)–2] and [X(Li,Na)1ZP1XR2+-1ZSi-1], are likely the most important and can account for incorporation of the majority of Y + REE, P, and Li + Na in pegmatite garnets. Whichever substitution becomes predominant in garnets is strongly dependent on the bulk chemistry of pegmatite-forming melts. Consequently, the first substitution dominates in garnets from NYF (niobium-yttrium-fluorine) pegmatites and mixed-type pegmatites with NYF affinity whereas the other one dominates in garnets from relatively P-rich LCT (lithium-cesium-tantalum) and barren pegmatites. Incorporation of Sc and Ti in garnet may also need Li + Na, although Sc and Ti are not as much as Y and REE in pegmatite garnet. The zonings of Y, REE, and Sc in garnet are mainly controlled by the composition of pegmatite-forming melt and consumption of these elements due to the growth of garnet itself and other minerals enriched in Y + REE (e.g., xenotime, fergusonite, monazite, etc.) or Sc (e.g., biotite). Additionally, our results, combined with the data in the literature, show that garnets from NYF pegmatites and mixed-type pegmatites with NYF affinity commonly contain elevated Y (up to ∼ 20000 ppm), REE (up to ∼ 20000 ppm), and Sc (up to ∼ 2400 ppm) but low P (<100 ppm) compared to those from LCT and barren pegmatites. Nevertheless, individual garnets with low concentrations of Y, REE, Sc, and P can occur in any types of pegmatites. Therefore, the coupled substitution between Li + Na and Y + REE or P along with zonings and concentrations of Y, REE, and Sc in garnet should be investigated on a regular basis to interpret pegmatite petrogenesis.

Continuous Flow Processes as an Enabling Tool for the Synthesis of Constrained Pseudopeptidic Macrocycles.

Herein we report our efforts to develop a continuous flow methodology for the efficient preparation of pseudopeptidic macrocyclic compounds containing the hexahydropyrrolo-[3,4-f]-isoindolocyclophane scaffold and involving four coupled substitution reactions in the macrocyclization process. Appropriate design of a supported base permitted the continuous production of the macrocycles even at large scales, taking advantage of the positive template effect promoted by the bromide anions. In addition, the use of flow protocols allowed a ca. 20-fold increase in productivity as well as reducing the environmental impact almost 2 orders of magnitude, in comparison with the related batch macrocyclization process.

Geology, mineralogy, and cassiterite geochronology of the Ayawilca Zn-Pb-Ag-In-Sn-Cu deposit, Pasco, Peru

The Ayawilca deposit in Pasco, Peru, represents the most significant recent base-metal discovery in the central Andes and one of the largest undeveloped In resources globally. As of 2018, it hosts an 11.7 Mt indicated resource grading 6.9% Zn, 0.16% Pb, 15 g/t Ag, and 84 g/t In, an additional 45.0 Mt inferred resource grading 5.6% Zn, 0.23% Pb, 17 g/t Ag, and 67 g/t In, and a separate Sn-Cu-Ag inferred resource of 14.5 Mt grading 0.63% Sn, 0.21% Cu, and 18 g/t Ag. Newly obtained U–Pb dates for cassiterite by LA-ICP-MS (22.77 ± 0.41 and 23.05 ± 2.06 Ma) assign the Ayawilca deposit to the Miocene polymetallic belt of central Peru. The polymetallic mineralization occurs as up to 70-m-thick mantos hosted by carbonate rocks of the Late Triassic to Early Jurassic Pucará Group, and subordinately, as steeply dipping veins hosted by rocks of the Pucará Group and overlying Cretaceous sandstones-siltstones of the Goyllarisquizga Group. Relicts of a distal retrograde magnesian skarn and cassiterite (stage pre-A) were identified in the deepest mantos. The volumetrically most important mineralization at Ayawilca comprises a low-sulfidation assemblage (stage A) with quartz, pyrrhotite, arsenopyrite, chalcopyrite, Fe-rich sphalerite, and traces of stannite and herzenbergite. Stage A sphalerite records progressive Fe depletion, from 33 to 10 mol% FeS, which is compatible with the observed transition from low- to a subsequent intermediate-sulfidation stage (B) marked by the crystallization of abundant pyrite and marcasite. Finally, during a later intermediate-sulfidation stage (C) sphalerite (up to 11 mol% FeS), galena, native bismuth, Cu-Pb-Ag sulfosalts, siderite, Mn-Fe carbonates, kaolinite, dickite, and sericite were deposited. This paragenetic evolution shows striking similarities with that at the Cerro de Pasco Cordilleran-type polymetallic deposit, even if at Ayawilca stage C did not reach high-sulfidation conditions. The occurrence of an early retrograde skarn assemblage suggests that the manto bodies at Ayawilca formed at the transition between distal skarn and skarn-free (Cordilleran-type) carbonate-replacement mineralization. Mineral assemblages define a T-fS2 evolutionary path close to the pyrrhotite-pyrite boundary. Buffering of hydrothermal fluids by underlying Devonian carbonaceous phyllites of the Excelsior Group imposed highly reduced conditions during stage A mineralization (logfO2 < − 30 atm). The low fO2 favored efficient Sn mobility during stages pre-A and A, in contrast to other known ore deposits in the polymetallic belt of central Peru, in which the occurrence of Sn minerals is minor. Subsequent cooling, progressive sealing of vein walls, and decreasing buffering potential of the host rocks promoted the shift from low- (stage A) to intermediate-sulfidation (stages B and C) states. LA-ICP-MS analyses reveal significant In contents in Fe-rich sphalerite (up to 1.7 wt%), stannite (up to 1908 ppm), and chalcopyrite (up to 1185 ppm). The highest In content was found in stage A sphalerite that precipitated along with chalcopyrite and stannite, thus pointing to the early, low-sulfidation assemblage as prospective for this high-tech metal in similar mineral systems. Indium was likely incorporated into the sphalerite crystal lattice via Cu+ + In3+ ↔ 2 Zn2+ and (Sn, Ge)4+ + (Ga, In)3+ + (Cu + Ag)+ ↔ 4 Zn2+ coupled substitutions. Indium incorporation mechanisms into the stannite and chalcopyrite crystal lattices remain unclear.

Influence of Pr3+ and CO32− Ions Coupled Substitution on Structural, Optical and Antibacterial Properties of Fluorapatite Nanopowders Obtained by Precipitation

Coupled substitution of fluorapatite (FAP) crystal lattice plays an important role in the engineering of optically active nanomaterials. Uniform fluorapatite nanopowders doped with praseodymium (Pr3+) and carbonate (CO32−) ions have been successfully synthesized by precipitation method under room temperature (25 °C). The structural, morphological, chemical and optical properties of monophase material were characterized by X-ray diffraction (XRD), Fourier Transform Infrared and Far Infrared Spectroscopy (FTIR and FIR, respectively), Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM/EDS), Transmission Electron Microscopy (TEM) and Photoluminescence Spectroscopy (PL). Coupled substitution of FAP crystal lattice with Pr3+ and CO32− reduces the crystallite size with a constant c/a ratio of 1.72. FTIR study showed that synthesized nanopowders were AB-type CO32− substitution, and FIR study revealed new Pr–O vibrations. TEM analysis was found that synthesized nanopowders were composed of irregular spheres in the nanometer range. The fluorescence of FAP nanoparticles was in the violet-blue region of the visible part of the spectrum. When Pr3+ was doped in a lattice, the violet-blue emission becomes sharper due to reabsorption. MCR–ALS analyses of fluorescence spectra indicated the shift of the maximum to the blue color with the increase in the concentration of Pr3+ ions. Additionally, luminescent nanopowders demonstrated significant antibacterial activity against Escherichia coli. As the obtained nanoparticles showed a good absorption of ultraviolet A light and reabsorption of blue-green luminescence, they are suitable for further development of optically active nanomaterials for light filtering. Optically active PrCFAP nanopowders with antibacterial properties may be promising additives for the development of multifunctional cosmetic and health care products.

Atomic-scale structure and non-stoichiometry of meteoritic hibonite: A transmission electron microscope study

Abstract Hibonite (CaAl12O19) is a common refractory mineral in Ca-Al-rich inclusions (CAIs) in primitive meteorites. Transmission electron microscope (TEM) studies have identified enigmatic planar defects in different occurrences of hibonite in the Allende meteorite that give rise to strong streaking along c* in electron diffraction patterns. Atomic resolution high-angle annular dark-field (HAADF) imaging and energy-dispersive X-ray (EDX) analyses were used to determine the nature and origin of these planar features. HAADF images of hibonite grains reveal lamellar intergrowths of common 1.6 nm spacing, and less commonly 2.0 and 2.5 nm spacings, interspersed in stoichiometric hibonite showing 1.1 nm (002) spacing. Stoichiometric hibonite consists of alternating Ca-containing (“R”) and spinel-structured (“S”) blocks stacked in a sequence RS. In contrast, the 1.6 nm layers result from a doubled S block such that the stacking sequence is RSS, while in the widest defect observed, the stacking sequence is RSSSS. These intergrowths are epitaxial and have coherent, low-strain boundaries with the host hibonite Meteoritic hibonite shows common Ti and Mg substitution for Al in its structure. Atomic-resolution EDX maps of hibonite grains in the Allende CAI confirm the preferred site occupancy of Mg on tetragonal M3 sites in S blocks and of Ti on trigonal bipyramidal M2 and octahedral M4 sites in R blocks. Mg is highly concentrated, but Ti is absent in the planar defects where wider S blocks show Al-rich compositions compared to stoichiometric MgAl2O4 spinel. Therefore, Mg likely played the major role in the formation and metastability of planar defects in hibonite. Electron energy loss spectroscopy data from the Ti L2,3 edge show the presence of mixed Ti oxidation states with ~15–20% of Ti as Ti3+ in hibonite, suggesting a direct substitution of Ti3+ ↔ Al3+ in hibonite. The remaining ~80–85% of Ti is present as Ti4+ and corresponding EDX analyses are consistent with the well-known coupled substitution 2Al3+ ↔ Ti4+ + Mg2+ being the major mechanism for Ti and Mg substitution in hibonite. The formation of planar defects in hibonite occurred during high-temperature nebular condensation or melting/crystallization processes. The occurrence of non-stoichiometric hibonite in the Allende CAI deviates from the mineral formation sequence predicted from equilibrium condensation models. Overall, our atomic resolution TEM observations signify non-equilibrium, kinetic-controlled crystal growth during the high-temperature formation of refractory solids in the early solar nebula.

Chemical Composition and Genesis Implication of Garnet from the Laoshankou Fe-Cu-Au Deposit, the Northern Margin of East Junggar, NW China

In order to reveal the formation mechanism of different garnets and its implications for the fluid evolution in the Laoshankou Fe-Cu-Au deposit in the northern margin of East Junggar (NW China), three types of garnet have been investigated in detail in this study. (1) Type 1 grossular, formed at Ca-silicate stage (stage I, the pre-mineralization stage), was replaced by Type 2 garnet and magnetite, and displays a compositional range of Grs44–53Adr44–53, which has relatively lower total REE (rare earth elements) contents (8.14–32.8 ppm) and markedly depleted LREE (light rare earth elements) with distinctive positive Eu anomaly (1.36–9.61). (2) Type 2 Al-rich andradite, formed at the early sub-stage of amphibole-epidote-magnetite stage (stage II, the main magnetite mineralization stage), can be divided into two sub-types, i.e., Type 2a and Type 2b. Type 2a garnets exhibit polysynthetic twinning and relatively narrow compositional variations of Adr63–66Grs31–34 with HREE-(heavy rare-earth elements) enrichment and positive Eu anomalies (3.22–3.69). Type 2b garnets own wide compositional variations of Adr55–77Grs21–43 with relatively higher REE contents (49.1–124 ppm), markedly depleted LREE and a distinctive positive Eu anomaly (2.11–4.61). (3) Type 3 andradite (Adr&gt;91) associated with sulfide stage (stage III, the main copper-gold mineralization stage) is different from other types of garnets in Laoshankou, which are characterized by lowest total REE contents (1.66–91.1 ppm), flat HREE patterns, LREE-enrichment and the strongest positive Eu anomalies (3.31–45.48). Incorporation of REE into garnet is largely controlled by external factors, such as fluid chemistry, pH, ƒO2 and water-rock ratios as well as its crystal chemistry. Type 1 and 2 garnets mainly follow the creation of X2+ (e.g., Ca2+) site vacancy, e.g., [X2+]−3VIII[]+1VIII[REE3+]+2VIII. The REE3+ substitution mechanism for Type 3 garnet is the Na+-REE3+ coupled substitutions, e.g., [X2+]−2VIII[X+]+1VIII[REE3+]+1VIII, without the evaluation of the creation of site vacancy. The compositional variations from Type 1 to Type 3 garnet indicate significant differences of fluid compositions and physicochemical conditions, and can be used to trace the fluid–rock interaction and hydrothermal evolution of garnet. Type 1 grossular was formed by magmatic fluid under low water–rock ratios and ƒO2, and neutral pH environment by diffusion metasomatism in a nearly closed system with the preferential incorporation into the grossular of HREE. As the long fluid pore residence and continuing infiltration metasomatism under nearly closed-system conditions, fluids with high water/rock ratios were characterized by increased ƒO2, more active incorporation of Fe3+ and REE, and formed Type 2 Al-rich andradite. In contrast, Type 3 garnet formed by oxidizing magmatic fluid under a mildly acidic environment with highest ƒO2 and water–rock ratios, and was influenced by externally derived high salinity and Ca-rich fluids in an open system. Thus, the geochemical features of different types and generations of garnets in the Laoshankou deposit can provide important information of fluid evolution, revealing a transition from neutral magmatic fluid to oxidizing magmatic fluid with addition of external non-magmatic Ca-rich fluid from the Ca-silicate stage to the sulfide stage. The above proved the fluid evolution process further indicates that the Laoshankou deposit prefers to be an IOCG-like (iron oxide-copper-gold) deposit rather than a typical skarn deposit.

Tracing mineralization history from the compositional textures of sulfide association: A case study of the Zhenzigou stratiform Zn-Pb deposit, NE China

The Qingchengzi Orefield located in northeastern China hosts more than ten Pb-Zn deposits and several Au-Ag deposits. Among those, the Zhenzigou Zn-Pb deposit displays a typical stratiform morphology, comprising ore bodies in layers and lenses. The ore minerals are simply composed of sphalerite, galena, pyrite, and pyrrhotite, together with minor arsenopyrite, marcasite, and argentite. A sulfide association of sphalerite-galena-pyrite-arsenopyrite was selected for this study; the interior compositional texture of the sulfide association was investigated by means of LA-ICPMS spot and mapping analysis, with the aim of constraining the ore genesis, fluid evolution history and element partitioning behavior. The results show that three distinct fluids were involved in the mineralization, namely an episode of sedimentary fluids at ∼2052 Ma which is responsible for the precipitation of pyrite core (Py0), an episode of metamorphic fluids at ∼1800 Ma which formed an overgrowth of pyrite (Py 1) and new crystallization of sphalerite (Sph1) and arsenopyrite (Aspy1), and a final episode of hydrothermal fluids related to the Triassic magmatism, forming another overgrowth of pyrite (Py2), sphalerite (Sph2) and arsenopyrite (Aspy2), together with minor precipitation of galena (Ga). The sedimentary fluids are relatively low in temperature (<150 °C), enriched in Cu, Pb, Zn, Ag, and depleted in Co, Ni, and As. Pyrite is the main precipitated sulfide mineral, scavenging most of the available trace elements. The late metamorphic and magmatic fluids are high in temperature (>300 °C) and have remobilized As, Co and Ni but only at mineral grain scale. The late metamorphic and magmatic fluids are enriched in In, Cd, Au, As, Ag, Cu, Pb, Sb, Sn, Co and Ni, and depleted in Bi, W, Mo, Te, and Se. The partitioning of most of the metals from the fluids into the sulfide minerals was controlled by the metals’ availability in the fluids and by the compatibility with crystal structure through direct or coupled substitutions. Pyrite acts as the main host of Co, Ni, Pb, and As; arsenopyrite hosts a major amount of Sb and Pb, and a minor amount of Au; sphalerite is the primary host of Cd, In, Cu, and the secondary host of Ag and Sn; galena is the primary host of Ag, Sb, Sn, and subordinately hosts Cd, In and As. In particular, a remarkable amount of In and Cd was carried by the hydrothermal fluids associated with the Triassic magmatism, indicating that the contribution of magmatic hydrothermal fluids to the mineralization is significant. In conclusion, our in-situ geochemical evidence of sulfide minerals reinforces a sedimentation-metamorphism-magmatic-hydrothermal reworking model for the Zhenzigou stratiform Zn-Pb mineralization. This case study underlines the significance of compositional textures of coexisting sulfide minerals when assessing the ore genesis. Overall, our study demonstrates that, by integrating trace element mapping and spot analysis on coexisting sulfide minerals, it is possible to re-construct the fluid evolution history, resolve the metal behaviors associated with fluids evolution, and review the contribution of individual fluids to mineralization.

Synthesis, Elasticity, and Spin State of an Intermediate MgSiO3‐FeAlO3 Bridgmanite: Implications for Iron in Earth's Lower Mantle

AbstractFe‐Al‐bearing bridgmanite may be the dominant host for ferric iron in Earth's lower mantle. Here we report the synthesis of (Mg0.5Fe3+0.5)(Al0.5Si0.5)O3 bridgmanite (FA50) with the highest Fe3+‐Al3+ coupled substitution known to date. X‐ray diffraction measurements showed that at ambient conditions, the FA50 adopted the LiNbO3 structure. Upon compression at room temperature to 18 GPa, it transformed back into the bridgmanite structure, which remained stable up to 102 GPa and 2,600 K. Fitting Birch‐Murnaghan equation of state of FA50 bridgmanite yields V0 = 172.1(4) Å3, K0 = 229(4) GPa with K0′ = 4(fixed). The calculated bulk sound velocity of the FA50 bridgmanite is ~7.7% lower than MgSiO3 bridgmanite, mainly because the presence of ferric iron increases the unit‐cell mass by 15.5%. This difference likely represents the upper limit of sound velocity anomaly introduced by Fe3+‐Al3+ substitution. X‐ray emission and synchrotron Mössbauer spectroscopy measurements showed that after laser annealing, ~6% of Fe3+ cations exchanged with Al3+ and underwent the high‐ to low‐spin transition at 59 GPa. The low‐spin proportion of Fe3+ increased gradually with pressure and reached 17–31% at 80 GPa. Since the cation exchange and spin transition in this Fe3+‐Al3+‐enriched bridgmanite do not cause resolvable unit‐cell volume reduction, and the increase of low‐spin Fe3+ fraction with pressure occurs gradually, the spin transition would not produce a distinct seismic signature in the lower mantle. However, it may influence iron partitioning and isotopic fractionation, thus introducing chemical heterogeneity in the lower mantle.

Coupled Substitutions of Minor and Trace Elements in Co-Existing Sphalerite and Wurtzite

The nature of couple substitutions of minor and trace element chemistry of expitaxial intergrowths of wurtzite and sphalerite are reported. EPMA and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses display significant differences in the bulk chemistries of the two epitaxial intergrowth samples studied. The sample from the Animas-Chocaya Mine complex of Bolivia is Fe-rich with mean Fe levels of 4.8 wt% for wurztite-2H and 2.3 wt% for the sphalerite component, while the sample from Merelani Hills, Tanzania, is Mn-rich with mean Mn levels in wurztite-4H of 9.1 wt% and for the sphalerite component 7.9 wt% In both samples studied the wurtzite polytype is dominant over sphalerite. LA-ICP-MS line scans across the boundaries between the wurtzite and sphalerite domains within the two samples show significant variation in the trace element chemistries both between and within the two coexisting polytypes. In the Merelani Hills sample the Cu+ + Ga3+ = 2Zn2+ substitution holds across both the wurztite and sphalerite zones, but its levels range from around 1200 ppm of each of Cu and Ga to above 2000 ppm in the sphalerite region. The 2Ag+ + Sn4+ = 3Zn2+ coupled substitution does not occur in the material. In the Animas sample, the Cu+ + Ga3+ = 2Zn2+ substitution does not occur, but the 2(Ag,Cu)+ + Sn4+ = 3Zn2+ substitution holds across the sample despite the obvious growth zoning, although there is considerable variation in the Ag/Cu ratio, with Ag dominant over Cu at the base of the sample and Cu dominant at the top. The levels of 2(Ag,Cu)+ + Sn4+ = 3Zn2+ vary greatly across the sample from around 200 ppm to 8000 ppm Sn, but the higher values occur in the sphalerite bands.

Highly Selective Anion Template Effect in the Synthesis of Constrained Pseudopeptidic Macrocyclic Cyclophanes.

Herein, we report the synthesis of a novel family of constrained pseudopeptidic macrocyclic compounds containing the hexahydropyrrolo[3,4-f]isoindolocyclophane scaffold and involving four coupled substitution reactions in the macrocyclization process. Although the increase in the number of steps involved in the macrocyclization could lead to a larger number of possible side products, the optimization of the methodology and the study of the driving forces have made it possible to obtain the desired macrocycles in excellent yields. A thorough computational study has been carried out to understand the macrocyclization process, and the results obtained nicely agree with experimental data. Moreover, the bromide anion had a clear catalytic template effect in the macrocyclization reaction, and surprisingly, the chloride anion had a negative template effect in opposition to the results obtained for analogous macrocycles. The parameters responsible for the specific kinetic template effect observed have been studied in detail.

Mitigating alkali-silica reaction induced concrete degradation through cement substitution by metakaolin and bentonite

This paper reports the results of an investigation on the efficiency of cement modification with clay minerals in preventing deteriorations of concrete due to alkali-aggregate reactions. Coupled substitution of metakaolin and bentonite in Portland cement was studied for a synergistic effect to mitigate alkali-silica reactivity (ASR). The expansion and cracking behavior of mortar bars were investigated. Hydration products of cement blends in two environments, immersion in saturated lime water and “mortar bar test” environment (i.e., in 1 N sodium hydroxide solution at 80 °C), were characterized. The role of the clay-based minerals in mitigating ASR was also evaluated by monitoring the strength development and characterizing the microstructural defects of the cement blends. The results indicate that cement modification with a combination of metakaolin and bentonite can effectively mitigate ASR-induced concrete deteriorations. By replacing 10% and 30% cement, expansion of mortar was decreased from deleterious to innocuous level. With 50% cement substitution, no ASR expansion was observed, and accordingly, no crack could be detected. The incorporated aluminum originating from the minerals slowed down the SiO2 dissolution, and formation of gel and its swelling potential. In addition, the decreased alkalinity and consumption of calcium in the modified cement matrix also contributed to the suppressed ASR. This paper demonstrates a low-cost approach to mitigating ASR; and metakaolin, bentonite, or both can effectively replace other supplementary cementitious materials used for the same purpose (e.g., fly ash) which may not be available or cost effective.

Indium Mineralization in the Volcanic Dome-Hosted Ánimas–Chocaya–Siete Suyos Polymetallic Deposit, Potosí, Bolivia

A volcanic dome complex of Miocene age hosts the In-bearing Ánimas–Chocaya–Siete Suyos district in SW Bolivia. Ore mineralization occurs as banded and massive infillings in sub-vertical, NE-SW striking veins. In this article, a detailed petrographic study is combined with in situ mineral geochemistry determinations in ore from the Arturo, Chorro and Diez veins in the Siete Suyos mine, the Ánimas, Burton, Colorada, and Rosario veins in the Ánimas mine and the Nueva vein in the Chocaya mine. A three-stage paragenetic sequence is roughly determined for all of them, and includes (1) an early low-sulfidation stage that is dominated by cassiterite, pyrrhotite, arsenopyrite, and high-Fe sphalerite (FeS &gt; 21 mol. %); (2) a second intermediate-sulfidation stage dominated by pyrite + marcasite ± intermediate product, sphalerite (FeS &lt; 21 mol. %), stannite, and local famatinite; and, (3) a late intermediate-sulfidation stage dominated by galena and Ag-Pb-Sn sulfosalts. Electron-probe microanalyses reveal high indium enrichment in stage-2 sphalerite (up to 9.66 wt.% In) and stannite (up to 4.11 wt.% In), and a moderate enrichment in rare wurtzite (up to 1.61 wt.% In), stage-1 sphalerite (0.35 wt.% In), cassiterite (up to 0.25 wt.% In2O3), and ramdohrite (up to 0.24 wt.% In). Therefore, the main indium mineralization in the district can be associated to the second, intermediate-sulfidation stage, chiefly in those veins in which sphalerite and stannite are more abundant. Atomic concentrations of In and Cu in sphalerite yield a positive correlation at Cu/In = 1 that agrees with a (Cu+ + In3+) 2Zn2+ coupled substitution. The availability of Cu in the mineralizing fluids during the crystallization of sphalerite is, in consequence, essential for the incorporation of indium in its crystal lattice and would control the distribution of indium enrichment at different scales. The highest concentrations of indium in sphalerite, which is found in the Diez vein in the Siete Suyos mine, occur in crustiform bands of sphalerite with local “chalcopyrite disease” texture, which has not been observed in the other studied veins. In stannite, the atomic concentrations of In are negatively correlated with those of Cu and Sn at Cu + In = 2 and Sn + In = 1. Thus, atomic proportions and correlations suggest the contextualization of the main indium mineralization in the sphalerite–stannite–roquesite pseudoternary system.

In-situ major and trace element chemistry of melanite from Tieshan Fe–Cu skarn deposit, Hubei Province, Eastern China: Implications for hydrothermal fluid evolution

The Tieshan Fe–Cu skarn deposit is located in the Edong ore district, the largest ore district of the Middle–Lower Yangtze River Metallogenic Belt (MLYRMB) in Eastern China. The Fe–Cu mineralization is spatially associated with the Tieshan pluton, which intruded the carbonates of the Lower Triassic Daye Formation. The orebodies are predominantly hosted along the intrusive contact zone between the (quartz)-diorite and (dolomitic)-marble. EMPA analysis shows that the Tieshan garnets belong to high-Ti andradite, with TiO2 contents ranging from 0.03 to 2.59 wt%, and is designated as melanite. The melanite contains low Na content, suggesting the absence of Na+-REE3+ coupled substitution. LA–ICP–MS analysis shows high total LREE (∑LREE), low ∑HREE and positive Eu anomalies, suggesting high fO2 and rapid growth of the melanite. Compared with other Ti-rich garnets worldwide, the Tieshan melanite grains are more Al-rich and Si-rich, and shows negative SiO2 vs. TiO2 correlation, which suggests predominant Ti-substitution in the tetrahedral site. The more positive Eu anomalies and higher percentage of andradite are present in the rim (than in the core), indicating lower salinities and higher fO2 of the late-stage ore-forming fluid. The magmatic-hydrothermal fluid source of these garnets supports the metasomatic ore genesis at Tieshan.

Spatial and Temporal Controls on the Distribution of Indium in Xenothermal Vein-Deposits: The Huari Huari District, Potosí, Bolivia

The Huari Huari deposit, Potosí Department in SW Bolivia, hosts polymetallic stratiform and vein mineralization of Miocene age with significant concentrations of the critical metal indium (In). Vein mineralization records document early crystallization of quartz and cassiterite followed by prominent associations of sulfides and sulfosalts. The earliest sulfide was arsenopyrite, followed by pyrrhotite, and progressively giving way to pyrite as the main iron sulfide, whereas Cu–Ag–Pb sulfosalts constitute late hypogene associations. Sphalerite is the chief ore mineral, and its crystallization is extended during most of the mineralization lifespan as evidenced by its initial cocrystallization with pyrrhotine, then with pyrite, and finally with Ag–Pb sulfosalts. The composition of sphalerite varies from early to late generations with a continuous decrease in FeS that attests to a decrease in temperature, which is constrained to vary from ~450 to &lt;200 °C, and/or an increase in f(S2), both congruent with the described paragenetic sequence. Indium concentrated mostly in the structure of Fe-rich sphalerite (up to 3.49 wt. %) and stannite (up to 2.64 wt. %) as limited solid solutions with roquesite in the (Zn,Fe)S–Cu2FeSnS4–CuInS2 pseudoternary system. In sphalerite, In shows a strong positive correlation with Cu at Cu/In = 1, suggesting its incorporation via a (Cu+ + In3+) ↔ 2Zn2+ coupled substitution, and it does not correlate with Fe. In stannite, In shows a moderate, negative correlation with Cu and Sn, and an In3+ ↔ (Cu+ + ½ Sn4+) coupled substitution is suggested. Coexisting sphalerite and stannite yielded the highest In concentrations and crystallized at temperatures between 350 and 250 °C. Copper activity probably played a major role in the accumulation of In in the structure of sphalerite since In-bearing sphalerite coexisted with the deposition of stannite, shows high concentrations of Cu (up to 0.13 atoms per formula unit (a.p.f.u.)) in its structure, and hosts exsolutions of stannite and chalcopyrite. Distribution on the district scale of In suggests an input of hydrothermal fluids richer in Cu in the central position of the mineralizing system, represented by the Antón Bravo vein.

Element enrichment characteristics: Insights from element geochemistry of sphalerite in Daliangzi Pb–Zn deposit, Sichuan, Southwest China

Sphalerite in the Daliangzai Pb-Zn deposit, one of the hundreds of Pb-Zn deposits in South China, was chosen as an case study for geochemical research using in situ laser-ablation inductively coupled mass spectroscopy analysis. Many kinds of minor and trace elements were investigated and subsequently treated by multivariate statistical analysis, especially principal component analysis (PCA). These research works indicate that the enrichments of Cu, Ga, As, Cd, Ge, Ag, Sb, Pb, Ni in sphalerite from different depths of the super-large No. I orebogy mutually imply a lower-temperature hydrothermal process dominating the mineralization in Daliangzi Pb-Zn deposit. A small temperature gradient controls the almost similar element distribution patterns indicated by the sphalerite samples which were enriched in Fe, Mn, Co, Cu, Ge, Ag, and Cd in the lower orebody whereas those in the upper orebody were enriched in Fe, Mn, Co, Cu, Ge, Ag, Cd, plus Ga and In. The enrichment of Fe in sphalerite matrix assists the incorporation of many metals, for example, Mn, Co, Ge, and (Ag+Sb). Some important coupled substitutions are discovered: 4Zn2+↔2Fe2++Ge4++□ (where □ denotes a vacancy), 2Zn2+↔Ag+ (Cu+)+Sb3+, and 3Zn2+↔2Ag+ (Cu+)+Ge4+. Moreover, tetrahedrite–tennantite and jordanite–geocronite solid solution series distinctly feature the sphalerite formed in Daliangzi Pb-Zn deposit which differentiates it from other metallogenic types, especially those related to magmatic activities.Results of PCA work to the selected sphalerite samples from deposits of skarn, syngenetic massive sulfide, MVT, special Jinding and Daliangzi reveal that significant differences, which are mainly defined by the concentration of Fe, Mn, Co, and depleted In, exist in the composition of sphalerite from different deposit types and lie along a single dimension of PC2*. Such a conclusion bases a framework established by PCA and proposed to study the base-metal deposit types and the relevant metallogenic process that are typical for deposits located in South China.

Distribution of In and other rare metals in cassiterite and associated minerals in Sn ± W ore deposits of the western Variscan Belt

We present data available on rare metal and indium distributions in cassiterite and associated minerals from thirteen Sn ± W granite-related ore deposits in the western Variscan Belt (Massif Central and Armorican Massif, France; Galicia, Spain; and SW England). Cassiterite and associated minerals including sulfides and titanium oxides were analysed using an electron probe micro-analyzer (EPMA). Significant indium contents were only measured in cassiterite from hydrothermal vein-type mineralizations associated with peraluminous granites of Montbelleux, Abbaretz (French Armorican Massif) and Marcofan (Galicia); they correlate with the highest Nb, Ta and Fe substitutions. Two coupled substitutions are proposed: (1) 2 (Sn4+, Ti4+)↔(Fe3+, In3+) + (Nb5+, Ta5+) and (2) Fe2+ + (Nb, Ta)5+↔In3++(Ti, Sn)4+ depending on Fe valence state. Sulfides (stannite, chalcopyrite, pyrite, arsenopyrite and sphalerite) and rutile associated with cassiterite contain significant amounts of indium, even when cassiterite is indium-free, suggesting preferential partitioning into the sulfides.

Ab initio calculations of uranium and thorium storage in CaSiO3-perovskite in the Earth’s lower mantle

Earth’s mantle convection is powered in part by the radiogenic heat released by the decay of 238 U, 235 U, 232 Th, and 40 K. We present ab initio calculations of uranium and thorium incorporation in CaSiO 3 -perovskite with and without aluminum, and propose that aluminous calcium silicate perovskite is the likely host of uranium and thorium in the lower mantle. At 15 GPa, the enthalpies of solution into aluminum-free CaSiO 3 -perovskite are 10.34 kJ/mol for U 4+ and 12.52 kJ/mol for Th 4+ in SiO 2 saturated systems, while the enthalpies are 17.09 kJ/mol and 19.27 kJ/mol, respectively, in CaO saturated systems. Coupled substitution of U 4+ and Th 4+ with aluminum is thermodynamically favored, with the enthalpies of solution negative for U 4+ and near 0 kJ/mol for Th 4+ throughout the stability field of CaSiO 3 -perovskite. Therefore, U incorporation into CaSiO 3 -perovskite is spontaneous in the presence of aluminum while Th forms a near ideal solid solution, implying these elements are potentially compatible with respect to partial melting in the transition zone and lower mantle. Furthermore, the solid solution reactions of U 4+ and Th 4+ are broadly similar to each other, suggesting a restriction on the fractionation of these actinides between the upper and lower mantle. U and Th compatibility in the presence of Al has implications regarding actinide transport into the deep mantle within subducting slabs and the geochemical content of seismic anomalies at the core-mantle boundary.