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American Mineralogist

Journal of Earth and Planetary Materials

ISSN: 1945-3027

Edited by: Don Baker, Hongwu Xu

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Volume 108 Issue 12

December 2023

November 30, 2023

Evidence for abundant organic matter in a Neoarchean banded iron formation

Zidong Peng, Jingbo Nan, Lianchang Zhang, Simon W. Poulton, Junlie Zhou, Yuan Yuan, Kaiwen Ta, Changle Wang, Mingguo Zhai

Page range: 2164-2181

Abstract

Microbial Fe(II) oxidation has been proposed as a major source of Fe minerals during deposition of banded iron formations (BIFs) in the Archean and Proterozoic Eons. The conspicuous absence of organic matter or graphitic carbon from BIFs, however, has given rise to divergent views on the importance of such a biologically mediated iron cycle. Here, we present mineral associations, major element concentrations, total carbon contents and carbon isotope compositions for a set of lower amphibolite-facies BIF samples from the Neoarchean Zhalanzhangzi BIF in the Qinglonghe supra-crustal sequence, Eastern Hebei, China. Graphite grains with crystallization temperatures (~470 °C) that are comparable to that predicted for the regional metamorphic grade are widely distributed, despite highly variable iron (12.9 to 54.0 wt%) and total organic carbon (0.19 to 1.10 wt%) contents. The crystalline graphite is interpreted to represent the metamorphosed product of syngenetic biomass, based on its co-occurrence with apatite rosettes and negative bulk rock δ 13 C organic values (–23.8 to –15.4‰). Moreover, the crystalline graphite is unevenly distributed between iron- and silica-rich bands. In the iron-rich bands, abundant graphite relicts are closely associated with magnetite and/or are preserved within carbonate minerals (i.e., siderite, ankerite, and calcite) with highly negative bulk rock δ 13 C carb values (–16.73 to –6.33‰), indicating incomplete reduction of primary ferric (oxyhydr) oxides by organic matter. By comparison, only minor graphite grains are observed in the silica-rich bands. Normally, these grains are preserved within quartz or silicate minerals and thus did not undergo oxidation by Fe(III). In addition, the close association of graphite with iron-bearing phases indicates that ferric (oxyhydr)oxides may have exerted a first order control on the abundance of organic matter. Combined, the biological oxidation of Fe(II) in the oceanic photic zone and subsequent burial of ferric (oxyhydr)oxides and biomass in sediments to form BIFs, suggests that a BIF-dependent carbon cycle was important in the Archean Eon. Although significant re-adsorption of phosphorus to ferric (oxyhydr)oxides and the formation of authigenic phosphate minerals at the sediment-water interface would be expected, oxidation of biomass in BIFs may have recycled at least a portion of the P (and other nutrients) released from reactions between organic matter and ferric (oxyhydr)oxides to the overlying water column, potentially promoting further primary productivity.

November 30, 2023

Electrical properties of iron sulfide-bearing dunite under pressure: Effect of temperature, composition, and annealing time

Michael J. Tauber, Suryansh Saxena, Emma S. Bullock, Hélène Ginestet, Anne Pommier

Page range: 2193-2208

Abstract

The detection and quantification of metal sulfides in host rocks by electrical measurements have been priorities for field and laboratory studies, motivated by mineral prospecting and fundamental interest in the mantle structure or core/mantle differentiation, among other reasons. Here, we reanalyze electrical data for a dunite host with added FeS or Fe-S-Ni (Saxena et al. 2021), and report additional experimental runs along with electron microprobe analyses. The applied pressure was 2 GPa; impedance spectra were acquired while annealing at 1023 K (below the metal-sulfide solidus), and while varying temperature from 570 to 1650 K. Addition of 6.5 or 18 vol% FeS strongly enhances conductivity of the bulk sample compared with that of the dunite host, though values are 100–100 000 times less than those of pure FeS. These results indicate that most metal sulfide content is not part of a viable conductive path, even for the 18 vol% quantity. Nevertheless, the relatively high conductivity and weak temperature dependence of the 18 vol% sample reveal that contiguous paths of solid or molten FeS span the electrodes. The sample with 6.5 vol% sulfide also exceeds the percolation threshold for temperatures as low as ~100 K below the eutectic melting point, likely because FeS softens. Conductivity is nearly unchanged upon crossing the eutectic temperature, however a decline over 1400–1500 K reveals that the 6.5 vol% molten FeS forms a fragile electrical network in dunite. Samples with Fe 50 S 40 Ni 10 or Fe 40 S 40 Ni 20 (at%) are less conductive than pure dunite at temperatures below ~1450 K. This surprising result, likely caused by a reducing influence of Fe or Ni metal, does not support the use of FeS as an analog for compositions with nickel or excess metal. Our findings suggest that probing the electrical network of metal sulfides as solids complements other studies focused on connectivity of molten metal sulfides.

November 30, 2023

Gas-mediated trace element incorporation into rhyolite-hosted topaz: A synchrotron microbeam XAS study

D.R. Schaub, Paul Northrup, Hanna Nekvasil, Tristan Catalano, Ryan Tappero

Page range: 2153-2163

Abstract

Magmatic gas exsolving during late-stage cooling of shallow magmas has been considered an important facilitator of low-pressure alteration and metal transport. However, the chemical properties of such gas, particularly its metal transport mechanisms and capacity, remain elusive. Trace elements in minerals produced by gas-mediated surface reaction or precipitation from gas capture details of gas composition and reaction pathways. However, interpretation of mineral trace element contents is dependent on understanding crystallographic controls on gas/mineral partitioning. This work investigates the structural accommodation of As, Mn, Ga, Ge, Fe, and Ti in vapor-deposited topaz of vesicular topaz rhyolite from the Thomas Range, Utah, through single-crystal synchrotron microbeam X-ray techniques on picogram quantities of those trace elements. X-ray absorption near edge structure (XANES) data indicates that these elements are incorporated into topaz as As 5+ , Fe 3+ , Mn 3+ , Ti 4+ , Ga 3+ , and Ge 4+ . Extended X-ray absorption fine structure (EXAFS) analysis for these trace elements, compared to EXAFS of structural Al and Si, reveals that As 5+ and Ge 4+ are incorporated directly into the tetrahedral site of the topaz structure, with the octahedral site accommodating Mn 3+ , Fe 3+ , Ga 3+ , and Ti 4+ . For As 5+ and Fe 3+ , the structural impact of substitution extends to at least second neighbors (other elements were only resolvable to first neighbors). Further interpretation of the EXAFS results suggests that the substitution of Ti 4+ results in increased distortion of the octahedral site, while the other trace elements induce more uniform expansion correlating in magnitude to their ionic radius. Comparison of quantified X-ray fluorescence (XRF) data for two topaz crystals from this rhyolite reveals variable trace element concentrations for As 5+ , Fe 3+ , Ga 3+ , and Ti 4+ , reflective of a source gas undersaturated in these trace elements changing in concentration over the period of topaz deposition. The identical Ge 4+ content of the two topaz crystals suggests that Ge 4+ in the gas was buffered by the growth of another Ge 4+ -bearing phase, such as quartz. The very low Mn 3+ content in the topaz crystals does not reflect the abundance of Mn 3+ in the gas (saturation of Mn is evidenced by coexisting bixbyite). Instead, it suggests a strong Jahn-Teller inhibitory effect to the substitution of Mn 3+ for Al 3+ in the distorted octahedral site of topaz. It is proposed that exsolution of an HF-enriched gas from cooling rhyolitic magma led to local scouring of Al, Si, and trace metals from the magma. Once topaz crystals nucleated, self-catalyzed reactions that recycle HF led to continued growth of topaz.

Open Access November 30, 2023

A dunite fragment in meteorite Northwest Africa (NWA) 11421: A piece of the Moon’s mantle

Allan H. Treiman, Julia Semprich

Page range: 2182-2192

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Abstract

A centimeter-sized fragment of dunite, the first recognized fragment of Moon mantle material, has been discovered in the lunar highlands breccia meteorite Northwest Africa (NWA) 11421. The dunite consists of 95% olivine (Fo 83 ), with low-Ca and high-Ca pyroxenes, plagioclase, and chrome spinel. Mineral compositions vary little across the clast and are consistent with chemical equilibration. Mineral thermobarometry implies that the dunite equilibrated at 980 ± 20 °C and 0.4 ± 0.1 gigapascal (GPa) pressure. The pressure at the base of the Moon’s crust (density 2550 kg/m 3 ) is 0.14–0.18 GPa, so the dunite equilibrated well into the Moon’s upper mantle. Assuming a mantle density of 3400 kg/m 3 , the dunite equilibrated at a depth of 88 ± 22 km. Its temperature and depth of equilibration are consistent with the calculated present-day selenotherm (i.e., lunar geotherm). The dunite’s composition, calculated from mineral analyses and proportions, contains less Al, Ti, etc., than chondritic material, implying that it is of a differentiated mantle (including cumulates from a lunar magma ocean). The absence of phases containing P, Zr, etc., suggests minimal involvement of a KREEP component, and the low proportion of Ti suggests minimal interaction with late melt fractionates from a lunar magma ocean. The Mg/Fe ratio of the dunite (Fo 83 ) is significantly lower than models of an overturned unmixed mantle would suggest, but is consistent with estimates of the bulk composition of the Moon’s mantle.

Open Access November 30, 2023

Hydrogen bond symmetrization and high-spin to low-spin transition of ε-FeOOH at the pressure of Earth’s lower mantle

Leslie Insixiengmay, Lars Stixrude

Page range: 2209-2218

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Abstract

We focus on the ferric end-member of phase H: ε-FeOOH using density functional theory at the PBEsol+U level. At 300 K, we find that ε-FeOOH undergoes a hydrogen bond symmetrization at 37 GPa and a sharp high-spin to low-spin transition at 45 GPa. We find excellent agreement with experimental measurements of the equation of state, lattice parameters, atomic positions, vibrational frequencies, and optical properties as related to the band gap, which we find to be finite and small, decreasing with pressure. The hydrogen bond symmetrization transition is neither first- nor second-order, with no discontinuity in volume or any of the elastic moduli. Computed IR and Raman frequencies and intensities show that vibrational spectroscopy may provide the best opportunity for locating the hydrogen bond symmetrization transition experimentally. We find that ε-FeOOH is highly anisotropic in both longitudinal- and shear-wave velocities at all pressures, with the shear wave velocity varying with propagation and polarization direction by as much as 24% at zero pressure and 43% at 46 GPa. The shear and bulk elastic moduli increase by 18% across the high-spin to low-spin transition.

November 30, 2023

CURIES: Compendium of uranium Raman and infrared experimental spectra

Tyler L. Spano, Travis A. Olds, Marshall McDonnell, Robert Smith, Jennifer L. Niedziela, Andrew Miskowiec, Roger Kapsimalis, Ashley E. Shields

Page range: 2219-2233

Abstract

Identification of radioactive materials is a critical goal of resource exploration, basic actinide science, and nuclear forensics, and we provide here new insights toward rapid, nondestructive analysis of uranium-containing minerals and technogenic phases. Raman and infrared spectroscopic data are powerful indicators of solid-phase U(VI) coordination chemistry. In addition, U(VI) minerals exhibit high chemical and structural diversity as artifacts of geochemical processes leading to ore formation. Spectral signals of axial U O 2 2 + U − O y l $\mathrm{UO}_{2}^{2+}\left(\mathrm{U}-\mathrm{O}_{\mathrm{yl}}\right)$bond lengths and the influences of additional oxyanions on these values are well documented for uranium oxide and oxysalt minerals and technogenic phases. Additional insight regarding the underlying crystallographic structure and chemical composition of uranium materials can be extracted through a survey of all available Raman spectroscopic data for these phases. To this end, we have developed the Compendium of Uranium Raman and Infrared Experimental Spectra (CURIES). CURIES was compiled via a thorough review of literature and databases, and for mineral species that lack measured and recorded spectra, data were obtained either from museum and academic collections or by direct syntheses. Characteristic Raman spectroscopic features for subgroups of uranyl minerals within CURIES were elucidated using multivariate statistical analyses. In addition, average spectra for groups of uranyl minerals were determined, providing insight into common spectroscopic characteristics that are indicative of the structural origins from which they arise. As of publication, 275 mineral species and technogenic phases have been entered in CURIES, and of these, 83 phases have published spectra that have been included in the CURIES database. Data collection is ongoing, and we have triaged missing data sets to assess CURIES for completion and to identify mineral groups that lack representation and should therefore be prioritized for data acquisition and inclusion in the database.

Open Access November 30, 2023

S 2 − and S 3 − radicals and the S 4 2 − polysulfide ion in lazurite, haüyne, and synthetic ultramarine blue revealed by resonance Raman spectroscopy

Stefan Farsang, Razvan Caracas, Takuji B.M. Adachi, Cédric Schnyder, Zoltán Zajacz

Page range: 2234-2243

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Abstract

Taking advantage of the Raman resonance effect, we employed 405 and 532 nm excitations to (1) identify sulfur species present in lazurite, haüyne, and synthetic ultramarine blue pigments and (2) investigate the enigmatic ~485 cm –1 band found previously in Raman spectra of lazurite and haüyne collected with 458 nm excitation. In spectra of lazurite and haüyne, bands of the sulfate ion and S 2 − a n d S 3 − $\mathrm{S}_{2}^{-}\, {\rm and} \,\mathrm{S}_{3}^{-}$radicals can be seen. Spectra collected using 405 nm excitation show the enhancement of the intensity of v 1 S 2 − $v_{1}\left( {\rm S}_{2}^{-}\right)$band and its nν 1 (n ≤ 7) progression. Spectra collected using 532 nm incident light show the enhancement of intensity of v 1 S 3 − , v 2 S 3 − , and v 3 S 3 − $v_{1}\left(\mathrm{~S}_{3}^{-}\right), v_{2}\left(\mathrm{~S}_{3}^{-}\right) \text {, and } v_{3}\left(\mathrm{~S}_{3}^{-}\right)$bands and the nν 1 (n ≤ 9) and ν 2 + nν 1 progressions of the v 1 S 3 − $v_{1}\left(\mathrm{~S}_{3}^{-}\right)$band. In spectra collected with 405 nm excitation, we also found features that we ascribe to the S 4 2 − $\mathrm{S}_{4}^{2-}$polysulfide ion. These include the ν 1 symmetric S-S stretching band at ~481 cm –1 , the ν 2 symmetric S-S stretching band at ~443 cm –1 (only present in spectra of some lazurite samples), the ν 3 symmetric S-S bending at 223 cm –1 and the nν 1 (n ≤ 5) and nν 1 +ν 3 progressions of the v 1 S 4 2 − $v_{1}\left(\mathrm{~S}_{4}^{2-}\right)$band. We observed that under laser illumination, the S 4 2 − $\mathrm{S}_{4}^{2-}$ polysulfide ion rapidly decomposes into two S 2 − $\mathrm{S}_{2}^{-}$radicals in lazurite while it remains stable in haüyne. In spectra of synthetic ultramarine blue pigments, only features of S 2 − a n d S 3 − $\mathrm{S}_{2}^{-}\, {\rm and} \,\mathrm{S}_{3}^{-}$radicals were observed. Finally, we verified the identity of the radical and polysulfide ions with ab initio molecular dynamics calculations. We conclude that Raman resonance spectroscopy is a powerful qualitative method to detect polysulfide and sulfur radical species with concentrations below the detection limit of conventional analytical techniques. Owing to the high stability of S 4 2 − $\mathrm{S}_{4}^{2-}$in haüyne, this mineral structure appears promising as a host material for S 4 2 − $\mathrm{S}_{4}^{2-}$entrapment, making it potentially useful for applications in optoelectronics.

November 30, 2023

Efect of faceting on olivine wetting properties

Yongsheng Huang, Takayuki Nakatani, Sando Sawa, Guoji Wu, Michihiko Nakamura, Catherine McCammon

Page range: 2244-2259

Abstract

Grain-scale pore geometry primarily controls the fluid distribution in rocks, affecting material transport and geophysical response. The dihedral angle (θ) in the olivine-fluid system is a key parameter determining pore fluid geometry in mantle wedges. In the system, curved and faceted olivine-fluid interfaces define θ, resulting in faceted-faceted (FF), faceted-curved (FC), and curved-curved (CC) angles. The efect of faceting on θ under various pressure and temperature ( P-T ) conditions and fluid compositions, however, has not been constrained, and mineralogical understanding remains unresolved. This study evaluated facet-bearing θ and their proportions in olivine-multicomponent aqueous fluid systems. Our results show that 1/3 of olivine-fluid θ are facet-bearing angles, regardless of the P-T conditions and fluid composition. Faceting produces larger dihedral angles than CC angles. The grain boundary plane (GBP) distribution reveals that the GBPs of faceted interfaces at triple junctions have low Miller index faces ({100}, {010}, and {101}). The misorientation angle/axis distributions of adjacent grain pairs are in accord with a theoretical distribution of random olivine aggregate. Moreover, the calculation of the FF angles for adjacent grain pairs with low Miller index GBPs reproduces measured angle values based on the olivine crystal habit. Therefore, our study suggests that the FF angle is strongly afected by olivine crystallography. The presence of faceting increases θ and a critical fluid fraction (φ c ) for percolation, lowering permeability. In the mantle wedge, where olivine crystallographic preferred orientation (CPO) is expected owing to corner flow, increasing the FF angle proportion with associated changes in fluid pore morphology will lead to permeability anisotropy, and controlling the direction of the fluid flow, and it will result in geophysical anomalies such as seismic wave attenuation and high electrical conductivity.

November 30, 2023

The obscuring efect of magma recharge on the connection of volcanic-plutonic rocks

Kai Zhao, Xisheng Xu, Zhenyu He, Yan Xia

Page range: 2260-2282

Abstract

The current debate on volcanic-plutonic connection is centered on whether eficient liquid-crystal segregation dominates the evolution of a mushy reservoir to produce evolved, crystal-poor rhyolite and cumulate leftover. However, magma recharge may remarkably influence the evolution of a mushy reservoir and obscure the evidence of liquid-crystal segregation. This complexity poses a challenge to exploring the connection of volcanic-plutonic rocks. This study investigates the Qinzhou Bay granitic complex (~250–248 Ma) from South China, which contains crystal-poor (<19 vol%) peraluminous rhyolites and subsequent crystal-rich (28–54 vol%) porphyries. Although the rhyolite and porphyry units have a close spatio-temporal link, they do not share a fractionation trend and similar whole-rock Sr-Nd-O isotopic compositions; thus, a direct connection is not evidenced. We further present textural analyses, mineral and melt inclusion compositions, thermobarometry (the combination of Ti-in-zircon thermometer and Ti-in-quartz thermobarometer), and thermodynamic modeling to examine the alternative interpretations, i.e., the two units may have intrinsically independent origins or the connection of the two units has been obscured. For the rhyolite unit, thermobarometric results reveal a polybaric storage system consisting of middle (>600 ± 80 MPa) and upper (~150 ± 40 and ~60 ± 20 MPa) crustal reservoirs. Variations in quartz Fe content and chlorine-rich, metaluminous melt inclusions suggest that magma hybridization with less-evolved metaluminous magmas occurred at both crustal levels. In particular, the elevated Fe contents in the quartz population that crystallized at the shallowest level (~60 ± 20 MPa) suggest that recharge magmas were directly injected into the shallowest reservoir. Deviation of the whole-rock composition from the liquid evolution trend recorded in melt inclusions suggests a combined efect of magma mixing and crystal-melt segregation processes in upper crustal reservoirs. Thermodynamic modeling and mass balance calculations suggest that the whole-rock composition of the rhyolite could be reproduced by mixing between regionally exposed dacites and segregated melts at crystallinities of 50–60% (using parental magma represented by the least-evolved melt inclusion). For the porphyry unit, thermobarometric results reveal magma storage at middle (more than 450 ± 40 to 550 ± 40 MPa) and upper (110 ± 20 to 140 ± 20 MPa) crustal levels. The small-scale oscillatory zonation of plagioclase, the pervasive resorption of quartz and alkali feldspar, and the presence of peraluminous microgranular enclaves in the porphyries suggest a recharge event of metasediment-sourced magmas, triggering reactivation and convection of the reservoir. Autoclastic and overgrowth textures of quartz, plagioclase, and alkali feldspar phenocrysts and development of columnar jointing suggest that the reactivated porphyritic magmas ascended and emplaced at ultrashallow levels (~30 ± 10 MPa). Because of the similar storage pressures, the porphyries may represent remobilized cumulates of rhyolitic magmas, whereas the texture and geochemistry of the cumulate-liquid pair were modified, a key factor rendering a cryptic connection between the rhyolite and porphyry. Alternatively, the plumbing systems feeding the rhyolite and porphyry units are horizontally independent or vertically discrete, but this circumstance is inconsistent with the same evolution trend of quartz Fe and Al contents of the rhyolite and porphyry. Our study highlights that whole-rock composition may record blended information of complex processes, and caution should be taken when whole-rock composition is used to extract information of a single process. Multi-method constraints are required to evaluate the influence of recharge processes on the modification of liquid-cumulate records, and big data analysis on the basis of geochemistry should be conducted with caution to avoid biased understanding.

Open Access November 30, 2023

In-situ study of microstructures induced by the olivine to wadsleyite transformation at conditions of the 410 km depth discontinuity

Estelle Ledoux, Matthias Krug, Jeffrey Gay, Julien Chantel, Nadège Hilairet, Maxim Bykov, Elena Bykova, Georgios Aprilis, Volodymyr Svitlyk, Gaston Garbarino, Nicolas Guignot, Carmen Sanchez-Valle, Sergio Speziale, Sébastien Merkel

Page range: 2283-2293

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Abstract

The olivine-wadsleyite transformation is believed to occur at depths of about 410 km in the Earth, producing a major seismic discontinuity in this region of the Earth’s mantle. The mechanism of this phase transition controls the microstructures of the newly nucleated wadsleyite, the major phase of the upper part of the mantle transition zone, and thus impacts seismic observations in the region. Here, we study the microstructures produced by the olivine-wadsleyite transformation using in situ laboratory experiments at pressures and temperatures relevant for the mantle transition zone. We transform pure olivine samples in laser-heated diamond-anvil cells at pressures ranging from 12.3 to 20.2 GPa and temperatures of 1400–1730 K. At different steps of the transformation we measure the orientation and size distribution of individual sample grains using multigrain crystallography at synchrotron radiation sources. We find that the olivine to wadsleyite transformation is incoherent at the conditions of the mantle transition zone, and is probably dominated by nucleation of wadsleyite at grain boundaries of the parent olivine. Thus, we expect that seismic anisotropy near 410 km would drop significantly due to the randomized lattice preferred orientation of newly nucleated wadsleyite induced by the incoherent transformation.

November 30, 2023

Effect of pre-existing crystals and melt homogeneity on the decompression-induced crystallization of hydrous rhyodacite magma

Kazuhisa Matsumoto, Satoshi Okumura, Akihiko Tomiya

Page range: 2294-2307

Abstract

Decompression-induced crystallization is an important process that controls the behavior of volcanic eruptions because it strongly affects magma rheology and degassing behavior in the shallow parts of volcanic conduits. Several decompression experiments have been performed to understand and model the crystallization processes; however, the effect of superheating (i.e., heating above the liquidus temperature for a definite period of time) before decompression has not been elucidated, despite the proposal of its importance in previous cooling experiments. As the superheating influences the number of pre-existing crystals and melt homogeneity, it is expected to control decompression-induced crystallization. In this study, we investigated the effects of pre-existing crystals and melt homogeneity on crystallization during the decompression of rhyodacitic magma at a temperature of 900 °C. The magma studied herein has a liquidus temperature of ~920 °C. Five starting materials were prepared via heating at different superliquidus temperatures (940, 970, 1050, and 1300 °C) and a sub-liquidus temperature (900 °C) using an internally heated pressure vessel and a cold-seal pressure vessel, respectively. Decompression experiments using these starting materials were conducted from 130 to 30 MPa at decompression rates of 5, 20, and 100 MPa h –1 . When the melt was completely homogenized (at 1050 and 1300 °C), no crystals were formed at 100 MPa h –1 and the small amounts of crystals heterogeneously formed along the capsule wall were found at 5 and 20 MPa h –1 . At the same decompression rate, the number density of plagioclase formed during decompression increased as the superheating temperature decreased from 970 to 900 °C, despite the higher number densities of pre-existing crystals before decompression in the samples with lower superheating. Such finding indicates that nucleation occurs easily when the number density is initially high. This result is inconsistent with the idea that nucleation occurs when supersaturation is sufficient to overcome the energy barrier for nucleation, and the growth of pre-existing crystals decreases supersaturation. In contrast, the results of our experiments can be explained by considering that higher superheating results in a more homogeneous melt structure with few pre-crystal clusters, which are growth sites, and ultimately the suppression of nucleation. Based on these results, we conclude that pre-existing crystals and melt homogeneity strongly affect the crystal texture formed by decompression. For application to natural systems, the high number density of microlites found in natural samples may be due to heterogeneous nucleation caused by the presence of pre-crystal clusters and other mechanisms. Furthermore, the superheating of magma in a reservoir caused by the injection of high-temperature mafic magma may influence the crystal texture during magma ascent and, hence, control the explosivity of the eruption.

November 30, 2023

Origin of clinopyroxene-ilmenite symplectites in mafic granulites from eastern parts of the Chotanagpur granite gneissic complex, East Indian shield

Somdipta Chatterjee, Shreya Karmakar, Subham Mukherjee, Sanjoy Sanyal, Pulak Sengupta

Page range: 2308-2322

Abstract

In this manuscript, we report a rare occurrence of rod-like intergrowths of clinopyroxene-ilmenite that variably replace titanite in a suite of high-pressure mafic granulites from the Chotanagpur Granite Gneissic Complex, Eastern Indian Shield. Garnet proximal to the clinopyroxene-ilmenite intergrowth is invariably replaced with symplectic clinopyroxene-plagioclase or a rind of plagioclase. Textural modeling of the reaction textures and mineral compositions suggests that the garnet-titanite became unstable and the ilmenite-clinopyroxene preferentially develop after titanite. The presence and texture of halogen rich apatites within the ilmenite-clinopyroxene symplectite are consistent with a fluid mediated process. Thermodynamic modeling in the NCFMAST (+H 2 O) system, demonstrates that the clinopyroxene-ilmenite symplectite was formed along a steeply decompressive retrograde P - T path (from ~13 to ~7 kbar, at ~800 °C), in the presence of partial melts. The study demonstrates that relative to Fe, Mg, and Ca, Ti was less mobile and that the mobility of Ti was restricted within the confines of titanite being replaced by the clinopyroxene-ilmenite symplectite. This study implies that besides the ligand activity, the fluid/rock ratio exerts a strong control on the length scale of Ti transport in natural rocks.

Open Access November 30, 2023

Single-crystal analysis of La-doped pyromorphite [Pb₅(PO₄)₃Cl]

Julia Sordyl, John Rakovan, Peter C. Burns, Justyna Topolska, Adam WŁodek, Jennifer E.S. Szymanowski, Ginger E. Sigmon, JarosŁaw Majka, Maciej Manecki

Page range: 2323-2330

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Abstract

Rare earth elements (REE) in calcium apatite have been widely described in the literature. Based on the investigations of minerals and their synthetic analogs, the mechanism of substitution of REE 3+ for Ca 2+ and their structural positions are well established. Although the presence of REE in natural pyromorphite has been reported, the structural response of substitution of REE 3+ for Pb 2+ is not established. A better understanding of REE-rich Pb-apatite may facilitate the potential use of this mineral in industrial processes. Two La-doped pyromorphite analogs [Pb 5 (PO 4 ) 3 Cl] and two control pyromorphite analogs (with the absence of La) were synthesized from aqueous solutions at 25 °C. Na + and K + were used as charge-compensating ions to facilitate the incorporation of trivalent REE cations (La 3+ + Na + ↔ 2Pb 2+ and La 3+ + K + ↔ 2Pb 2+ ). Microprobe analysis, scanning electron microscopy, and Raman spectroscopy were used to confirm the purity of obtained phases. High-precision crystal structure refinements (R 1 = 0.0140–0.0225) of all four compounds were performed from single-crystal X-ray diffraction data. The La content varied from 0.12(1) to 0.19(1) atoms per formula unit with the counter ions of K + and Na + , respectively. Both substituting ions were accommodated at the Pb1 site only. By comparing the La-doped pyromorphite analogs with their control samples, it was possible to detect small changes in bond distances and polyhedral volumes caused by the La substitution. Variations in individual and mean interatomic distances reflected the cumulative effect of both the amount of substitution and ionic radii of substituting ions (La 3+ , Na + , and K + ).

November 30, 2023

Crystal structure of calcium-ferrite type NaAlSiO₄ up to 45 GPa

Fei Qin, Ye Wu, Shengchao Xue, Dongzhou Zhang, Xiang Wu, Steven D. Jacobsen

Page range: 2331-2337

Abstract

Alkali-rich aluminous high-pressure phases including calcium-ferrite (CF) type NaAlSiO 4 are thought to constitute ~20% by volume of subducted mid-ocean ridge basalt (MORB) under lower mantle conditions. As a potentially significant host for incompatible elements in the deep mantle, knowledge of the crystal structure and physical properties of CF-type phases is therefore important to understanding the crystal chemistry of alkali storage and recycling in the Earth’s mantle. We determined the evolution of the crystal structure of pure CF-NaAlSiO 4 and Fe-bearing CF-NaAlSiO 4 at pressures up to ~45 GPa using synchrotron-based, single-crystal X-ray diffraction. Using the high-pressure lattice parameters, we also determined a third-order Birch-Murnaghan equation of state, with V 0 = 241.6(1) Å 3 , K T0 = 220(4) GPa, and K T 0 ′ = 2.6 ( 3 ) $K_{T 0}^{\prime}=2.6(3)$for Fe-free CF, and V 0 = 244.2(2) Å 3 , K T0 = 211(6) GPa, and ′′ K T 0 ′ $K_{T 0}^{\prime}$= 2.6(3) for Fe-bearing CF. The addition of Fe into CF-NaAlSiO 4 resulted in a 10 ± 5% decrease in the stifest direction of linear compressibility along the c -axis, leading to stronger elastic anisotropy compared with the Fe-free CF phase. The NaO 8 polyhedra volume is 2.6 times larger and about 60% more compressible than the octahedral (Al,Si)O 6 sites, with K 0 N a O 8 = 127 G P a and K 0 ( A l , S i ) O 6 ∼ 304 G P a . $K_{0}^{\mathrm{NaO}} 8=127 \mathrm{GPa}\, \text{and} \,K_{0}^{(\mathrm{Al}, \mathrm{Si}) \mathrm{O}_{6}} \sim 304 \mathrm{GPa}.$Raman spectra of the pure CF-type NaAlSiO 4 sample shows that the pressure coefficient of the mean vibrational mode, 1.60(7) cm –1 /GPa, is slightly higher than 1.36(6) cm −1 /GPa obtained for the Fe-bearing CF-NaAlSiO 4 sample. The ability of CF-type phases to contain incompatible elements such as Na beyond the stability field of jadeite requires larger and less-compressible NaO 8 polyhedra. Detailed high-pressure crystallographic information for the CF phases provides knowledge on how large alkali metals are hosted in alumina framework structures with stability well into the lowermost mantle.

November 30, 2023

Revision of the CaMgSi₂O₆-CO₂ P-T phase diagram at 3–6 GPa

Anton Shatskiy, Yulia G. Vinogradova, Anton V. Arefiev, Konstantin D. Litasov

Page range: 2338-2347

Abstract

We reexamined the phase relationships in the system diopside-CO 2 in the range of 3–6 GPa and 850–1500 °C in multi-anvil experiments, including reversal ones lasting up to 169 h. The reaction CaMgSi 2 O 6 (clinopyroxene) + 2CO 2 (fluid) = 2SiO 2 (quartz/coesite) + CaMg(CO 3 ) 2 (dolomite) passes through 3 GPa/950 °C with a slope of 6 MPa/°C and terminates at an invariant point near 4.5 GPa/1200 °C, where carbonate liquid coexists with clinopyroxene, coesite, dolomite, and CO 2 fluid. The newly determined boundary has the equation P (GPa) = 0.006 × T (°C) – 2.7. As temperature increases to 1250 °C at 4.5 GPa, liquid, dolomite, and coesite disappear, and clinopyroxene coexists with CO 2 fluid. As pressure increases to 6 GPa, the solidus temperature increases to 1300 °C revealing a slope of 15 MPa/°C. At 4.5 and 6 GPa, solidus melts contain about 1 wt% SiO 2 . As temperature increases to 1400 and 1500 °C at 6 GPa, the silica contents in the carbonate melt increase to 6 and 13 wt%, respectively. Our data, combined with that of Luth (2006), indicate that above 4.5 GPa the liquidus reaction involving clinopyroxene and CO 2 sweeps down through 350 °C via a pressure maximum near 5.3 GPa to meet the invariant point at 4.5 GPa. The shape of the diopside-CO 2 solidus resembles that of lherzolite-CO 2 (Wyllie and Huang 1975a) but shifted by 2 GPa to higher pressure. Thus, the deep depression along the solidi in the system CaO-MgO-SiO 2 -CO 2 is a fundamental feature of both ultramafic and mafic assemblages at depths of 70–150 km.

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Editorial

Editors
Hongwu Xu; Los Alamos National Laboratory; Los Alamos, New Mexico, USA; hxu@lanl.gov
Don R. Baker; McGill University; Montreal, Quebec, Canada; don.baker@mcgill.ca
Ian Swainson; National Reseach Council of Canada, Ontario; Canada; Ian.swainson@usask.ca

Special Editors
Kimberley T. Tait; New Mineral Names Editor
Ross John Angel / Hannes Kruger / Biljana Lazic; Technical Editors Crystal Structures　
John C. Schumacher; Book Review Editor　

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Managing Editor, American Mineralogist　
Rachel A. Russell　
Mineralogical Society of America　
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(Deutsch)

Online ISSN: 1945-3027
Type: Journal
Language: English
Publisher: De Gruyter
First published: January 20, 1995
Publication Frequency: 12 Issues per Year
Audience: Researchers interested in Earth Sciences

American Mineralogist

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Single-crystal analysis of La-doped pyromorphite [Pb5(PO4)3Cl]

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Crystal structure of calcium-ferrite type NaAlSiO4 up to 45 GPa

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Crystal structure of calcium-ferrite type NaAlSiO₄ up to 45 GPa

Revision of the CaMgSi₂O₆-CO₂ P-T phase diagram at 3–6 GPa