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<jats:title>Abstract</jats:title> <jats:p>Advances in crystal nucleation and growth over the past three decades have led to the understanding that crystallization proceeds through a variety of pathways, ranging from the conventional atom-by-atom model to the particle aggregation- or amorphous transformation-based non-classical modes. Here, we present a novel mineralization mechanism exemplified by a lunar chromite formed via solid-liquid interface reactions, through investigations towards a lunar breccia returned by the Chang'e 5 mission. The chromite occurs in the middle of a whisker-shaped intergrowth structure made by olivine at the bottom and nanospheres of troilite and metallic iron at the top. Morphological observation and size statistics of the nanospheres, including those within the whisker structure and the others dispersed in glass, suggest the nanophases attached to olivine with coherent crystallographic orientations, possibly through an oriented aggregation process. The chromium deficiency in the olivine near the interface between olivine and chromite suggests that Cr in chromite originated from olivine, but the significantly reduced ferrous concentration in the glass surrounding chromite indicates the iron was derived from surrounding impact-induced glass. Based on laboratory observations and simulated calculations of energy and lattice mismatch, we propose that chromite crystallized at the interface between troilite and olivine in the impact melts, during which the nanospheres were lifted up and transported away from olivine surface and form a mushroom-shaped structure. This finding suggests that oriented attachment growth, chiefly confined to homogeneous systems thus far, can also take place in heterogeneous systems far from equilibrium, such as that produced by the impacts. It is conceivable that the studied crystallization pathway occurring on the heterogeneous interfaces may have been a common mineralization mode at highly-nonequilibrium conditions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>This study investigates the effect of pressure on diffusion creep of dry San Carlos and synthetic (prepared by sol-gel method) olivine. We prepared dry (water content &amp;lt; 9 ppm wt) fine-grained (&amp;lt; 1 μm grain-size) olivine and deformed the samples (both San Carlos and solgel olivine in the same assembly) in the same sample assembly under high-pressure (P = 2.9–8.8 GPa) and modest temperatures (T = 980–1250 K) at a fixed strain-rate. Evolution of strength was studied using the radial X-ray diffraction from various diffraction planes. We found that San Carlos and sol-gel olivine show similar rheological behaviour (when normalized to the same grain-size). Stress estimated by the radial X-ray diffraction increases with time and initially shows similar values for all diffraction planes. In many cases, stress values start to depend on the diffraction planes in the later stage and time dependence becomes minor. The micro-structural observations show that grain-size increases during an experiment. The results are interpreted using a theory of radial X-ray diffraction and the theoretical models of diffusion and dislocation creep. We conclude that the initial stage of deformation is by diffusion creep, but deformation in the later stage is by dislocation creep. For dislocation creep, our results are in reasonable agreement with previous low temperature dislocation creep results after a correction of temperature effect. For diffusion creep, we obtain an activation volume of 7.0 ± 2.4 cm3/mol that is substantially smaller than the values reported on dislocation creep but agrees well with the results on grain-growth. By comparing the present results on dry olivine with the previous results on wet (water-saturated) olivine, we found that water enhances diffusion creep but only modestly in comparison to dislocation creep. The difference in the pressure and water content dependence between diffusion and dislocation creep has an important influence on the dominant deformation mechanisms of olivine in the upper mantle.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Fission-track analysis is a thermochronologic method for dating rocks and reconstructing their low-temperature thermal histories. We investigate the influence of the apatite composition on the etching of fossil confined fission tracks, and its consequences for the fission-track method. We conducted step-etch experiments with 5.5 M HNO3 at 21 °C on samples with etch pit diameters (Dpar) spanning most of the range for natural apatites (Panasqueira: 1.60 µm, Slyudyanka: 2.44 µm, Brazil: 3.92 µm, and Bamble: 4.60 µm) to determine their apatite etch rates vR (the rate at which each lattice plane is displaced parallel to itself) as a function of crystallographic orientation (ϕ’). Our measurements revealed significant differences between the four samples. We fitted three-parameter functions, vR = a(Dpar) ϕ’ eb(Dpar)ϕ’ + c, describing vR as a function of the angle to the apatite c-axis for our hexagonal samples (excluding Bamble) and Durango apatite. The parameters a and b both exhibit a linear correlation with Dpar, whereas the constant c is small (~0.1 µm.min-1) and its between-sample variation negligible at the resolution of our measurements. Bamble exhibits a different, bimodal relationship between vR and ϕ’, which we fitted with a sum of two sine functions. In all cases, including Bamble, there is a striking correlation between the angular frequencies of horizontal confined tracks and the magnitude of the apatite etch rate vR perpendicular to the track axes. This shows that the sample of confined tracks selected for measurement and modeling is to a much greater degree determined by the etching properties of the apatite sample than by geometric or subjective biases. The track etch rate vT is constant along most of the track length but varies from track to track. The mean vT correlates with Dpar, so that tracks etch to their full lengths in a shorter time in faster etching apatites. The mean rate of length increase between etch steps, vL, also correlates with Dpar. The length increments of individual tracks are however irregular. This points to an intermittent structure at the ends of the tracks.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Germanates are often used as structural analogs of planetary silicates. We have explored the high-pressure phase relations in Mg2GeO4 using diamond anvil cell experiments combined with synchrotron X-ray diffraction and computations based on density functional theory. Upon room temperature compression, forsterite-type Mg2GeO4 remains stable up to 30 GPa. At higher pressures, a phase transition to a forsterite-III type (Cmc21) structure was observed, which remained stable to the peak pressure of 105 GPa. Using a 3rd order Birch Murnaghan fit to the experimental data, we obtained V0 = 305.1 (3) Å3, K0 = 124.6 (14) GPa and K0′ = 3.86 (fixed) for forsterite- and V0 = 263.5 (15) Å3, K0 = 175 (7) GPa and K0′ = 4.2 (fixed) for the forsterite-III type phase. The forsterite-III type structure was found to be metastable when compared to the stable assemblage of perovskite/post-perovskite + MgO, as observed during laser-heating experiments. Understanding the phase relations and physical properties of metastable phases is crucial for studying the mineralogy of impact sites, understanding metastable wedges in subducting slabs and interpreting the results of shock compression experiments.</jats:p>

<jats:title>Abstract:</jats:title> <jats:p>High calcic (~95% anorthite) plagioclase is the key mineral comprising the primary lunar crustal suites that cover over 60% of the Moon’s surface. Pristine crystals of similar high calcic plagioclase are rare occurrences on Earth, which creates a roadblock to using terrestrial material as lunar crustal analogs. We discuss the potential of a particular megacrystic anorthite (An95.51 ± 0.31) occurring in the basaltic lava flows of island arc volcano in Miyake-jima, Japan as an material analog. A comprehensive analytical routine for the Miyake-jima anorthites has been performed to explore intra- and inter-crystalline heterogeneities in terms of major, minor, and trace elements. These anorthites show flat concentration gradients across core profiles for all major elements (Si, Al, Ca, Na), minor elements (Mg, Fe), and most trace elements (La, Ce, Pm, Nd, Eu). Comparing the chemical composition of the samples with that of different lunar crustal suites like ferroan anorthosites, high-magnesium suites, and high alkali suites show that the Miyake-jima anorthites are overlapping or depleted in most minor and trace elements except for a slight enrichment in Li, Ti, Fe, Sr, Eu, Ba, and Pb. Given the low abundance of most trace elements in the Miyake-jima anorthites we can treat this sample suite as a “blank slate”, which provides the opportunity to dope the crystalline matrix with the elements of interest at different levels and use them for geochemical, petrologic, and spectroscopic studies. The lack of typical magmatic zoning and overlapping elemental compositions across the different megacrysts make the Miyake-jima anorthites very well-suited as a lunar crustal material analog. Highly calcic, crystalline anorthite is shown to have unique spectral signatures from less calcic anorthite, and intermediate and sodic compositions of plagioclase feldspar as calcium and iron contents control the wavelength position and shape of the diagnostic spectral features in the thermal infrared region of the electromagnetic spectrum. Thus, near and thermal infrared spectral measurements of the Miyake-jima anorthites highlight the importance of developing chemically and mineralogically consistent terrestrial material analogs for remote sensing studies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The present paper reports the first finding of oriented triphylite (LiFe2+PO4) rods in fluorapatite. This observation was made in the contact zone between a metamorphosed rare-element pegmatite and its amphibolite wall rock at the Stankuvatske Li-ore deposit in the Ukrainian Shield. This contact zone consists of an exocontact, in which hornblende was altered to biotite, and an endocontact, which comprises four parallel mineral zones (aplitic, apatite, triphylite, and transitional). The needle-shaped triphylite inclusions were observed in greenish-blue apatite within the apatite zone. They are oriented parallel to structural nanochannels along the c-axis in apatite, and were formed due to infiltration of Li-rich, pegmatite-derived fluids into the apatite zone. Small amounts of pyrite, U-Th-rich and Fe-rich phases, as well as small mono-phase fluid inclusions of CO2, CO, and N2 are associated with the oriented triphylite inclusions and record the character of fluid.</jats:p> <jats:p>The exo- and endocontacts were formed as a result of interaction between metasomatic fluids derived from the pegmatite (enriched in K, Na, Li, Rb, F, P, and Mn) with the host amphibolite. At the contact, the amphibolite was altered into the biotite zone during the first metasomatic stage; alteration of hornblende and plagioclase released Ca, Fe, and Mg towards the pegmatite, where these elements reacted with P, Li and Mn to produce the apatite and triphylite zones during the second metasomatic stage. Acting like a geochemical barrier, the apatite zone in the endocontact, inhibited the further escape of Li from the pegmatite, which now is a Li-ore deposit. The metasomatic processes observed in the Stankuvatske Li-ore deposit represent an example of apatite and triphylite formation at the contact between a pegmatite and a metabasite, which has metallogenetic implications.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The coloration mechanism of Oregon sunstone is a classic and controversial topic in mineralogy because of the unique co-existence of anisotropic (green-red) and isotropic (red) color zones within single feldspar crystals. After nearly 50 years of research, no models proposed to date have satisfactorily accounted for all observed optical phenomena. Here we present high-resolution transmission electron microscopy analyses of samples prepared by focused ion beam extraction along specific crystal directions. In both the anisotropic and the isotropic color zones, we observed Cu nanoparticles (NPs) included within plagioclase but with different geometries. In the isotropic (red) zone, NPs were randomly distributed nano-spheres or nano-ellipsoids (8.7-12 nm in diameter) with an aspect ratio of 1-1.3. In contrast, in dichroic (green/red) zones, NPs were directionally-aligned nano-rods (8.5-21 nm along the long axis) with an aspect ratio of ~2.5. We applied localized surface plasmon resonance (LSPR) theory to simulate absorption spectra, and we developed a model to explain the observed optical properties. LA-ICP-MS and polarized UV-Vis spectroscopy were also performed to confirm our conclusions. This study systematically reveals the existence and optical influence of variably shaped metal-NP inclusions in feldspar crystals. Furthermore, it demonstrates the necessity of including LSPR in the canon of mineral coloration mechanisms. Cu-NP-bearing labradorite has been shown to exhibit third-order non-linear optical properties, and approaches that incorporate NP shapes as well as sizes will assist in the design of NP-embedded optical materials with tailored optical behaviors.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Solfataric alteration at the South Sulfur Bank of the former Kilauea caldera produced opal, Mg- and Fe-rich smectites, gypsum, and jarosite through silica replacement of pyroclastic Keanakako’i ash and leaching of basaltic lavas. This site on the island of Hawaii serves as an analog for formation of several minerals found in altered deposits on Mars. Two distinct alteration environments were characterized in this study including a light-toned, high-silica, friable outcrop adjacent to the vents, and a bedded outcrop containing alternating orange/tan layers composed of smectite, gypsum, jarosite, hydrated silica, and poorly crystalline ferric oxide phases. This banded unit likely represents deposition of pyroclastic material with variations in chemistry over time that was subsequently altered via moderate hydrothermal and pedogenic processes and leaching of basaltic caprock to enhance the Si, Al, Mg, Fe, and Ca in the altered layers. In the light-toned, friable materials closest to the vents along the base of the outcrop glassy fragments were extensively altered to opal-A plus anatase.</jats:p> <jats:p>Lab measurements of samples returned from the field were conducted to replicate recent instruments at Mars and provide further characterization of the samples. These include elemental analyses, sample texture, XRD, SEM, VNIR/mid-IR reflectance spectroscopy, TIR emittance spectroscopy, and Mössbauer spectroscopy. Variations in the chemistry and mineralogy of these samples are consistent with alteration through hydrothermal processes as well as brines that may have formed through rain interacting with sulfuric fumes. Silica is present in all altered samples and the friable pyroclastic ash material with the strongest alteration contains up to 80 wt.% SiO2.</jats:p> <jats:p>Sulfate mineralization occurred at the South Sulfur Bank through fumarolic action from vents and likely included solfataric alteration from sulfuric gases and steam, as well as oxidation of sulfides in the basaltic caprock. Gypsum and jarosite are typically present in different layers of the altered wall, likely because they require different cations and pH regimes. The presence of both jarosite and gypsum in some samples implies high sulfate concentrations and the availability of both Ca2+ and Fe3+ cations in a brine percolating through the altered ash. Pedogenic conditions are more consistent with the observed Mg-smectites and gypsum in the tan layers, while jarosite and nontronite likely formed under more acidic conditions in the darker orange layers. Assemblages of smectite, Ca-sulfates, and jarosite similar to the banded orange/tan unit in our study are observed on Mars at Gale crater, Noctis Labyrinthus, and Mawrth Vallis, while high-silica outcrops have been identified in parts of Gusev crater, Gale crater, and Nili Patera on Mars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Zhonghongite (IMA2023-046), ideally Cu29(As, Sb)12S33, is a new mineral discovered in the high-sulfidation vein of the Jiama deposit (E 91°45′, N 29°42′), southern Tibet, China. It forms complex intergrowths with watanabeite and tennantite-tetrahedrite, creating veined or massive aggregates ranging from millimeters to centimeters in size. The single crystals of zhonghongite are anhedral grains, with sizes ranging from several micrometers to approximately 100 micrometers. The mineral is gray in color with a black streak and metallic luster. It is brittle, with uneven fractures, and has a calculated density of 4.925g/cm3. The average values of electron microprobe analyses (wt.%) are as follow: Cu 42.19, As 11.11, Sb 16.09, S 25.45, Hg 3.73, Mn 0.67, and Te 0.28. The empirical formula, based on 33 sulfur apfu, is (Cu27.60Hg0.77Mn0.51Fe0.07Ag0.02)Σ28.97(As6.16Sb5.49Te0.09)Σ11.74S33. In zhonghongite, the substitution of Sb for As is limited, with the atomic ratio of As/(As+Sb) ranging from 0.457 to 0.629. Hg, Mn, and Fe, together with minor Cu, are divalent and serve for charge balance. Zhonghongite is orthorhombic, space group F2mm (42), a = 10.37741(5) Å, b = 14.69821(9) Å, c = 36.7645(2) Å and V = 5607.66(5) Å3. The crystal structure has been solved and refined by single-crystal X-ray diffraction with a final R1 = 0.0235 for 27028 (2467 unique) reflections. It is composed of individual AsS3 tripyramids and clustered tripyramids As4S7, CuS4 tetrahedra and CuS3 planar triangles, connected through corner S atoms in tetrahedral coordination and octahedral coordination with Cu and/or As. The structure is a derivative of tetrahedrite-type structure. Zhonghongite was formed under high-temperature conditions and is classified as an intermediate-sulfidation state mineral.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Aggregates of ilmenite with varying amounts of rutile, ferropseudobrookite, and pseudorutile in suevites from the Ries impact structure have been analyzed by light microscopy, analytical scanning electron microscopy, electron microprobe analysis, and Raman spectroscopy to constrain their formation conditions. The tens to hundreds of micrometer aggregates comprise isometric ilmenite grains up to 15 µm in diameter that form a foam structure (i.e., smoothly curved grain boundaries and 120° angles at triple junctions). Grains with foam structure show no internal misorientations, indicating a post-impact formation. In contrast, ilmenite grains with internal misorientation occurring in the core of the aggregates are interpreted as shocked remnant ilmenite originating from the target gneisses. They can contain twin lamellae that share a common {1120} plane with the host, and the c-axis is oriented at an angle of 109° to that of the host. Similarly, the new grains with foam structure display up to three orientation domains, sharing one common {1120} plane for each pair of domains and c-axes at angles of 109° and 99°, respectively. This systematic orientation relationship likely reflects a cubic supersymmetry resulting from the transformation of the initial ilmenite upon shock (&amp;gt;16 GPa) to a transient perovskite-type high-pressure phase (liuite), subsequent retrograde transformation to the polymorph wangdaodeite, and then back-transformation to ilmenite. Whereas, the new grains with foam structure formed from complete transformation, the twin domains in the shocked ilmenite are interpreted to represent only partial transformation. Ferropseudobrookite occurs mostly near the rim of the aggregates. An intergrowth of ferropseudobrookite, ilmenite, and rutile, as well as magnetite or rarely armalcolite occurs at contact with the (devitrified) matrix. The presence of ferropseudobrookite indicates high temperature (&amp;gt;1140 °C) and reducing conditions. The surrounding matrix provided Mg2+ to form the ferropseudobrookite-armalcolite solid solution. Rutile can occur within the aggregates and/or along the ilmenite boundaries; it is interpreted to have formed together with iron during the decomposition of ilmenite at lower temperatures (850–1050 °C). We suggest magnetite in the rims formed by electrochemical gradients driven by the presence of a reducing agent, where Fe2+ within ilmenite diffused toward the rim. Subsequent cooling under oxidizing conditions led to the formation of magnetite from the iron-enriched rim as well as pseudorutile around ilmenite grains.</jats:p> <jats:p>Our study demonstrates that the specific crystallographic relationships of ilmenite grains with foam structure indicate a back-transformation from high (shock) pressures &amp;gt;16 GPa; moreover, the presence of associated Fe-Ti-oxides helps indicate local temperature and oxygen fugacity conditions.</jats:p>