Jianxi Zhu

Highly fractionated granites that serve as protoliths for ion-adsorption HREE deposits commonly exhibit extensive hydrothermal alteration. The structural stability of HREE-rich minerals in protoliths and HREE liberation from their breakdown are crucial to the formation of ion-adsorption HREE deposits. However, the impact of hydrothermal alteration on the structure and composition of HREE-rich minerals and its constraint on supergene HREE mineralization remain poorly understood. In the Dabu muscovite granites of South China, magmatic xenotime-(Y) and samarskite-(Y) have undergone pervasive hydrothermal alteration, as evidenced by significant textural and compositional variations. The alteration of xenotime-(Y) is characterized by randomly distributed pores and tiny thorite and apatite inclusions resulting from fluid-mediated dissolution-reprecipitation processes. Samarskite-(Y) with high U and Th concentrations is more susceptible to fluid-induced alteration due to radiation damage, exhibiting patchy zoning and microfractures as a result of hydrothermal alteration. Raman spectra show a significant degradation of crystallinity in xenotime-(Y) and samarskite-(Y) after hydrothermal alteration. During the alteration processes, HREEs in xenotime-(Y) and samarskite-(Y) were leached out by F-, CO2-, and Ca-rich fluids, with only minor fractions immediately precipitating as weathering-susceptible synchysite-(Y) along the rims of some altered grains. Transmission electron microscopy (TEM) investigations reveal that Th released during xenotime-(Y) dissolution-reprecipitation initially precipitated as metastable nanometric huttonite, which subsequently recrystallized into thorite inclusions. Furthermore, high-resolution TEM shows numerous structural defects in the altered domains of xenotime-(Y) and samarskite-(Y), including lattice dislocations, randomly oriented nanocrystals, and amorphous material. Therefore, hydrothermal fluids not only leach HREEs from robust HREE minerals to form weathering-susceptible synchysite-(Y), but also compromise their structural integrity, reducing weathering durability. Such alteration processes render the host granites favorable protoliths for ion-adsorption HREE deposits. This study indicates that the robust xenotime-(Y) and samarskite-(Y) altered by hydrothermal fluids can serve as potential sources of ion-exchangeable HREEs in the weathering crusts, thereby expanding the source spectrum for ion-adsorption HREE deposits.

Clay minerals, ubiquitous across diverse geological systems, exert fundamental controls on critical geochemical processes, particularly through their adsorptive interactions with economically vital elements such as heavy rare earth elements (HREEs). The adsorption of HREEs onto clay surfaces is not only the key process that enriches these critical resources but also a potential archive for dating significant weathering events. However, the atomic-scale mechanisms governing the preferential adsorption and fractionation of rare earth elements (REEs) at mineral–water interfaces have remained poorly constrained. Here, employing in situ atomic-scale imaging via high-resolution atomic force microscopy, we systematically investigate the adsorption behaviors of light and heavy REEs (La, Y, Yb) onto key crystallographic planes—Si-basal, Al-basal, and edge terminations—of representative clay minerals (kaolinite, illite, mica) under environmentally relevant conditions (pH 4.8 to 6.4). We directly visualize and quantitatively reveal that Si-basal planes selectively enrich HREEs with pH-enhanced adsorption capacity, whereas Al-basal planes exhibit negligible REE adsorption below pH 5.8 but preferential HREE adsorption at pH 6.4. Integrating synchrotron X-ray absorption spectroscopy, we identify the adsorption mechanism of REE on Si-basal and Al-basal/edge planes as outer-sphere complexation and inner-sphere binding, respectively. Furthermore, we quantify the contributions of distinct planes via surface complexation modeling, revealing Si-dominated to Al- and edge-controlled adsorption with elevating pH. These atomic-scale findings not only resolve long-standing debates about clay mineral-mediated REE fractionation but also redefine fundamental paradigms of surface reactivity at silicate mineral–water interfaces, offering insights into the cycling and metallogeny of other critical metals in aqueous and sedimentary environments.

In Gale crater, the Curiosity rover has documented a sequence of ∼3500 million year old sedimentary rocks containing smectite, indicating the presence of liquid water. However, the paleoenvironment in Gale crater, particularly the question of whether a paleolake ever existed, remains hotly debated. Here, we compare the interlayer cations of smectite from terrestrial basalt weathering sequences and marine environments. Marine smectites are considered key analogs for smectites detected in martian paleolakes due to perceived similarities in formation processes and geochemical contexts between terrestrial marine settings and ancient lacustrine environments on Mars. Numerous observations show that K+/Na+-rich smectite is associated with marine environments, whereas Ca2+-rich smectite is typical of terrestrial basalt weathering sequences. We find that the basal spacings of K+- and Na+-smectite are ∼10.0 Å, whereas the basal spacings of Ca2+- and Mg2+-smectite range from 13.1 to 13.8 Å at a relative humidity (<1%) equivalent to that of the Curiosity rover on Mars. The XRD profiles measured by Curiosity in Gale crater show a basal spacing of ∼10.0 Å, which is attributed to K+/Na+-rich smectites. Our results support the idea that the presence of K+/Na+-rich smectites from Yellowknife Bay to Glen Torridon indicates that a lake existed in the 3500 million-year-old Gale crater, confirming that the habitable state lasted longer than previously thought.

The Moon’s highly reduced nature contrasts with recent findings of ferric iron (Fe ³⁺ )‐bearing phases, suggesting an active redox cycle on the Moon. While solar wind proton reduction and impact‐vaporization oxygen loss from minerals are proposed drivers for lunar reduction, direct nanometer‐scale evidence for impact‐driven deoxygenation has been lacking. Here we report the discovery of oxygen‐deficient magnetite within impact melt glass in Chang’e‐6 lunar farside samples from the South Pole‐Aitken basin. This subhedral magnetite grain (Length 2.5 μm; width 1.5 μm.) exhibits a non‐stoichiometric composition and crystal chemical formula [(Fe ²⁺ _1.158(7) Mg _0.070(5) ) _∑1.228 (Fe ³⁺ _1.534(9) Cr ³⁺ _0.137(14) Al _0.101(8) ) _∑1.772 ] _∑3.000 (O _3.886 □ _0.114 ) _∑4.000 , confirming significant oxygen vacancies (□). Magnetite heating experiments (1000°C, in Ar atmosphere) replicated these features, demonstrating thermal deoxygenation generates oxygen vacancies through O ₂ evolution and concurrent Fe ³⁺ reduction. Our findings provide the first mineral structural evidence that post‐shock heating drives deoxygenation in oxygen‐bearing minerals, establishing impact‐induced oxygen removal as a fundamental mechanism making the localized lunar surface chemically reduced.

Manganese (Mn) oxides are crucial for the cycling of trace metals and nutrients. Over 30 types of Mn oxide minerals with distinct crystal structures have been identified across a variety of biogeochemical settings, yet the mechanisms underlying their formation are poorly understood. By incubating three layered Mn oxides of varying particle size and crystallinity under identical solution conditions, we observed the formation of distinct tunneled Mn oxides. The nanoscale mineral property, particularly vacancy density and surface energy, modulates the adsorption geometry of Mn(II) and subsequent electron transfer and structural rearrangement. These findings indicate that mineral structural properties associated with crystallinity are critical and previously underappreciated for the transformation of Mn oxides. Our study provides a mechanistic basis for understanding the natural diversity of Mn oxide minerals, suggesting that they likely derive from the transformation of biogenic layered Mn oxides that experienced dynamic variations in mineral crystallinity and local environmental conditions.

Nontronite, a Fe ³⁺ ‐rich smectite widely identified on Mars, serves as a key mineral indicator for reconstructing paleo‐redox and paleo‐aqueous environments. However, uncertainties in interpreting its spectral data hinder a precise understanding of its formation conditions and paleoenvironmental implications. To fill this gap, the present study investigated the controls of nontronite formation and its crystallographic‐spectral relationships by synthesizing a series of Fe‐Si‐Al samples with varying Fe/Si molar ratios under hydrothermal conditions. Results demonstrated that crystalline nontronite forms exclusively within a Fe/Si molar ratio of 0.21–0.48 under the simulated alkaline conditions. Incorporation of Fe ³⁺ into tetrahedral sites as ^[IV] Fe ³⁺ reduced the tetrahedral‐octahedral sheet mismatch, thereby enhancing the crystallinity of nontronite. This crystallographic evolution was systematically observed in Mid Infrared and Visible‐Shortwave Infrared spectra: ^[IV] Fe ³⁺ content negatively correlated with the Si‐O vibration wavenumber (near 1,000 cm ⁻¹ ) but positively correlated with the 2Fe ³⁺ ‐OH band position (∼1,430 nm) and depth. Furthermore, band depths at ∼1,430 and ∼2,290 nm are robust proxies for the crystallinity of nontronite in the absence of byproducts. These findings constrain the formation of nontronite on Mars to oxidizing, alkaline subsurface hydrothermal environments during the early Noachian, which represents one of the possible pathways for nontronite formation. These results provide a refined framework for interpreting orbital and in situ spectral data, advancing the understanding of clay mineral formation and environmental evolution on Mars.