American Mineralogist September 2026 issue paper highlights Dear American Mineralogist Readers, Below are the Paper Highlights for this month’s issue of the American Mineralogist: International Journal of Earth and Planetary Materials. You may also view the American Mineralogist Paper Highlights list at https://msaweb.org/MSA/AmMin/ and click the “Editor’s Notes” tab, which will be available shortly after the issue is live. The DOI links below will take you to the abstract on GeoScienceWorld. If you have “IP” access via your institution’s library, it should reveal the whole paper. Consult your institution’s IT department or friendly librarian. If you have an MSA membership, authenticate your login from the American Mineralogist website at http://www.msapubs.org/. On the portal page, click the American Mineralogist link and enter your username (e-mail address) and your password (membership number). Then, search for the paper you want to read via your browser's search tools. (On most PCs, it is control-F, but that may vary for you.) Note that on GSW, you can sign up for a table of contents to be sent to you when the issue is live -- this feature is available to anyone who registers on the site. Thank you for reading American Mineralogist. Sincerely, Fabrizio Nestola Paul Tomascak Chief Editors, American Mineralogist American Mineralogist Volume 111; Number 9; 09-01-2026 The first barium-HREE fluorocarbonate new mineral bayanoboite-(Y), Ba2Y(CO3)2F3, from Bayan Obo deposit, Inner Mongolia, China Yuan Xue, Ningyue Sun, Guowu Li, Jinhua Hao, Peng Liu, Wenlei Song, Xianhua Li, Junfeng Shen, Li Yang, Zhaojing Wang, Wenxiang Meng, Yonggang Zhao, Yun Liu The mineral diversity of the Bayan Obo deposit is attributed to its prolonged and complex magmatic-hydrothermal and metamorphic evolutionary history. The discovery of this new HREE-rich mineral, bayanoboite-(Y), provides novel mineralogical insights into the HREE occurrence within the deposit. This finding not only offers valuable mineralogical evidence for understanding the intricate post-ore modification processes but also serves as an indicator to guide future HREE exploration in the region. https://doi.org/10.2138/am-2025-10008 Si-Al disorder in natural and synthetic prehnite (Ca2Al2Si3O10(OH)2) clarified by multi-nuclear NMR and first-principles calculations Xianyu Xue, Masami Kanzaki, Jonathan F. Stebbins Prehnite (Ca2Al2Si3O10(OH)2) is a key mineral in low-grade prehnite-pumpellyite facies metamorphic rocks, and thus understanding its thermodynamic and structural properties is important to constrain the pressure/temperature relationships of metamorphic rocks, such as oceanic subduction complexes. The 29Si NMR results of this study clearly revealed an extra peak for Si in the T2 sites with one less Al next-nearest neighbors (NNN) (i.e., 3Si,1Al) than the main peak for T2 sites (2Si, 2Al NNN), indicative of local Si-Al disorder within the silicate chains, violating the Al-avoidance principle for the synthetic prehnite, but not the natural prehnite samples. This suggests the synthetic samples may not have reached equilibrium, possibly due to slow kinetics at the relatively low experimental temperatures. The data also revealed a pair of extra 29Si NMR peaks of equal intensities, attributable to Si in T1 sites with 2Al and 2Si NNN, respectively, in addition to a main peak attributable to Si in T1 sites with 1Al and 1Si NNN as expected in an ordered structure, for all the samples. For the natural prehnite samples, these extra peaks may be explained by the presence of a different type of Si-Al disorder, i.e., Si-Al exchange in the T2 chains under the constraint of strict alternating Si-Al (chain disorder). https://doi.org/10.2138/am-2025-10082 High-pressure spectroscopic signatures of the distortion and iron spin transition in Mg-rich MgCO3 ferromagnesite Izumi Mashino, Shigeru Yamashita, Kosuke Fujiwara, Takaya Mitsui High-pressure spectroscopic measurements on Fe-bearing magnesite ((Mg,Fe)CO3) were conducted up to ~71 GPa. Above ~29 GPa, pressure-induced lattice distortion is observed. These results indicate that even low Fe contents modify the local CO32– bonding environment and influence the stability of the MgCO3-FeCO3 solid solution. Mössbauer spectra reveal a high-spin to low-spin transition between ~45–50 GPa. Raman and IR measurements show discontinuous shifts in lattice and internal (ν1 and ν4) modes across the spin transition. The relative changes in the Grüneisen parameters (Δγ) of the lattice mode remain linear with Fe content, whereas Δγ of internal modes exhibits a second-order trend, particularly for ν4. Since lattice modes primarily control elastic wave propagation and internal-mode contributions are limited, the Fe-content dependence of sound velocities after the spin transition is expected to be approximately linear. https://doi.org/10.2138/am-2025-10097 Growth twins in chalcopyrite Massimo Nespolo, Cheick Fanta Mady Diakite The difference in the frequency of occurrence of the two known growth twins in chalcopyrite is explained by the structure at the interface between the twinned crystals. The prevalence of {204} twinning over {112} is rationalized by distinct Fe and Cu coordination effects of the structure of the respective composition planes. https://doi.org/10.2138/am-2025-10123 Matrix effects on secondary ion emissions for H and OH from natural spinel-structured oxides: Implications for H2O quantification by secondary ion mass spectrometry Georg F. Zellmer, Maria Rosa Scicchitano, Isao Sakaguchi, Takeshi Kuritani, Frédéric Couffignal Zellmer et al. derive Relative Sensitivity Factors (RSFs) for deuterium in spinel-structured oxides, which allow the determination of H contents in natural spinel-structured oxides through standardless analysis of the ¹⁶O¹⁶H/¹⁶O ratio in samples by ion microprobe. The trace concentration of H in a variety of silicate minerals has been used in magmatic hygrometry and to assess the H₂O content of the Earth's mantle, but few such data exist for magnetite, and none for the other common spinel-structured oxides, including mantle spinels and magmatic chromites. This study opens up the hygrometry of these minerals, which are highly resistant to weathering and thus may provide insights on the hydrogen cycle of Earth through deep time. https://doi.org/10.2138/am-2025-10129 Quantitative analysis of critical metal distribution and recoverability from serpentinite skarn tailings from Lord Brassey mine, Tasmania Makoto J. Honda-McNeil, Sasha Wilson, Andrew J. Locock Honda-McNeil et al. examine the distribution and abundance of Ni, Co, and Cr in weathered tailings from the historical Lord Brassey nickel mine in Tasmania, Australia. They tested the utility of three analytical approaches: quantitative X-ray diffraction (QXRD), electron probe microanalysis (EPMA), and whole-rock elemental analysis, for assessing the grade and recoverability of these metals from tailings. Heazlewoodite (Ni3S2) and awaruite (Ni3Fe) were detectable using XRD in a subset of samples, and spinels were detectable in every sample, but it was not possible to determine their stoichiometry using XRD alone. The calculated results revealed that serpentine is another major source of Ni due to the high abundance despite low oxide wt%. Measured whole-rock elemental analysis results show that tailings from Lord Brassey qualify as low-grade nickel and cobalt ore, while the calculated results from integrated QXRD and EPMA data provide accurate and precise geometallurgical context for the distribution of nickel and cobalt amongst sulfides, alloys, oxides, silicates and other minerals. The results demonstrate that integrating QXRD and EPMA datasets yields whole-rock geochemical estimates comparable to standard industry whole-rock analyses while providing much-needed context on the complex and evolving mineralogy of critical metals in mine tailings. https://doi.org/10.2138/am-2024-9496 Evaluation of the Mg-in-magnetite geothermometer and development of new magnetite geothermometers using machine learning Xing-Hao Gao, Hao Hu, Guo-Xiong Chen, David Lentz, Heng Luo, Xiao-Ming Li, Zhe Ren Through an integrated investigation of 40 representative deposits, Gao et al. reveal that while the TMg geothermometer demonstrates robust applicability in reconstructing thermal conditions for magmatic Fe-Ti-V deposits and some high-temperature hydrothermal systems, it exhibits fundamental limitations when applied to hydrothermal skarn deposits. A novel TTi+V geothermometric model establishes a versatile framework capable of yielding reliable thermal constraints across diverse genetic categories of magnetite mineralization and shows particular efficacy in skarn-type environments. https://doi.org/10.2138/am-2025-9778 Apatite fingerprints from the Dengfuxian Ore District and their implications for W–Sn mineralization in South China Xiaojun Hu, Huan Li, Biao Liu, Audrey Bouvier, Fan Kang, Dapeng Zhu Hu et al. show that apatite fingerprints reliably distinguish magmatic from hydrothermal origins, trace magma sources, and identify W-Sn fertile vs. barren granites. Using a combination of trace elements concentrations and Nd isotopes, the results establish apatite as a powerful tool for understanding W-Sn metallogeny and guiding exploration in South China. https://doi.org/10.2138/am-2025-9799 Wettability behavior of montmorillonite and pyrophyllite in CO2 geological sequestration: Insights from molecular dynamics simulation Xingcheng Long, Chunquan Li, Huiquan Tian, Jiaxin Liang, Shanqi Liu, Yongbing Li This study used molecular dynamics simulations to examine clay mineral behavior relevant to CO2 storage site selection. Results reveal that hydrophobic surfaces exhibit enhanced CO2 trapping capacity; however, the concomitant pressure buildup raises leakage risks. Additionally, the presence of cations and isomorphic Al-Si substitutions was found to amplify surface wettability. https://doi.org/10.2138/am-2025-9848 Vibrational spectroscopy of dense oxyhydroxides: Hydrogen bond disorder and symmetrization at high pressure Benjamin Strozewski, Zhenxian Liu, George R. Rossman, Wolfgang Sturhahn, Johannes Buchen, Jennifer M. Jackson Knowledge of the water content of Earth's mantle is often difficult to measure directly due to the chemical processing of rocks as they are exhumed. Previous estimates of the water content of the lower mantle have indicated it is relatively dry. However, recent experiments have indicated that dense oxyhydroxide phases may in fact retain hydrogen at the high pressures and temperatures of the lower mantle, challenging this assumption. In this work, Strozewski et al. use infrared and Raman spectroscopy to discern whether dense oxyhydroxides undergo hydrogen bond disorder and symmetrization at high pressure. Observations of hydrogen-bond symmetrization are critical as it is likely a necessary process for creating a strong enough hydrogen-bonding environment to prevent dissociation in the lower mantle. https://doi.org/10.2138/am-2025-9898 Molecular mechanisms of magnesium sulfate crystallization: Bond-length inversion and the role of hydration in mineral formation Aaron J. Celestian, Wenqian Xu, James D. Kubicki, Scott M. Perl Magnesium sulfate crystallization shows that S-O bonds are shorter in solution than in crystals, revealing how crystal packing influences molecular geometry. Time-resolved spectroscopy and diffraction capture prenucleation cluster formation and reorganization during crystal growth. Results support non-classical nucleation pathways and improve interpretation of Mg sulfate deposition on Earth and Mars, as well as assisting in the development of novel materials synthesis techniques. https://doi.org/10.2138/am-2025-9913 In situ geochemical and S-Pb-Sr isotopic constraints on fluid origin and evolution of the Heilongtan-Xiejiagou intrusion-related gold deposit, Tongbai Orogen, central China Ying Ma, Shao-Yong Jiang, Cheng Yang, Chao Chen The textural, chemical, and isotopic features of pyrite can yield significant insights into the composition of auriferous fluids in lode gold deposits, which are posited to reflect episodic fluctuations in source reservoirs and/or the physicochemical conditions of the fluids. The findings of Ma et al. underscore that variations in internal physicochemical parameters, such as temperature, pressure, and oxygen fugacity, in conjunction with external fluid-rock interactions, may elucidate the observed diversity in mineralization within a lode gold system originating from a single-source fluid. https://doi.org/10.2138/am-2025-9922 Thermochemical measurements on Mg2SiO4 wadsleyite and SiO2 stishovite and assessment of their self-consistent thermodynamic datasets for Gibbs energy expressions and equations of state Hiroshi Kojitani, Yuki Tsunekawa, Toru Inoue, Masamichi Noda, Masaki Akaogi Standard enthalpies of formation for Mg2SiO4 wadsleyite and SiO2 stishovite and high-temperature heat capacity for Mg2SiO4 wadsleyite were redetermined precisely by differential scanning calorimetry. The obtained data were used for re-assessments of their thermodynamic parameters, including self-consistent thermoelastic parameters of thermal expansivity, isothermal bulk modulus at the standard state, and its temperature derivatives. The phase diagram in the MgSiO3 system thermodynamically calculated using the present thermodynamic datasets suggests that the stability field of MgSiO3 akimotoite extends to lower pressure regions by 2–4 GPa than those proposed in previous studies. https://doi.org/10.2138/am-2025-9924 Juxingite, Bi6Cu140Fe30S125, a new copper sulfide mineral from the Jiama porphyry-skarn Cu-polymetallic deposit, Tibet, China Feng-Hua Gu, Yong-Mei Zhang, Xiang-Ping Gu, Guang Fan, Si-Hong Jiang, Yu Zhao, Zhong-Liang Cui, Xiang Fang, Aorigele Zhou, Zheng-Kun Yang Juxingite was discovered in the Jiama world-class porphyry-skarn deposit in Tibet. It mainly formed at the contact zone between the Lower Cretaceous Linbuzong Formation and the Miocene porphyry. Juxingite is a complex metal sulfide, cubic, space group F43m, with a Mohs hardness of approximately 3. It is opaque in transmitted light with a metallic luster. Under reflected light, juxingite is pale yellow-white in color with no anisotropy. It formed under conditions of medium-low temperature and intermediate sulfur fugacity with abundant trace elements and a close relation to gold mineralization in Jiama deposit. https://doi.org/10.2138/am-2025-9944 New insights into antimony isotopic fractionation mechanisms from experimental investigations Da Wang, Ryan Mathur, Hong-Jie Wu, Wayne Powell, Peter Baran, Zi-An Liu, Huchao Ma This research explores how antimony (Sb) isotopes naturally fractionate during geological processes, including those acting near the Earth’s surface. While many theories exist, there's limited experimental data to confirm how this fractionation work. To address this, Wang et al. conducted lab experiments to study how different conditions (e.g., ligand type, pH, and the minerals involved) affect Sb isotope fractionation. They found that chloride-based solutions were most effective at leaching and caused heavier Sb isotopes to concentrate in the liquid during weathering or leaching. Nitric acid and pure water showed lesser capacity to transport Sb or fractionate its isotopes. In evaporation experiments, no clear isotope separation was observed, but factors like temperature, heating time, and the chemical state of Sb influenced the process. Redox reactions were also important, with factors presence of biotic/abiotic mechanisms and the type of chemical bonds affecting fractionation. https://doi.org/10.2138/am-2025-9999