American Mineralogist July 2025 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 is a feature open to anyone who registers on the site. Thank you for reading American Mineralogist. Sincerely, Hongwu Xu Paul Tomascak Editors, American Mineralogist American Mineralogist July 2025 issue paper highlights Kyanite-muscovite-dumortierite vein mineralization mechanisms from advanced microstructural analysis using EBSD Sneha Dandekar, Elisabetta Mariani, Tushar R. Dandekar, Rajesh Khatirkar, Kavita Pande, Joseph Gardner, Heath Bagshaw, Kirtikumar Randive, Dilip Peshwe This is the first comprehensive microstructural EBSD study of dumortierite, an aluminous silicate containing boron, a critical element for the energy transition. Dumortierite crystallizes from reactive fluids transported by transient porosity waves generated along muscovite basal surfaces by new defects known as ripplocations. This mineralization mechanism has important implications for understanding the occurrence and distribution of dumortierite in muscovite-rich rocks. https://doi.org/10.2138/am-2024-9380 A first-principles investigation on the enthalpy landscape for the hibonite solid solution: Implications for a nebular barometer Pierre-marie Zanetta, Abhishek Thakur, Venkat Manga, Krishna Muralidharan, Thomas Zega The abundance of titanium (Ti) in hibonite, a mineral forming very early in solar nebula condensation, can be important to understanding the chemical environment of this system. In part, the Ti content reflects fO2 of the equilibrating environment, considering the variable oxidation states of the element (Ti3+ and Ti4+). However, bulk composition of the system also mitigates the valence state of Ti in hibonite. Zanetta and coauthors applied density functional theory to known hibonite compositions to determine that coupled substitution (Ti4++Mg2+ for 2Al3+) was the most energetically favorable mechanism for Ti incorporation. The research is an important step toward comprehensive thermodynamic modeling of early solar system materials. https://doi.org/10.2138/am-2024-9449 Deciphering the origin of low-grade W mineralization and hydrothermal fluids in the oxidized Fujiashan W skarn deposit using garnet geochemistry Guiqing Xie, Yunhao Ji, Simon Jowitt, Denis Fougerouse, Wei Li, Bin Fu Garnet plays a crucial role in skarn deposits, with its chemical and isotopic composition serving as valuable indicators for understanding the origin and evolution of mineralizing fluids. Fluorine potentially facilitated the transportation of Al to the distal skarn in this deposit, leading to an elevation in the grossular component from the proximal to distal skarn zones. These variations suggest that the zoning patterns within W skarn systems may differ from those observed in other types of skarn systems, such as Cu skarns. This study highlights high concentrations of W within andradite and lacks evidence of re-equilibration during retrograde alteration. This suggests W within W-rich garnet is unlikely to be remobilized by fluids and reconcentrated as scheelite. This may explain the low W grade in oxidized skarns. https://doi.org/10.2138/am-2024-9452 Oboniobite (Mg4Nb2O9), a new magnesic-niobic oxide mineral from the Bayan Obo deposit (China) Hai-Dong She, Hong-Rui Fan, Xiangping Gu, Xiao-Chun Li, Kui-Feng Yang, Qi-Wei Wang, Hai-Long Jin, Biao Chen, Yong-Xin Pan A new magnesic-niobic mineral has been discovered in the Bayan Obo deposit, China. The ideal formula of the mineral is Mg4Nb2O9, and it has been named oboniobite. Both the mineral and its name have been approved by the IMA-CNMNC (2023-118a). The mineral occurs in a Meso-Proterozoic dolomite-calcite carbonatite dike and is associated with fersmite, columbite, pyrochlore, apatite, magnetite, dolomite, and calcite. Oboniobite shares a similar elemental composition with columbite-(Mg) and ternovite, yet it is notably different in its crystallographic, physical, and optical properties compared to the other two minerals. https://doi.org/10.2138/am-2024-9469 Equilibrium oxygen isotope fractionation in Mg(OH)2-Ca(OH)2-H2O: Implication from in situ high-temperature and high-pressure vibrational spectroscopy Xi Zhu, Yu Ye, Yancheng Hu, Sha Wang, Yunfan Miao, Dan Liu, Qingbo Wang, Zhen Wu, Yanming Pan Vibrational spectra, including Raman and FTIR, are useful for modeling equilibrium isotope fractionations, especially for non-metallic isotopes, like C, O, H, Si. Synthetic reactions between Mg and Ca oxides and isotopically labeled water yielded hydroxides at high temperatures and pressure, permitting calculation of equilibrium oxygen isotope •(P,T) fractionation factors. The results were further used to better understand kinetic isotope fractionation in this system at lower temperatures. https://doi.org/10.2138/am-2024-9474 Boron coordination in omphacite and glaucophane derived from inter-mineral B isotope fractionation in natural rocks Jie Xu, Horst Marschall, Axel Gerdes, Anette Meixner, Simone Kasemann, Alan Woodland The determination of boron coordination in the nominally boron-free minerals (e.g., clinopyroxene and amphibole) in natural samples is important for geochemical modeling of subduction. However, a direct determination of boron coordination at the trace-element level is analytically not feasible. Xu et al. provide an indirect way to estimate the proportions of trigonally and tetrahedrally coordinated boron in omphacite and glaucophane using their boron isotopic fractionations against tourmaline and phengite. Based on their B isotope fractionations, the authors concluded that 88 ±9% of the incorporation in clinopyroxene proceeds through the B(F,OH)Si-1O-1 substitution, and 12 ±9% of the B is incorporated by replacement of SiO4 tetrahedra by BO3 triangles. In contrast, B in glaucophane is exclusively incorporated in the tetrahedrally coordinated sites. https://doi.org/10.2138/am-2024-9482 Analcime-wairakite formation during experimental cement-bentonite alteration at 200–300 °C Amber Zandanel, Marlena Rock, Kirsten Sauer, Florie Caporuscio, Carlos Jove-Colon, Edward Matteo Understanding the environmental conditions that control radionuclide-sorbing properties of zeolites is a critical piece of assessing the resiliency of spent nuclear fuel repository design concepts. Zandanel et al. present experimental work characterizing the formation of zeolites during hydrothermal interactions between generic clay and cement barrier materials. Focus is given to the analcime-wairakite zeolite series that has known radionuclide sorption and exchange properties, and characterization of the mineral reaction products. The experimental approach combined engineered repository system materials (cement, clay buffer, steels) with a natural clay rock and synthesized groundwater solution to simulate water-saturated conditions in an argillaceous rock repository. Reactants were heated to 200 or 300 °C at 15 MPa, for 8–24 weeks. After reaction with cements, they observed the formation of analcime-group minerals in all experiments. The results show a fully realized analcime-wairakite solid solution that falls between Si/Al = 2 (ideal) and a trend of analcime minerals that have increasing Si/Al ratios with increasing Na/(Na+Ca). Additionally, these results illustrate the repository material interactions that may promote the formation of zeolites in the analcime-wairakite solid solution during heating events in the subsurface. https://doi.org/10.2138/am-2024-9490 Structural mechanisms of pressure-induced isosymmetric second-order phase transitions Qifa Zhong, Jingui Xu, Dongzhou Zhang, Shanrong Zhang, Mengzeng Wu, Hailing Gu, Yi Zhou, Dawei Fan Pressure-induced polymorphic phase transitions play a pivotal role in fundamental physics and chemistry. Among these transitions, isosymmetric phase transitions are particularly intriguing as they maintain crystal symmetry throughout the process and display characteristics of both first- and second-order transitions. To address why an increase in chemical bonding induced by pressure triggers isosymmetric first-order phase transitions in specific crystalline materials, while simultaneously exhibiting isosymmetric second-order phase transitions in others, Zhong et al. conducted a meticulous investigation focusing on cerite, a rare earth element (REE) mineral with unique structural characteristics that suggest its potential to undergo bonding increases under high pressure. The study examined the structural evolution of cerite under high pressures and revealed an isosymmetric second-order phase transition occurring at 5.5 GPa, attributed to the formation of new REE-O bonds. The results present novel support for the occurrence of bonding increase-triggered isosymmetric second-order phase transitions under pressure and allow a comprehensive classification of the five types of pressure-induced isosymmetric phase transitions. https://doi.org/10.2138/am-2024-9497 Correlations between the structural and impurity features revealing fluctuant growth conditions of natural fibrous diamonds Chengyang Sun, Mingyue He, Taijin Lu, Shinji Muraishi, Yi Deng This study systematically correlated growth structures, plastic deformation, residual stress, and impurity, as well as micro-inclusion distribution features of coated diamonds from the Democratic Republic of Congo. Growth structures observed under CL were related to VN3H defects, while plastic deformation and residual stress were mainly associated with nitrogen impurities. Nitrogen content in the core could be contrastingly related to VN3H defects due to different annealing conditions. In the coat, nitrogen contents and VN3H defects were conventionally positively correlated. However, positive anomalies of nitrogen and VN3H contents appeared not only at dark fibrous layers with high micro-inclusion densities but also at light-yellow layers with low micro-inclusion densities. Considering factors influencing growth rates and the incorporation of nitrogen and hydrogen atoms during diamond growth, it was speculated that high water contents of growth mediums led to the formation of dark layers, and growth pressure decreases should be the major reason causing the formation of micro-inclusion-poor but nitrogen- and hydrogen-rich light-yellow fibrous layers. This study advanced our understanding of the unique fibrous growth of natural diamonds and exemplified the occurrence of multiple fluctuations in growth conditions during the rapid crystallization of diamonds. Therefore, careful attention should be paid to the layer position of analyzed micro-inclusions when investigating the compositions of mantle fluids trapped in fibrous diamonds https://doi.org/10.2138/am-2024-9503 A machine learning-based approach using clinopyroxene data to improve accuracy and efficiency in predicting tectonic settings: Implications for Rodinia supercontinent breakup triggered by mantle plume events Hengxu Li, Zhaochong Zhang, Ruixuan Zhang, Marc K. Reichow, Zhiguo Cheng, Zhenjie Zhang, Madhava Santosh Clinopyroxene is a pervasive rock-forming mineral in basalt, carrying essential information about tectonic settings. In this study, three machine learning algorithms—Artificial Neural Network, Support Vector Machine, and Random Forest—were employed to analyze global big geochemical data of clinopyroxene to discriminate five tectonic environments, including continental within-plate, island arc, oceanic island, oceanic floor, and continental flood province. The findings demonstrate the efficacy of Support Vector Machine, which attains an accuracy of 92.1% (major element-based) and 95.2% (major and trace elements-based) for tectonic discrimination. https://doi.org/10.2138/am-2024-9535 Effect of layer charge density and charge location on the swelling of smectite: Implications for geological storage CO2 and high-level nuclear waste Paolo Andre Benavides, Stephen Guggenheim Smectite is a mineral present in geological reservoirs considered for the storage of CO2 and high-level nuclear waste (HLNW). XRD experiments show that increasing temperature and brine concentration result in the contraction of smectite, whereas increasing pressure is of less importance. Geochemical modeling shows that the extent of dissolution of smectite is affected by its composition. Contraction or dissolution of smectite may affect the ability of geological reservoirs to effectively store CO2 or HLNW. https://doi.org/10.2138/am-2024-9557 Mineralogical characterization of the Seffner fulgurite, Florida: Insights into lightning-induced petrogenesis Luca Bindi, Marta Morana, Matthew Pasek, Tiziano Catelani Fulgurites provide unique evidence of high-temperature processes and rapid quenching in natural settings. The present study can help geologists understand the conditions under which minerals can melt and rapidly solidify, offering insights into the extreme thermodynamic environments that are otherwise challenging to replicate in laboratory settings. https://doi.org/10.2138/am-2024-9558 The valence states of Nb and Ta in magmatic-hydrothermal systems: Insights from combined SXAS and XPS analyses of titanite and ilmenite in two Chinese anorogenic suites Yuxiang Zhu, Yuanming Pan, Lianxun Wang, Zhenbing She, Renfei Feng, Ning Chen, Sarah Purdy, Kenan Cao Significant fractionations of Nb and Ta have recently been documented in diverse magmatic-hydrothermal and metamorphic systems, but their origin remains unclear. Our SXAS and XPS data showed that Nb5+ (and minor Nb4+) dominates in ilmenite with Nb/Ta > 30, whereas titanite and ilmenite with Nb/Ta < 30 contain mixed Nb5+ and Nb4+ states. The presence of high proportions of Nb4+ in magmatic titanite from the Fangcheng and Ziyang alkaline intrusions points to the possible existence of tetravalent niobium in the Earth's mantle. The significant Nb/Ta fractionations (5.3 to 65) in magmatic-hydrothermal titanite and ilmenite are governed by the prevailing oxygen fugacity. Therefore, the applications of Nb/Ta systematics as a geochemical tool for discriminating magma sources and tracing Earth and other planetary processes must be treated with circumspection. https://doi.org/10.2138/am-2024-9610 Hoperanchite, (NH4)2(S2O3), a new mineral from an active vent in a burning bituminous shale Anthony Kampf, Paul Adams, Chi Ma, David Lynch An active vent in a naturally burning bituminous shale near Santa Barbara, California, U.S.A., has produced the new mineral hoperanchite (ammonium thiosulfate). Hoperanchite (IMA2024-017), has the formula (NH4)2(S2O3). It is the first thiosulfate mineral that does not contain essential Pb. https://doi.org/10.2138/am-2024-9625