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 far right tab. 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 MSA membership, then authenticate in from the American Mineralogist menu (here<http://www.msapubs.org/> directly). Once at the portal page, click the right-side American Mineralogist link, enter your user name (e-mail address), and your password (membership number). Then search via your browser’s search tools for the paper you want to read. (On Rachel’s computer, it is control-f but we think that is little different for everyone.) Note that on GSW you can sign up for a table of contents to be sent 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 Don Baker Reduced charge transfer in mixed-spin ferropericlase inferred from its high-pressure refractive index https://doi.org/10.2138/am-2023-9100 The refractive index of a material is the macroscopic expression of its electronic structure; thus, it can provide valuable insights into related physical properties, such as electrical conductivity. Here, Schifferle et al. studied the refractive index of ferropericlase, Earth’s second most abundant mineral, across the pressure range of the mantle. Their results constrain the electronic structure of this mineral at high pressure with implications for the electrical conductivity of the lower mantle. Stability of magnesite in the presence of hydrous fluids up to 12 GPa: Insights into subduction zone processes and carbon cycling in the Earth’s mantle https://doi.org/10.2138/am-2023-8982 CO2 is brought into the deep Earth in subduction zones. Transporting carbon from the Earth’s surface into the deep Earth is fundamental, as otherwise carbon would accumulate in the atmosphere, contributing to global warming. The fluxes and storage of carbon in the deep Earth are, however, controversial. By laboratory simulations of deep Earth conditions, Sieber et al. here report that magnesite (MgCO3), one of the main carriers of carbon (C), can be transported deep into the Earth without undergoing extensive melting even in the presence of water. Influence of Fe(II), Fe(III), and Al(III) isomorphic substitutions on acid-base properties of edge surfaces of cis-vacant montmorillonite: Insights from first-principles molecular dynamics simulations and surface complexation modeling https://doi.org/10.2138/am-2023-9057 In this contribution from Gao et al., the structure of smectitic clay mineral layers is shown to influence the acid-base properties of their edge surfaces. Molecular-level information makes it possible to predict these properties and build larger-scale surface reactivity models that take into account the presence of isomorphic substitutions and the position of structural vacancies in the layer. The kinetic effect induced by variable cooling rate on the crystal-chemistry of spinel in basaltic systems revealed by EPMA mapping https://doi.org/10.2138/am-2023-9014 The crystal chemistry of spinel is markedly affected by ΔT/Δt (1 to 180 °C/h), and it can be captured only by EPMA maps. At sluggish rates, the average spinel is relatively rich in Ti and the Fe2+ + Mg sum and close to equilibrium with its basaltic liquid, but at progressively higher ΔT/Δt, these values decrease, and the Fe3+ + Al sum increases. In this paper by Gennaro et al., this behavior indicates that only the amounts of Me4+, Me3+, and Me2+ cations record kinetics of solidification suffered by magmas and lavas on Earth. Machine-learning oxybarometer developed using zircon trace-element chemistry and its applications https://doi.org/10.2138/am-2023-8978 Zou et al. use trace elements in zircons and their independent fO2 constraints to train machine learning models to construct an oxybarometer. The machine-learning oxybarometer has higher accuracy than the traditional method and can be applied in a variety of geologic settings. Web-based software has been developed to make the machine-learning oxybarometer easily accessible to the research community. Experimental determination of Si, Mg, and Ca isotope fractionation during enstatite melt evaporation https://doi.org/10.2138/am-2023-9111 Lu et al. find that during high-T evaporation of enstatite melt, light Si and Mg isotopes favor vapor over melt, but Ca does not show evaporation or isotopic fractionation. Their modeling shows that if the Mg/Ca and Si/Ca ratios and isotopes in the bulk silicate Earth are attributed to the evaporation of enstatite chondrite-like precursors, evaporation temperatures > 5000 K are required. Quartz texture and chemical composition fingerprint ore-forming fluid evolution at Bilihe porphyry Au deposit, NE China https://doi.org/10.2138/am-2022-8840 Porphyry Au-only deposits represent an important but understudied sub-type of porphyry deposits. However, this type of deposit is rarely developed globally and former studies mainly focused on porphyry Cu and Mo endmembers, indicating that the ore-forming and Au enrichment processes of porphyry Au-only deposits are poorly understood. In this contribution, Hong et al. report systematic SEM-CL textures and in-situ trace element concentrations of different quartz generations from the magmatic to hydrothermal stages in the Bilihe porphyry Au deposit. Their results suggest that different quartz generations could reveal different growth rates, temperatures, pH, and fluid compositions. This study advances the application of CL textures combined with quantitative trace element data of successive quartz generations to comprehend specific ore-forming, physiochemical environments, and mineralization processes, which helps to better understand the processes of porphyry Au ore formation. Zhengminghuaite, a new sulfosalt mineral from the Zimudang Carlin-type gold deposit in southwestern Guizhou, China https://doi.org/10.2138/am-2023-9078 Sulfosalts are a large family of minerals that have been reported in a wide variety of hydrothermal gold and other metal deposits, including Carlin-type gold deposits that often contain Hg-Tl-As-Sb sulfides and sulfosalts. Zhengminghuaite, ideally Cu6Fe3As4S12, is a new Cu-Fe arsenosulfosalt found in the Zimudang Carlin-type gold deposit in southwestern Guizhou, China, and reported here by Gu et al. Ore paragenetic sequence and phase relationship indicate that zhengminghuaite and associated Hg-Tl-As sulfosalts formed after the bulk gold mineralization of the main ore stage, during the transition from Fe-Cu sulfides (pyrite ± arsenopyrite ± chalcopyrite) to As-sulfides (realgar ± orpiment) as the fS2 of the ore fluid increased by 1-2 log units and the temperature decreased by up to 100 °C. The occurrence of zhengminghuaite and its paragenetic relationship to other ore and gangue minerals provide insights into the evolutionary history of hydrothermal fluids and the physicochemical conditions of ore formation for Carlin-type gold deposits. Magmatic degassing and fluid metasomatism promote compositional variation from I-type to peralkaline A-type granite in late Cretaceous Fuzhou felsic complex, SE China https://doi.org/10.2138/am-2023-8936 Zhang et al. show how magmatic degassing and fluid metasomatism promote compositional variation from I-type to peralkaline A-type granites. They found that apatite in both I- and A-type granites show large H2O and δD variations as a result of magmatic degassing, while F-rich alkaline fluid metasomatism promotes the compositional change from I-type to peralkaline A-type magmas. The relatively lower water content and stronger H isotope fractionation observed in A-type than I-type granite was likely due to more intense degassing in an open system. The new mineral cuprozheshengite, from Yunnan province, China, with site-selective As-P substitution https://doi.org/10.2138/am-2023-8942 Sun et al. describe the properties of new mineral cuprozheshengited. It is a member of the dongchuanite group and is a rare example of P-As ordering in different structural sites. The structural geometric parameters and the DFT calculation study of cuprozheshengite reveal that the occupancy propensity of As at the X1 site benefits structural stability. The structure and stability studies of cuprozheshengite may have implications for environmental governance. A neutron diffraction study of the hydrous borate inderborite https://doi.org/10.2138/am-2023-9162 Gatta et al. reinvestigated the crystal chemistry of inderborite, a B-rich mineral (B2O3 ~ 41 wt%) with ideal formula CaMg[B3O3(OH)5]2(H2O)4∙2H2O by a multi-methodological approach (single-crystal X-ray and neutron diffraction, EPMA-WDS, LA-ICP-MS). The inderborite from the Inder deposit shows a positive δ11B value, within the range in which the source of boron is ascribable to marine reservoirs. In addition, the fraction of potential isomorphic substituents is substantially insignificant: B is, therefore, the only industrially relevant element occurring in this mineral. X-ray and neutron structure refinements show that ten over eleven independent oxygen sites in the structure of inderborite are involved in H-bonds as donors or acceptors, and this reflects the pervasive effect of the H-bonding network. The role played by the complex H-bond network is expected to be substantial on the stability of the crystalline edifice. The potential utilization of inderborite, as a B-bearing mineral, are also discussed. Bobfinchite, a new Na-bearing member of the schoepite family https://doi.org/10.2138/am-2023-9031 Many competing mechanisms work to control the migration, precipitation, and speciation of uranium minerals formed in U-bearing systems. The description of bobfinchite by Olds et al. provides an essential foundation for defining crucial crystal-chemical and structure-property relationships in the schoepite family, which include some of the most frequently encountered alteration phases in uranium mines and the technogenic environments of the nuclear fuel cycle. Kenorozhdestvenskayaite-(Fe): A new tetrahedrite group mineral containing a natural [Ag6]4+ cluster and its relationship to the synthetic ternary phosphide (Ag6M4P12)M'6 https://doi.org/10.2138/am-2023-9074 This manuscript by Qu et al. reports an extremely rare [Ag6]4+ cluster-containing tetrahedrite group mineral. In addition, the comparison to the isostructural ternary phosphides (Ag6M4P12) M’6 and the origin of the naturally occurring silver clusters are discussed. The work could represent a reference work for studies on the genesis of the natural subvalent hexasilver clusters, which in the future could represent an important source of information for both the mineralogical community and material science. Compressibility and pressure-induced structural evolution of kokchetavite, hexagonal polymorph of KAlSi3O8, by single-crystal X-ray diffraction https://doi.org/10.2138/am-2023-9120 Kokchetavite (KAlSi3O8, IMA-2004-011) was earlier identified as mineral inclusions in a wide range of crustal rocks and massifs, including those with an ultra-high pressure origin. However, their crystal structures were only guessed on the basis of powder X-ray diffraction patterns. The absence of data on the high-pressure stability and equation of state of this mineral was problematic for people involved in the study of metamorphic crustal complexes. In this work, Romanenko et al. present results of room-temperature compression and observations of pressure-induced structure transitions of kokchetavite up to 11.8 GPa by in situ, single-crystal, synchrotron X-ray diffraction. Observational data provide important information for the identification and interpretation of this phase in mineral inclusions. The authors demonstrate the high structural stability of kokchetavite, therefore, it can be taken into account in thermodynamic modeling of the composition of the fluid-saturated crust. Local strain heterogeneity associated with Al/Si ordering in anorthite, CaAl2Si2O8, with implications for thermodynamic mixing behavior and trace element partitioning in plagioclase feldspars https://doi.org/10.2138/am-2023-9197 Atkinson et al. used infrared spectroscopy to demonstrate that the most significant aspect of ordering of Al and Si in plagioclase feldspars relates to strain energies associated with local strain heterogeneity on a length scale of at least 1-5 unit cells. Using samples of synthetic anorthite, they establish the principle that there is a clear correlation between line width parameters extracted from primary IR spectra, macroscopic strain data determined from lattice parameter variations, and enthalpy data from solution calorimetry, with implications for trace element partitioning and thermodynamic mixing properties. The glass transition temperature of anhydrous amorphous calcium carbonate https://doi.org/10.2138/am-2023-9191 Bissbort et al. report the first observation and quantification of a glass transition in anhydrous amorphous calcium carbonate (ACC). ACC has been known to be an amorphous solid, which is produced by a precipitation and freeze-drying synthesis. However, it was not identified as a glass due to the absence of a glass transition in previous calorimetric analyses, which used conventional differential scanning calorimetry. Fast Differential Scanning Calorimetry with very fast heating and cooling rates allowed us to observe a glass transition at 339 °C. Hence, ACC is not only an amorphous solid but also a structural glass, although it was not produced in the classical way of glass formation, e.g., quenching from a melt. Further, this implies that a structural glass can also be formed from a simple single-component carbonate system and not only in more complex systems like ACMC or specific K-Mg-carbonates. Book Review https://doi.org/10.2138/am-2024-B109713