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 here (http://www.minsocam.org/MSA/Ammin/AM_NotableArticles.html). 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 The composition of mackinawite https://doi.org/10.2138/am-2023-8943 A critical statistical meta-analysis by Rickard shows that the composition of the tetragonal iron sulfide, mackinawite, is stoichiometric Fe1.0S. In the 60 years since its discovery, various formulations of mackinawite composition, such as Fe1+xS, have been listed in mineralogical databases and used by researchers. These formulations are wrong. Most mackinawites are simple FeS, but distinct cobaltian, nickelian, and (possibly) cupriferous varieties exist where minor elements substitute for Fe in the mackinawite structure. The result is fundamental to understanding the role of mackinawite in geologic, environmental, biologic, and industrial processes. Halogen fractionation during vapor-brine phase separation revealed by in situ Cl, Br, and I analysis of scapolite from the Yixingzhai gold deposit, North China Craton https://doi.org/10.2138/am-2022-8826 Gao et al. examine scapolite from the Yixingzhai deposit, which is formed from oxidized and high salinity magmatic-hydrothermal fluids under high temperature and low-pressure conditions. The extremely high Br/Cl and I/Cl values of the scapolite mainly result from halogen fractionation during vapor-brine phase separation where Br and I preferentially partition into the brine phase compared to Cl the separation. The effects of oxygen fugacity and sulfur on the pressure of vapor-saturation of magma https://doi.org/10.2138/am-2022-8739 The volatile content of the melt can be used as a barometer if the melt is vapor-saturated (pressure of vapor-saturation: Pvsat). Most Pvsat calculations assume the melt and vapor contain only oxidized C-O-H species. Hughes et al. show that the exclusion of reduced C-O-H species results in large Pvsat underestimations at reduced conditions while excluding S causes underestimates of Pvsat at intermediate redox conditions. Hence, magma depths are deeper than previously calculated at these conditions. A revisit to phase transition behavior of K-feldspar at high-pressure and high-temperature: Implications on metastable K-feldspar in cold subduction https://doi.org/10.2138/am-2022-8877 To better understand the phase relations of K-feldspar in cold subduction, He and Li carried out a series Raman measurements and quenching experiments to systematically study the effect of pressure and temperature on the phase behavior of K-feldspar and produced a more comprehensive P-T phase diagram of K-feldspar than previously known. K-feldspar undergoes a pressure-induced transition from C-1 to P-1 (metastable K-feldspar) symmetry at ~10.2 GPa. Metastable K-feldspar is an intermediate phase from K-feldspar to K-holl-I in extremely cold subduction slabs (<2 °C/km) or the center of some old and cold subducting slabs. Metastable K-feldspar is stable even at 11.4 GPa and 400°C in enriched hydroxyl group (OH) environments, and shows greater high P-T stability than K-feldspar. This study could help us to explore the possible formation paths and conditions of K-holl-I, the existing depth of K-feldspar stability, and have furthered the understanding of continental subduction. Equation of state and structural evolution of manganese dolomite (kutnohorite) under high pressures https://doi.org/10.2138/am-2023-8949 Xu et al. studied manganese dolomite at various high pressures. They report: (1) X-ray diffraction patterns of single-crystal manganese dolomite at pressures up to 51.2 GPa; (2) The equation of state of manganese dolomite from 1 bar to ~13.3 GPa; and (3) The split and disappearance of diffraction spots up to 51 GPa indicating the highly distorted rhombohedral structure of manganese dolomite. A possible origin of the lunar spinel-bearing lithologies as told by the meteorite NWA 13191 https://doi.org/10.2138/am-2022-8728 Pink spinel anorthosite (PSA) and pink spinel troctolite (PST) are two lunar lithologies known to contain Mg-rich spinel. They are widely distributed on the Moon's near and far sides. The study of their petrogenesis is important to understand the evolution of the Lunar crust. Xie et al. report their study of NWA 13191, a recently approved lunar meteorite that contains a large amount of spinel-bearing clasts, and discuss its origin. The characteristics of its petrology and mineralogy indicate that the Mg-Al-rich melt may be produced by the impact melting of Mg-rich anorthosite precursors. The spinel is a metastable crystallization product along with plagioclase and vitric melt near the Moon's surface. This realization provides observational evidence for previous simulated crystallization experiments and theoretical speculations. Vergasovaite to cupromolybdite topotactic transformation with crystal shape preservation https://doi.org/10.2138/am-2022-8753 Whereas SC-SC transformations have been studied in detail to date, well-documented topotactic transitions in pure inorganic compounds and minerals with crystal shape preservation are as yet very rare. The thermal behavior of vergasovaite, Cu3O(SO4)(MoO4), and its synthetic analog was studied by Nazarchuk et al. by high-temperature single-crystal X-ray diffraction. The mineral is stable until 950; at 975 K, the topotactic transformation of vergasovaite into cupromolybdite, Cu3O(MoO4)2 occurs. Thermal conductivity of aluminous garnets in Earth's deep interior https://doi.org/10.2138/am-2023-8953 Hung et al. precisely determined the thermal conductivity of aluminous garnets under high pressure-temperature conditions, which significantly advances our understanding of the thermal properties of garnets and their impacts on the complex thermochemical evolution in Earth's deep interior. Their results also offer an important basis for future novel design and fabrication of laser optics and electronic devices based on a variety of synthetic non-silicate garnets. Interaction of seawater with (ultra)mafic alkaline rocks -- Alternative process for the formation of aegirine https://doi.org/10.2138/am-2023-8928 Rapprich et al. present a new model for the origin of aegirine in (ultra)mafic and relatively Na-poor alkaline rocks from Central Europe, which did not experience significant magmatic differentiation. Their data suggest that the late- to post-emplacement alteration of a mafic alkaline rock due to its contact with seawater leads to the degradation of diopside, resulting in the growth of secondary aegirine. The observed transformation of diopside into aegirine does not lead to a significant enrichment of the rock in Na (sourced from the seawater), as the process is blocked by a lack of available ferric iron. This effect may explain the limited Na-enrichment of submarine alkaline rocks, which is negligible compared to large-scale sodic-metasomatism of MORB tholeiites acquired during spilitization. Experimental study on the kinetics of magnesiohornblende dehydration and its implications https://doi.org/10.2138/am-2022-8692 This experimental study by Han et al. provides a further understanding of seismic and high-electrical-conductivity anomalies in the subduction zone. This research will be of interest to colleagues who study the physical and chemical properties of hydrous minerals in subduction zones. Predicting olivine formation environments using machine learning and implications for magmatic sulfide prospecting https://doi.org/10.2138/am-2022-8893 The natural formation from various mantle melts at diverse environments makes olivine one of the most informative messengers from Earth's mantle and a recorder of the fate of magma related to important mineral resources. Here, Xue et al. train a machine-learning-based model using a global olivine dataset to discriminate between olivine-forming environments and evaluate the ore deposit potential of magmatic sulfide mineralization. Thisr study documents that this model is excellent in discriminating between olivine origins and provides a reliable prospecting indicator for global Ni-Cu-PGE exploration. Reaction between volatile-bearing eclogite and harzburgite as a function of degree of interaction: experimental constraints at 4 GPa https://doi.org/10.2138/am-2022-8799 The mantle is known to be heterogeneous, mainly composed of peridotite and eclogite. Eclogite-derived hydrous melts may interact with harzburgite in subduction zones and in the sub-continental lithospheric mantle. Elazar and Kessel studied the interaction of volatile-bearing eclogite-derived melt and harzburgite as a function of the degree of interaction. They experimentally simulated such interaction by using a capsule containing an eclogite layer juxtaposed next to a harzburgite layer in the presence of H2O and CO2 at 4 GPa and 1200 °C. A diamond trap was placed between the two layers to trap the fluid or melt, allowing direct determination of its composition. The multi-anvil apparatus was rotated at different frequencies to examine the effect of the increasing degree of interaction on the melt composition as well as the mineral compositions. The interaction of eclogite-derived hydrous melt with harzburgite results in a Opx + garnet reaction layer at the interface between the two lithologies. The interaction of the melt with the harzburgite refertilizes it, forming a lherzolite rock containing olivine, Opx, Cpx, and garnet. Melt is formed during this interaction. Both the eclogitic and the peridotitic garnet compositions approach each other and become intermediate between the composition of the garnet in the eclogite+H2O+CO2 system and the garnet in the harzburgite+H2O+CO2 system at these conditions. The Mg# of the peridotitic olivine and Opx is reduced. The alkalinity of the melt increases with increasing degrees of interaction. With increasing interaction, the mode of the melt fraction, peridotitic Opx, Cpx and garnet increase on the expense of peridotitic olivine and the eclogitic garnet. These experiments demonstrate the influence of the degree of interaction on the range of melt compositions found in volcanic arcs as well as the degree of metasomatism in the mantle found in the sub-continental lithospheric mantle. Thermal behavior of borax Na2B4O5(OH)4·8H2O https://doi.org/10.2138/am-2023-8970 Borate minerals display extensive structural complexity resulting from the great diversity of their fundamental building blocks (FBBs), which are similar to that of silicates. The FBB in borax is an isolated B4O5(OH)4 cluster consisting of two BO2(OH) triangles and two BO3(OH) tetrahedra linked by common oxygen atoms to form a double ring with two common tetrahedra. Nishiyasu and Kyono reveal that a large electron cloud shared on the two borate tetrahedra leads to an extremely rigid B4O5(OH)4 cluster. Thus, once the B4O5(OH)4 cluster is formed, it is very stable as long as it is not exposed to strong acidic environments. In conclusion, the existence of borate minerals containing the B4O5(OH)4 cluster should provide evidence of the existence of moderately acidic or alkaline water where the borate minerals were grown via the incorporation of B4O5(OH)4 clusters. These types of clusters, consisting of BO3 triangles and BO4 tetrahedra, in borate minerals can potentially be used as a paleoenvironmental indicator. Multiple magmatic processes revealed by distinct clinopyroxene populations in the magma plumbing system: A case study from a Miocene volcano in West Qinling, Central China https://doi.org/10.2138/am-2022-8744 Clinopyroxene (Cpx) with various origins (phenocryst, antecryst, xenocryst, etc.) and textures (e.g., zoning texture and spongy texture) in a basaltic plumbing system could provide information about the deep storage, mixing, migration, and physicochemical conditions of magmas. Zhang et al. carried out detailed petrographic, mineralogical, and whole-rock geochemical analyses to unravel the origins of various Cpx crystals and the reaction between Cpx xenocrysts/antecrysts and melts/fluids in the lithosphere. The detailed investigation of compositional and textural features of Cpx antecrysts/xenocrysts in this study suggests that the interactions between various interconnected magma reservoirs are widespread in the magmatic plumbing system. Their study emphasizes the importance of Cpx-melt/fluid reactions that can significantly modify the whole-rock compositions and lead to the formation of spongy textures without the need for fractures or cracks in minerals. Genetic implications, composition, and structure of trioctahedral micas in xenoliths related to Plinian eruptions from the Somma-Vesuvius volcano (Italy) https://doi.org/10.2138/am-2022-8782 Balassone et al. present a detailed, systematic mineralogical and petrographic study of mica-bearing xenoliths (mainly skarns and composite skarns-marbles, with minor cumulates and a syenite) from the Somma-Vesuvius volcano (Roman Magmatic Province, southern Italy), that are related to the Pompeii (AD 79) and Avellino (3945 ± 10 cal yr BP) eruptions. The crystal-chemical and structural features of the Pompeii and Avellino micas are characterized, extending the known range of the other Vesuvian micas from literature. Genetic inferences are also given and provide valuable information for reconstructing the environmental conditions in similar geological systems. For example, the Ti-depletion and wide degree of dehydrogenation of phlogopites from skarns and composite skarns-marbles suggest that the studied samples originated under variable pressure conditions. Magmatic and hydrothermal controls on diverse Nb mineralization associated with carbonatite-alkaline complexes in the southern Qinling orogenic belt, Central China https://doi.org/10.2138/am-2022-8651 Few carbonatites and/or alkaline rocks contain significant amounts of Nb minerals for economic exploitation; pyrochlore-group minerals generally are the dominant primary Nb-rich minerals in these rocks. In some cases, the occurrence of diverse Nb mineralization, such as columbite and Nb-rutile, may impede beneficiation and metallurgy. Ma et al. provide strong evidence that late-stage external or authigenic hydrothermal fluids could not remobilize Nb on a large scale as is observed for REE. This could result in the decoupling of REE and Nb during magmatic evolution or hydrothermal overprinting, suggesting that hydrothermal veins are potential targets for REE exploration whereas the complex itself could be the target for Nb exploration. In particular, this indicates that more attention should be paid to the country rocks, especially those rich in U or reduced, that may contribute to U enrichment and columbitization during magma ascent and hydrothermal overprinting, respectively, which may result in the diversity of Nb mineralization. Potassium isotope fractionation during silicate-carbonatite melt immiscibility and phlogopite fractional crystallization https://doi.org/10.2138/am-2022-8898 Potassium (K) isotopes have been precisely measured since 2016 and explored in geochemistry and cosmochemistry. The available K isotope data reveal large δ41K variations in igneous rocks, which were mostly attributed to control of source composition. Magma differentiation is considered to have an insignificant effect on K isotope fractionation, mainly because of lack of studies on more-evolved magmatic rocks. Su et al. measured K isotopic compositions of a magmatic suit of alkaline silicate-carbonatite affinity, which were formed from melt immiscibility and subsequent phlogopite fractionation. Significant K isotope fractionation (1‰) induced by melt immiscibility and phlogopite fractionation span the δ41K range of most igneous rocks. This reveals an alternate mechanism of K isotope fractionation, which is important to constrain K recycling and K isotope variation in geological processes. This paper highlights (1) K isotope fractionation during silicate-carbonatite melt immiscibility; (2) K isotope fractionation induced by phlogopite fractional crystallization; and (3) Distinct mechanisms of K recycling and K isotope variation in various tectonic settings. Yuchuanite-(Y), Y2(CO3)·H2O, a new hydrous yttrium carbonate mineral from the Yushui Cu deposit, South China https://doi.org/10.2138/am-2023-8971 Yao et al. report a new hydrous yttrium carbonate mineral, yuchuanite-(Y), Y2(CO3)·H2O. Yuchuanite-(Y) has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA2022-120). Moreover, this new mineral has important implications for heavy rare earth mineralization in Yushui and other sediment-hosted Cu deposits. Nature and timing of Sn mineralization in southern Hunan, South China: Constraints from LA-ICP-MS cassiterite U-Pb geochronology and trace element composition https://doi.org/10.2138/am-2022-8823 Ren et al. present three points of interest related to Sn mineralization the rocks they studied: (1) The chronology of cassiterite defines three stages of magmatic mineralization in tin deposits in southern Hunan; (2) Cassiterite compositions are a physicochemical pathfinder of magmatic-hydrothermal processes; and (3) Multiple types of cassiterite need to be studied to establish an idealized tin mineralization model. A simple method for obtaining heat capacity coefficients of minerals https://doi.org/10.2138/am-2023-9109 Bowman et al. developed a simple method for obtaining the heat capacity coefficients of minerals. Determination of heat capacity coefficients through this direct calculation yields comparable results to those obtained from conventional methods. Accurate heat capacity measurements can be utilized to calculate geothermal gradients in active basins, geothermometry, and study geodynamic transformations in tectonically active regions. Molybdenum isotopic fractionation in the Panzhihua mafic layered intrusion in the Emeishan large igneous province, southwest China https://doi.org/10.2138/am-2023-9154 The large Mo isotopic fractionations between different geological reservoirs make this isotopic system a potential useful tool for constraining the origins of magmatism. However, the effect of magmatic differentiation on Mo isotopes is still controversial. Zhang et al. focused on the effect of the fractional crystallization of Fe-Ti oxides on Mo isotope fractionation. They obtained Mo isotope data for the Panzhihua gabbroic intrusion and mineral separates, which exhibit large Mo isotopic fractionations with δ98/95Mo values as follows: magnetite < clinopyroxene < ilmenite < plagioclase. Iron-Ti oxides have Mo contents that are one order of magnitude higher than those of clinopyroxene and plagioclase. Rayleigh fractionation modeling shows that the removal of magnetite and ilmenite results in significant Mo isotopic fractionation in the residual magma. Due to the low Mo contents of clinopyroxene and plagioclase, Mo isotopes are not significantly fractionated by the removal of these minerals. Their study highlights that fractionation of Fe-Ti oxides can cause considerable Mo isotopic fractionation; consequently, caution is needed when using Mo isotopes to infer magma origins. ___ Rachel Russell Managing Editor, American Mineralogist rrussell@minsocam.org Am Min Website, including Information for Authors and Tab to Permissions: http://www.minsocam.org/msa/AmMin/AmMineral.html Join MSA: https://msa.minsocam.org/membership.html (hint: members get free online color in Am Min) New Main MSA Website view: https://msaweb.org/ Because of changes in Outlook365, we are having trouble emailing "gmail" client users. Thank you in advance for patience. Holiday/Vacation/Out of Office/etc.: March 8, 29