Amaterasuite [(Sr_xBa_1−x)₄Ti₆Si₄O₂₃(OH)Cl] is a jadeitite mineral with a dual nature encapsulating two sets of atom sites. The sets of atom sites are not equivalent; 85% of the crystal is in one set and 15% is in the other, and the Sr:Ba ratio differs between the two sets. Universal neural network potential calculations were performed to evaluate defect formation energies. Amaterasuite favors phase separation into the end members (x = 0 and 1); however, if this is not allowed in a single particle, some phase boundaries (PBs), which separate regions with different Sr:Ba ratios, form with almost no energy penalty. Thus, no strong driving force exists to segregate Sr and Ba once the local Sr:Ba ratio is determined during formation. Stacking fault (SF) formation energies normal to the c-axis are sufficiently low to form spontaneous SFs, and SFs preferentially form next to each other. The SF direction differs from the low-formation-energy PB directions. This complexity can explain the coexistence of two sets of atom sites and the possibility of regions with different Sr:Ba ratios, although the difference in average Sr:Ba ratios between the two sets of atom sites is not anticipated from the calculations.

Mg-amesite and Al-serpentine with cylindrical microstructures were found in white patches near the edge of rodingite, which were mixtures of white and opaque parts comprising fine grains and light-brown transparent parts comprising coarse grains, in a serpentinite body in the Yakiyama area of Iizuka, Fukuoka Prefecture, Japan. In the transmission electron microscopy images, the white parts comprised a cylindrical structure with a large hollow in the center (cylindrical Mg-amesite), space-filling irregular grains, and fibrous grains. Cylindrical Mg-amesite may have one or more lath grains with widths close to the diameter of the hollow or none at all. The selected area electron diffraction pattern of cylindrical Mg-amesite shows diffraction spots on b* and c* aligned in a straight line. Therefore, we conclude that the cylindrical Mg-amesite has a cylindrical structure elongated along the a-axis, similar to chrysotile or polygonal serpentine. Although the lath grain inside the cylindrical Mg-amesite shows mainly 7 Å reflections, slight reflections are present, indicating the presence of a long-period structure. The chemical composition shows trends corresponding to the microstructure, which can be roughly divided into three phases: most parts of the cylinder and some space-filling irregular grains are of Mg-amesite composition, whereas the inner wall of the cylinder and irregular grains are of Al-serpentine composition; lath grains inside the cylinder and fibrous grains show compositions that can be explained by serpentine-kaolin substitution or a mixed layer of serpentine-amesite and talc.

The Nomo metamorphic rocks in western Kyushu, Japan, are distributed in the southwesternmost part of the Sanbagawa metamorphic belt; yet quantitative pressure-temperature (P-T) constraints have remained limited. Here, we apply Raman spectroscopy of carbonaceous material (RSCM) thermometry and quartz-in-garnet Raman barometry to pelitic schists from the biotite zone of the Sanwa Formation (metamorphic ages of 90-70 Ma) to constrain peak P-T conditions. RSCM thermometry of three samples yields peak metamorphic temperatures of 463-471 °C. Raman spectra of 92 quartz inclusions in garnet from three samples provide residual pressures of 0.46-0.57 GPa, which correspond to quartz entrapment pressures of 1.03-1.28 GPa within the garnet growth temperature range of ∼ 400-470 °C. These results suggest that the Sanwa Formation experienced peak P-T conditions of ∼ 460-470 °C at ∼ 1.0-1.3 GPa. The estimated temperatures overlap those reported for the garnet zone of the Sanbagawa belt in central Shikoku, whereas the inclusion-based pressures are higher than thermobarometric estimates for that zone. This discrepancy suggests that quartz inclusions in garnet may record conditions during prograde subduction along a Sanbagawa-type subduction-zone geotherm prior to exhumation.

Elastic wave velocities of sodium disilicate (Na₂Si₂O₅; NS2) glass were determined up to 7 GPa using an ultrasonic pulse-echo overlap technique combined with synchrotron X-ray imaging in a Kawai-type multi-anvil apparatus. NS2 glass represents an intermediate degree of polymerization (NBO/T = 1), bridging the compositional gap between fully polymerized and depolymerized silicate glasses. The compressional wave velocity (V_P) increases monotonically with pressure, whereas the shear wave velocity (V_S) remains nearly constant up to ∼ 4 GPa before rising at higher pressures. In comparison with fully polymerized (silica, jadeite, albite) and depolymerized (diopside, enstatite) glasses, the pressure dependence of NS2 glass resembles that of depolymerized compositions. These results indicate that the transition between polymerized and depolymerized elastic behavior occurs near NBO/T = 1. Consequently, mafic melts (NBO/T < 1) may exhibit anomalous elastic properties under upper-mantle conditions, whereas ultramafic melts (NBO/T > 1) are expected to stiffen monotonically with increasing pressure.

The distributions of Mn²⁺, Fe²⁺, and Fe³⁺ in babingtonite Ca_2.00Fe_0.64²⁺Fe_0.98³⁺Mn_0.27²⁺Mg_0.09Al_0.02Si₅O₁₄(OH) from the Yakuki mine, Fukushima, Japan are revealed via single-crystal anomalous X-ray scattering (AXS). The ordered distribution of Mn²⁺ at the Fe1 site is readily confirmed via Mn-AXS analysis based on the intensity difference observed in a pair of datasets measured at E_M1 = 6.388 keV and E_M2 = 6.513 keV below the Mn K absorption edge. The distributions of Fe²⁺ and Fe³⁺ are quantitatively determined for the first time using a dataset measured at E_F3 = 7.116 keV, where a significant difference in the anomalous scattering factor is observed between Fe²⁺ and Fe³⁺. This study quantitatively determines the site occupancies of Mn²⁺, Fe²⁺, and Fe³⁺ in babingtonite based on experimental evidence obtained from advanced AXS analysis. The present AXS analysis confirms the structural-chemical formula Ca_2.00(Fe_0.64²⁺Mn_0.27²⁺Mg_0.09²⁺)^Fe1(Fe_0.98³⁺Al_0.02³⁺)^Fe2Si₅O₁₄(OH) for babingtonite from the Yakuki mine.

The crystal structure of uedaite-(Ce), ideal formula ^A1Mn^2+A2Ce^M1-2Al₂^M3Fe²⁺(SiO₄)(Si₂O₇)O(OH), was studied using single crystals from the Karasugawa mine, Fukushima, Japan (hereafter, KG), and the Heftetjern pegmatite, Norway (HJ), by means of electron microprobe and X-ray diffraction. Both the specimens have similar composition, and are rich in MnO (8.1-8.5 wt%) and FeO (13.8-14.8 wt%). The Fe²⁺/total Fe ratios were estimated as 0.93. Structure refinements converted to R₁ = 1.70 and 1.55% for KG and HJ, respectively. The unit-cell parameters are a = 8.8200(2), b = 5.69346(12), c = 9.9745(3) Å, β = 114.016(3)°, and V = 457.52(2) ų for KG, and a = 8.8212(3), b = 5.69723(11), c = 9.9821(3) Å, β = 114.083(2)°, and V = 458.00(2) ų for HJ. The cation assignments of the A and M sites are as follows: ^A1Mn²⁺_0.71Fe²⁺_0.17Ca_0.12, ^A2Ce_0.40Nd_0.27La_0.09Pr_0.06Sm_0.06Y_0.05Gd_0.03Na_0.01Ca_0.03, ^M1Al_0.85Fe³⁺_0.09Fe²⁺_0.05Ti_0.01, ^M2Al_1.00, and ^M3Fe²⁺_0.91Mg_0.02Al_0.07 for KG, and ^A1Mn²⁺_0.70Fe²⁺_0.16Ca_0.14, ^A2Ce_0.39Nd_0.28La_0.09Sm_0.07Pr_0.06Y_0.06Gd_0.04Na_0.01, ^M1Al_0.83Fe³⁺_0.09Fe²⁺_0.07Ti_0.01, ^M2Al_1.00, and ^M3Fe²⁺_0.90Mg_0.02Al_0.08 for HJ. The unit-cell volume of uedaite is the smallest among allanite-group minerals, resulting from the smallest combination of dominant cations at the A1 and M3 sites, Mn²⁺ and Fe²⁺. In the uedaite structure, the substitution of Ca for Mn²⁺ induces a topological change at the A1 site, as reported in previous studies, and the structure also shows an elongation of the A1-O5 bond. This elongation results from the edge-sharing coordination polyhedra of Fe²⁺-dominated M3 and Al-dominated M1 sites, being slightly tilted to maintain bonding with adjacent SiO₄ tetrahedra. Moreover, a positive correlation between the β angle and the O4-M3-O8 angle was observed. This correlation is suggested to be a general feature across the entire epidote supergroup minerals.

Contact metamorphism of ultramafic rocks provides a valuable natural laboratory for investigating fluid-assisted metamorphic reactions under well-constrained thermal conditions. In such settings, fluid behavior associated with dehydration reactions can be recorded as systematic spatial variations in mineral assemblages. In this study, we investigated a contact aureole in the Sanbagawa high-pressure metamorphic belt in the northern Akaishi Mountains, central Japan. In this aureole, the boundary between the Kifune plutonic body and the surrounding metamorphic rocks cuts almost perpendicularly across the structural arrangement of the ultramafic body. This geometry allows spatial variations in mineral assemblages to be examined primarily as a function of distance from the intrusive body. The ultramafic rocks were classified into four zones according to their proximity to the intrusive boundary: lizardite (non-contact metamorphic zone), antigorite, olivine + talc, and Ca-amphibole zones. The antigorite to Ca-amphibole zones, located within approximately 2.5 km of the intrusive boundary, were affected by contact metamorphism. Mineral assemblages from the non-contact metamorphic zone to the olivine + talc zone can be explained by metamorphic reactions in the MgO-SiO₂-H₂O (MSH) system, whereas those in the Ca-amphibole zone involve Ca-bearing components. The lower temperature limits of the antigorite and olivine + talc zones correspond to key reactions: the transition from lizardite to antigorite defines the base of the antigorite zone, whereas the dehydration breakdown of antigorite defines the base of the olivine + talc zone. The Ca-amphibole zone is characterized by the formation of Mg-hornblende associated with the dehydration of tremolite and chromium-aluminum (Cr-Al) spinel, indicating higher temperature conditions. The upper temperature limit of the olivine + talc zone is constrained to be below the anthophyllite-forming reaction, as indicated by the absence of anthophyllite. Amphibole compositions in the Kifune body indicate pressure conditions of ∼ 2 kbar, under which the temperature ranges are estimated as <350 °C for the lizardite zone, ∼ 350-500 °C for the antigorite zone, >∼ 500 °C for the olivine + talc zone, and >∼ 700 °C for the Ca-amphibole zone. Localized occurrences of olivine veins, calcic plagioclase, and Na-bearing Ca-amphibole (pargasite) were also observed near the intrusive boundary. These features, together with the discrepancy between the estimated temperatures of the ultramafic rocks and the results of conductive thermal modeling, suggest that the intrusion induced large-scale dehydration reactions within the ultramafic body and the inflow of external fluids. Such fluid influx likely played a major role in the enhanced transport of heat and chemical components.

Ti-in-zircon geothermometry is widely applied to silicic magmas. However, reliable temperature estimates depend on the activity of TiO₂ in the melt relative to rutile saturation (aTiO₂), which may change significantly during magma crystallization. We constrain realistic aTiO₂ values for high-silica rhyolitic magmas by integrating Ti concentration measurement in zircon with thermodynamic modeling of the Miocene Kinpusan granite, a shallow intrusion emplaced at ∼ 50 MPa in central Japan. Zircons show uniformly low Ti contents (interquartile range of ∼ 2.0-4.5 µg/g). MAGEMin phase-equilibrium modeling combined with zircon saturation calculations indicates that zircon saturation occurs at <800 °C and melt fractions ≤90 vol%. Within the zircon-melt coexistence interval (∼ 750-800 °C), both thermodynamic predictions and the observed Ti concentrations in zircon require low aTiO₂ values of ∼ 0.1-0.4, consistent with the petrographic absence of rutile and ilmenite. The models also predict a sharp increase in Ti concentration in zircon immediately above the solidus, implying that the highest Ti contents reflect eutectic crystallization. These results also support the idea that aTiO₂ must vary during crystallization and cannot be assumed constant in a rhyolitic magma system.

Horiite is a new member of the bafertisite group within the seidozerite supergroup, discovered at the Taguchi mine in Aichi Prefecture, Japan. The mineral was named in honor of Dr. Hidemichi Hori (1934-2019), a distinguished Japanese mineralogist and founder of Hori Mineralogy, Ltd. Horiite crystals are thin, elongated plates with long axes up to 300 µm in length and 20 µm in thickness. They are closely layered and tend to aggregate, sometimes forming aggregates with hejtmanite. The Mohs hardness of horiite is 4-5, its tenacity is brittle, and its calculated density is 4.08 g·cm⁻³. Under plane-polarized light, the mineral shows very weak pleochroism, changing from nearly colorless to pale brown. The empirical formula, calculated on the basis of 24 O and 2 (OH + F) atoms per formula unit, is (Ba_1.98Sr_0.01)_Σ1.99(Mn_1.97Ca_0.04)_Σ2.01(Mn_3.34Fe_0.36Mg_0.31)_Σ4.01(Ti_1.98Sn_0.03)_Σ2.01Si_4.00P_1.99O₂₄[(OH)_1.92F_0.08]_Σ2, ideally Ba₂Mn₂Mn₄Ti₂(Si₂O₇)₂(PO₄)₂O₂(OH)₂. The refined unit-cell parameters determined by single-crystal X-ray diffraction are a = 5.35090(10) Å, b = 13.89170(10) Å, c = 14.2777(2) Å, α = 98.7260(10)°, β = 93.8420(10)°, γ = 90.0570(10)°, and V = 1046.60(3) ų, and Z = 2, in conjunction with space group P1 (#2), whereas the lattice parameters determined by synchrotron-radiation powder X-ray diffraction are a = 5.3349(9) Å, b = 13.884(2) Å, c = 14.270(3) Å, α = 98.785(15)°, β = 93.836(16)°, γ = 90.029(16)°, and V = 1043.7(3) ų. Although the crystal structure of horiite is partially related to that of yoshimuraite, it represents a new structural type. Consequently, according to the nomenclature concept of the seidozerite supergroup, the crystal structure of horiite is identified as the fifth basic structure within the bafertisite group. Horiite is the only species among more than 50 in the seidozerite supergroup that contains species defining Mn-occupied sites in the I block.

In a protoplanetary disk, the interactions between fluids and meteoritic materials are influenced by their distance from the Sun. This is because of the varying abundance of organic molecules in aqueous fluids. However, a few studies have explored the effects of organic compounds in aqueous alteration fluid. Therefore, we aimed to observe the fractionation behavior of major elements in the early solar system, by focusing on the reaction between organic matter and meteoritic material, particularly on the interaction between organic compound-rich fluids and meteoritic materials and on their role in magnesium/iron (Mg/Fe) fractionation. This experiment is intended to simulate the reactions that are considered to have occurred in dwarf planets. We used two solutions: Fluid A with ∼ 16 wt% formic acid and 10 wt% ethanol, and Fluid B with 30 wt% formic acid and 19 wt% ethanol. The solid phase was the powdered Allende CV chondrite. Experiments were conducted over 10, 40, and 120 days at 150-350 °C. In both solutions, dashkovaite, an Mg formate mineral, formed metastably at 150 °C. During its temporary presence and subsequent extinction, Mg and Fe exchanged between dashkovaite and insoluble Si-Mg-Fe product phases. The extinction of dashkovaite and changes in its Mg/Fe ratio led to an insoluble Si-Mg-Fe phase with an Mg# value as low as 10 in Fluid B. These reactions suggest that organic-rich fluids can influence Mg/Fe fractionation by separating Mg-rich organic minerals.

Quaternary adakitic volcanism along the volcanic front of Western Honshu and northern Kyushu has been attributed to slab melting of the young, hot Shikoku Basin oceanic crust. However, the timing and magnitude of magma production at the northernmost end of the Kyushu volcanic front, represented by the Himeshima volcanic group, remain insufficiently constrained. In this study, we present new K-Ar ages obtained using the unspiked sensitivity method combined with petrographic and whole-rock geochemical data to refine the volcanic history of the Himeshima volcanic group.

The new K-Ar dating results indicate that volcanic activity in Himeshima was short-lived and concentrated between 89 and 59 ka. By integrating these age constraints with previously reported erupted volumes (0.05 km³), we estimate an average eruption rate of 1.7 × 10⁻³ km³/kyr. This rate is the lowest among the volcanoes of the Kyushu volcanic front and is one to two orders of magnitude lower than those of the adakitic volcanoes in Western Honshu, Japan, such as Daisen, Sanbe, and Aonoyama. We infer that the notably low eruption rate at Himeshima reflects the combined influence of shallow slab dehydration processes of the Shikoku Basin crust, a relatively cool mantle wedge beneath Himeshima, and a prevailing compressional stress regime.

We present detailed petrographic observations, modal compositions of phenocrysts, and whole-rock major and trace element data (Ba, Rb, Nb, Sr, Zr, and Y) for volcanic rocks of the Kuju Volcanic Group (KVG), northeast Kyushu Island, southwest Japan Arc. We classified these volcanic rocks into three groups based on the mineral assemblage and modal abundance of phenocrysts: (1) a group containing ≥2.9 vol% olivine and lacking hornblende (Ol-g); (2) a group containing hornblende (Hbl-g); and (3) a group containing ≥2.0 vol% orthopyroxene with a small amount of olivine (≤1.3 vol%) and lacking hornblende (Opx-g). In addition, based on the characteristics of plagioclase (Pl) and clinopyroxene (Cpx) phenocrysts, Hbl-g can be subdivided into three types: samples containing abundant Pl with dusty cores are defined as Hbl-g1, whereas those poor in Pl with dusty cores are divided into Cpx-rich Hbl-g2 and Cpx-poor Hbl-g3. The petrographic features of Opx-g, Hbl-g1, and Hbl-g2 indicate they experienced mixing between mafic and silicic magmas, as indicated by the occurrence of Pl phenocrysts with honeycomb and dusty cores, as well as the coexisting olivine and quartz. Fractional crystallization cannot explain the geochemical relationship between Ol-g and the other groups. In addition, the geochemical characteristics of each group show that Opx-g, Hbl-g1, and Hbl-g2 cannot be explained by simple binary mixing between the most mafic Ol-g and most silicic Hbl-g3 samples from the KVG. Furthermore, Opx-g, Hbl-g1, and Hbl-g2 are not related to each other by magma mixing. The variations in Y contents and Sr/Y ratios suggest that Hbl-g1 and Hbl-g3 were derived by partial melting of a subducting slab (i.e., slab melts), and that Hbl-2 may be petrogenetically related to a bajaitic magma that was generated by interactions between slab melt and mantle material. The variations in SiO₂, TiO₂, P₂O₅, and Nb contents, and the trend defined by Opx-g towards the compositional field of high-Nb basalts, similar to volcanic rocks erupted in areas around the KVG, are ascribed to interactions between slab melt and the mantle above the subducting slab.

Ferro-holmquistite with high Fe²⁺ content, which was previously predicted to be unlikely to occur, has been identified from a dyke composed of quartz and microcline in limestone at Mt. Hayamadake, Tamura City, Fukushima Prefecture, Japan. The specimen was collected in the 1980s and had been preserved in a private collection as an unnamed amphibole species; however, its name and features have now been revealed as ferro-holmquistite. The empirical formula, estimated on the basis of the constraints of Si = 8 is (□_0.93Na_0.07)_Σ1(Li_1.97Mg_0.03)_Σ2(Fe²⁺_2.62Mg_0.20Mn_0.18Al_1.75Fe³⁺_0.25)_Σ5Si₈O_22.05(OH_1.96F_0.04)_Σ2 for the platy-crystal sample. The unit-cell parameters obtained from single-crystal X-ray diffraction measurements are a = 18.4080(3) Å, b = 17.7942(3) Å, c = 5.29463(11) Å, and V = 1734.29(6) ų, with placement in the orthorhombic Pnma space group. Comparison of the single-crystal X-ray diffraction results with those for the holmquistite group reveals that the M1 and M3 octahedra expand with increasing Fe²⁺ content. In particular, the M3 octahedron exhibits marked distortion and anisotropic elongation, most notably along the b-axis. Conversely, an increase in the Fe³⁺ content enlarges and distorts the M2 octahedron, while mitigating deformation at the M1 and M3 sites. The tetrahedral chain demonstrates structural flexibility in response to these changes, either by adjustment of the tetrahedral volume and/or alteration in the degree of chain kinking. Through these changes, the low bond valence sums for O4A and O4B, which were once considered the starting point of structural limitation, remain consistently low. As a result, the previously held notion that geometric constraints inhibit the formation of Fe²⁺- and/or Fe³⁺-rich members should be reconsidered.

We report prehnite-forming reactions in scapolite-bearing calc-silicate granulites from the high-grade metamorphic terrane of Lützow-Holm Complex, East Antarctica. Based on textural observations, three possible reactions were identified: 1) Prehnite forming from andradite-grossular garnet, 2) Prehnite formation from anorthite, quartz, and calcite, and 3) Prehnite replacing meionite, quartz, and calcite. Phase equilibrium modeling indicates that prehnite formed under H₂O-rich conditions at T = 320-390 °C and X_CO2 < 0.004. The occurrence of Fe-rich prehnite reflects Al-Fe³⁺ partitioning that occurred during low-grade metamorphism and may serve as an indicator of oxygen fugacity conditions. The prehnite formation reactions exemplify the role of aqueous fluid infiltration during retrograde metamorphism and provide important constraints on changes in fluid conditions from fluid-absent to H₂O-rich.

Zircon U-Pb dating was conducted on granite, granodiorite, and gabbro from the central part of Yamaguchi Prefecture, Southwest Japan. The granodiorite and gabbro are recognized as components of the Susuma-Nagaho Plutonic Complex (SNPC). The zircon U-Pb dating yielded ages of 103 Ma for the granite and 92 Ma for the granodiorite and gabbro. These ages coincide with the magmatic flare-up event documented in Southwest Japan. A compiled dataset of Cretaceous gabbro-diorite from Southwest Japan shows that initial-εNd values (εNdI) decrease progressively to as low as −5.1 between 107 and 90 Ma. In addition, Th/La ratios exhibit a negative correlation with initial-Nd isotopic ratios. These geochemical trends, together with their temporal variation, suggest that subducted sediments increasingly interacted with the mantle wedge, producing progressively enriched mantle sources throughout the Cretaceous. This secular change may reflect variations in plate motion, such as change in the rate of subduction.

We investigated pyrite formation in modern stromatolite formed along the flow path of hot spring water in Fukiagesawa using detailed SEM and TEM analyses. The stromatolite consists of alternating white layers composed of silicified cyanobacteria, and dark layers composed mainly of clay minerals and other clastic particles. Pyrite is present in all layers, but shows considerable variation in frequency, morphology and occurrence depending on location. In the white layers, pyrite is generally scarce, occurring only in small amounts as isolated euhedral crystals and framboids near diatom shells, consistent with a relatively oxidizing environment maintained by cyanobacterial photosynthesis. By contrast, the dark layers are enriched in pyrite, mostly concentrated within pores between clastic particles and showing various morphologies including euhedral and framboidal forms, suggesting that localized sulfate reduction facilitated pyrite formation. The presence of jarosite within some framboids and euhedral crystals, along with the overgrowth of secondary pyrite indicates that reducing and oxidizing conditions alternated within the stromatolite over relatively short timescales.

Petrological and chemical analyses were conducted on garnet-bearing rhyolites of the Darumayama and Shiroyama volcanoes in the Hime-shima Volcanic Group (HVG) on Kyushu Island, southwest Japan arc. In the Darumayama rhyolite, garnet occurs at a frequency of approximately one grain per several thin-sections. It is mostly euhedral and dispersed within the groundmass, and locally exhibits resorption textures. Garnet in the Shiroyama rhyolite occurs as multiple euhedral grains that are dispersed within the groundmass and show no distinct resorption features. The almandine, spessartine, and pyrope proportions of the garnets in the Darumayama and Shiroyama rhyolites are nearly identical, indicative of a common origin. The Shiroyama rhyolite contains a plagioclase-rich aggregate containing garnet, sillimanite, and zircon, indicating a metamorphic origin. The garnet in this plagioclase-rich aggregate has a solid-solution composition identical to those outside the aggregate, indicating that all garnet in the HVG rhyolites has a common origin. The high MnO (5.6-7.0 wt%) and low CaO (0.5-1.2 wt%) contents of the garnet are indicative of metamorphic origin rather than a magmatic origin. These observations indicate that the garnets in the HVG rhyolites are xenocrysts derived from metapelites rather than magmatic phenocrysts.

The identification of dislocations and associated Burgers vectors in minerals is important to discuss the mechanisms of crystal plastic deformation of rocks. Observation of dislocations is normally carried out using transmission electron microscopy (TEM). However, the necessary sample preparation and observation can be challenging in particular for hydrous minerals that are sensitive to beam damage. In addition, the spatial scale at which TEM can be used to observe dislocations is limited, making it difficult to evaluate the process of bulk-rock deformation. Recently, the improved accuracy and indexing rate in Electron Backscatter Diffraction (EBSD) mapping and the ease of analyzing mapping data using a toolkit for analysis provided in programming software have made it possible to observe the microstructural features related to dislocations using EBSD measurements, such as small misorientations less than a few degrees of angular differences within a single grain. In this study, we examine how EBSD observations of natural antigorite-rich serpentinite samples can be used to derive information about the nature of dislocations and associated antigorite deformation mechanisms. In particular, we focused on the effect of different lengths in the crystal undulations, expressed as M- and m-values, of antigorite on the deformation processes and the resulting crystal preferred orientation (CPO) patterns of antigorite in antigorite schist samples. The resulting antigorite CPOs are all like the B-type CPO patterns that have been widely reported from natural antigorite schists regardless of the wavelength of the curved antigorite crystals. However, misorientation analyses using the EBSD maps suggest dislocations characteristic of the [100](001) slip system, responsible for A-type CPO formation, are more common in antigorite-rich serpentinites with shorter wavelengths, whereas dislocations characteristic of the [hk0](001) slip systems, responsible for G-type CPO formation, are more common in those with longer wavelengths. Therefore, the dislocation microstructures observed in this study do not provide evidence that the B-type CPO was formed by dislocation creep. However, these dislocation microstructures may preserve evidence of different deformation stages before and after the formation of the B-type CPO. This implies that, as M- and m-values tend to decrease under high-pressure conditions, A- and G-type CPOs are likely to form in deeper and relatively shallower domains, respectively. These results suggest the strength of schistosity, grain shape, and CPO strength in ductilely deformed antigorite-rich serpentinites may be affected by differences in the dominant deformation mechanisms and dislocation microstructures influenced by the wavelength of antigorite.

Arsenotučekite (Ni₁₈Sb₃AsS₁₆), a rare mineral of the hauchecornite group, is newly identified from carbonated serpentinite and schistose listvenite in the Median Tectonic Line (MTL) shear zone in eastern Ehime Prefecture, Japan. This is the second occurrence after the type locality and the first report from outcrop samples. At the Urayama River, arsenotučekite occurs in carbonated serpentinite and in schistose listvenite, and at the Ichiba River it occurs in schistose listvenite. Arsenotučekite is associated with millerite in antigorite, magnesite, and dolomite in carbonated serpentinite, and in quartz and dolomite in schistose listvenite. The chemical compositions are (Ni_16.08Fe_1.61Co_0.23)(Sb_3.47As_0.80)S_15.81 (carbonated serpentinite, Urayama River), (Ni_16.20Fe_1.14Co_0.57)(Sb_3.50As_0.60)S_15.98 (schistose listvenite, Urayama River), and (Ni_15.84Fe_1.73Co_0.28)(Sb_3.55As_0.59)S_16.02 (schistose listvenite, Ichiba River). The unit-cell parameters of arsenotučekite in carbonated serpentinite from the Urayama River are a = 10.184(4), c = 10.758(6) Å, and V = 1115.8(8) ų. The a-axis length is markedly larger than that reported from the type locality. The mineral paragenesis indicates that arsenotučekite formed in relation to the carbonation of serpentinite and pelitic schist by CO₂-rich slab-derived fluids.

Continental fragments with Australian or Gondwanan affinities are widely distributed in eastern Indonesia. Metamorphic rocks intruded by Permo-Triassic granitoids have long been considered remnants of Paleozoic basement. However, recent geochronological studies in eastern Indonesia have revealed Mesozoic metamorphism with Cenozoic overprints, challenging previous interpretations. The Banggai-Sula microcontinent (BSM), a prominent continental fragment at the eastern tip of Sulawesi, preserves such Paleozoic basement rocks. Although the sedimentary cover sequences of the BSM are well studied, the metamorphic history of its Paleozoic basement remains underexplored. This study integrates petrography and monazite U-Th-Pb geochronology to investigate the metamorphic evolution of the BSM. In contrast to the Cenozoic resetting observed in neighboring continental fragments, this study reveals multiple tectonothermal episodes from the Mesoproterozoic to the Permian. Garnet-sillimanite-biotite gneiss records the earliest metamorphism at ∼ 1500 and 1400 Ma, consistent with major tectonothermal events in the North Australian Craton, suggesting a Gondwanan affinity. Regional Ordovician-Silurian metamorphism (∼ 460-433 Ma) occurred under medium P-T conditions. This metamorphism is linked to the eastern continuation of the Proto-Tethyan oceanic subduction zone from south of the Tarim Block along the northern Gondwana margin. Devonian-Carboniferous metamorphism (∼ 360 Ma) correlates with the evolution of the Tasman Orogen in eastern Australia, followed by Permian-Triassic granitic intrusion (∼ 260 Ma) representing an extension of continental arc magmatism from eastern Australia into New Guinea and the Sula Spur. Comparable tectonothermal histories in the Bird’s Head region of New Guinea indicate a shared tectonic history since at least the Devonian. These results establish the BSM as a fragment of the North Australian Craton crust that underwent significant reworking during the Paleozoic-Triassic evolution of the Gondwana margin.

Fluid behavior during grain growth was observed in real time using a simple technique. Camphor as a rock analogue was ground in a mortar with ∼ 10 vol% ethanol (a fluid phase). A total of ∼ 5 mg of the mixture was pressed onto a glass slide, and the textual development of the dense aggregate was observed at room temperature under a polarizing optical microscope. Videos were taken, and snapshots were analyzed. The mean initial size of the camphor grains was 7.0 µm with 2.6 area% of ethanol. The sizes of fluid inclusions, expressed as the diameter of a circle with the same area, were ≤0.6-4.0 µm with a mode of 0.8 µm. During the growth of camphor grains, three distinct types of fluid behavior were observed: (1) the coalescence of neighboring fluid inclusions along a migrating grain boundary, (2) an inclusion originally at a grain boundary being left behind into the grain interior, and (3) the incorporation of a fluid inclusion from a grain interior into a migrating grain boundary. After ∼ 1 h of the experiment, the mean grain size had increased to 19.8 µm. The total area of the fluid inclusions in the viewing area did not change, whereas the total number of inclusions decreased. In addition, the size of the inclusions increased to ≤0.6-5.0 µm with a mode of 1.6 µm due to the coalescence of fluid inclusions. This study provides insights into the redistribution of fluid inclusions during textural development relating to grain growth, which can occur in the Earth’s crust where water is the dominant fluid phase.

We investigated the synthesis conditions of metastable α-cristobalite using amorphous silica (silica-gel) with and without V₂O₅ as a flux. Secondary electron microprobe observations for synthesized samples with flux revealed the formation of faceted α-cristobalite crystals (2-19 µm), whereas those synthesized without flux did not exhibit a clear facet. Powder X-ray diffraction analysis showed that the addition of flux lowered the formation temperature of metastable α-cristobalite to 1000 °C, which contrasts with the fact that the crystallization occurred at 1200 °C in the case without flux. These observations demonstrate that the V₂O₅ flux alone promotes crystallization by lowering the activation energy of the crystallization process of metastable cristobalite as well as other kinds of flux materials.

New allanite-group minerals, akasakaite-(Ce), akasakaite-(La), vanadoakasakaite-(Ce), and vanadoakasakaite-(La), associated with small quartz lenses within rhodonite-enriched rocks of the stratiform manganese deposits at the Mogurazawa mine, Gunma, Japan, were studied using electron microprobe analysis, Raman spectroscopy, and single-crystal X-ray diffraction methods. Akasakaite is defined by the ideal formula, ^A1Ca^A2REE^3+M1Me^3+M2Al^M3Mn²⁺(SiO₄)(Si₂O₇)O(OH) (Z = 2, space group P2₁/m). The four new minerals are distinguished by the dominant cation at the M1 site (Al or V³⁺) and the dominant REE³⁺ at the A2 site (Ce or La). The unit-cell parameters are a = 8.885-8.925, b = 5.694-5.744, c = 10.100-10.153 Å, β = 113.62-113.79°, and V = 468.0-476.4 ų. Structure refinements converged to R1 values of 2.20-2.86%. At the M1 site, Al content reaches 85% in akasakaite-(La), whereas V³⁺ reaches 82% in vanadoakasakaite-(La). The cation assignments for these specimens are as follows: ^A1Ca_0.54Mn²⁺_0.46, ^A2La_0.53Ce_0.18Nd_0.05Pr_0.02Sr_0.04Ca_0.18, ^M1Al_0.85V³⁺_0.13Ti_0.02, ^M2Al_1.00, and ^M3Mn²⁺_0.75Fe_0.01V³⁺_0.16Al_0.07 for akasakaite-(La) and ^A1Ca_0.60Mn²⁺_0.40, ^A2La_0.45Ce_0.17Nd_0.06Pr_0.03Sr_0.15Ca_0.14, ^M1V³⁺_0.82Al_0.18, ^M2Al_0.64V³⁺_0.33Ti⁴⁺_0.03, and ^M3Mn²⁺_0.71V³⁺_0.26Al_0.02 for vanadoakasakaite-(La). All studied specimens are enriched in Mn²⁺, which predominates at the M3 site and also occupies over 40% of the A1 site. The unit-cell dimensions of vanadoakasakaites are larger than those of akasakaites. This variation appears to be attributed mainly to the elongation of <M1-O> caused by the substitution of Al for V³⁺ at the M1 site. Moreover, the expansion of the M3 site by the substitution of divalent octahedral cations, such as Mn²⁺, elongates the M3-O8 distance, which shortens the A2-O8 distance despite no cation substitution at the A2 site. This suggests that the variation in the akasakaite structure is largely influenced by cation substitution at the M3 and M1 sites. The topological change of the A1 polyhedra due to Mn²⁺ substitution for Ca at the A1 site, as reported in previous studies, was also observed.

Quartz occurs ubiquitously in felsic plutons. The development of a methodology to reveal the quartz growth process in a granitic body provides essential insights into magma chamber processes. Cathodoluminescence (CL) characterization combined with Ti concentration of quartz crystals in granite is a prevalent tool for identifying the crystal growth and crystallization temperature of quartz in granitic magmas. This study focused on quartz crystal growth in the Kuki granite (KKG), northeast Japan, and Kurobegawa granite (KBG), central Japan. In the KKG quartz crystals, the difference in luminescence corresponds to Ti concentration: high luminescence corresponds to high Ti concentrations, which is consistent with our previous studies on the Toki and Okueyama granites. In the KBG quartz crystals, the Ti concentrations do not correspond to the luminescence level, whereas the Al concentrations are negatively correlated with the luminescence level. Therefore, quartz crystallizations in KKG and KBG are characterized by Ti- and Al-diffusion-controlled growth, respectively. Using the difference in the time-temperature history among the target rock bodies, this study provides insight into the nature of the relationship between quartz crystallization and magma chamber processes. Quartz crystallizations in the KKG were accompanied by gradual variations from oscillatory zonation through gradual zonation to homogeneous CL patterns with decreasing temperature in the cooling magma chambers. The quartz internal structure of the KBG was resulted from the rapid cooling of the KBG magma and the scarcity of Ti and enrichment of Al in the magma during quartz crystallization.