Basis, conditions, and practical techniques for scanning electron microscopy (SEM) are described, to obtain high-resolution secondary-electron images (SEIs) which give valuable information for fine morphology of mineral specimens. Although minimization of the electron-probe diameter on specimens is the primary request for high-resolution imaging, a smaller scattering volume in the specimen with a lower acceleration voltage is often important to acquire clear SEIs to show surface structures of the specimens. The operators are recommended to optimize the acceleration voltage and other operating conditions of SEM, as well as specimen preparation conditions, depending on their samples and purpose of the observation.

New Minerals and Occurrences in Japan (2022)

Magma rheology is a key factor in understanding and modelling volcanic eruptions. Until now, macroscopic rheology experiments reveal the viscosity of the magma and conditions at which shear thinning and brittle failure occur. However, it remains unclear what mechanisms control complex magma rheology from the atomic and molecular-scale structure perspective. More specifically, no experimental data on molecular-scale structure have been obtained for deforming magma in the non-Newtonian regime. To resolve this situation, we have developed an experimental system for time-resolved X-ray diffraction and scattering at SPring-8, Japan. Based on the experiments on this system, we found that intermediate-range ordering (IRO), which is related to the size of the ring formed by SiO₄ tetrahedra, expands under tensional deformation. In particular, the IRO shows elastic and anisotropic deformation in the non-Newtonian regime. On the other hand, the short-range ordering such as T-O and T-T distances, where T and O represent Si and Al in the T-site and oxygen, respectively, shows no clear change during the deformation. These results imply that shear thinning and brittle failure may originate from the expansion of the ring size because the large ring is relatively weak and its formation results in cavitation. According to this model, the magma fails when the stress is large enough, rather than the strain rate, because the IRO deforms according to the stress applied to the structure. Recent experiments also observed that small and anisotropic rings form under compression. Previous rheology experiments did not confirm the difference between the conditions, at which shear thinning and brittle failure occur, under tension and compression, but the experimentally-determined molecular-scale structure clearly shows different behavior. To fully understand the mechanism of magma rheology from the view of the molecular-scale structure, we need to perform additional studies including the experiments and theoretical approaches.

Information collected from geological newsmagazines in 2022 is reviewed. First topic is mineral resources such as mineral commodity summary in USA, geospatial analyses of lode gold potential in Alaska, and mineralogy of Eudialyte. Second topic is mineral matters, that is, attribution of minerals to science, technology, and society, and notation history from alchemy to modern mineralogy. Third topic is regional geology such as large igneous provinces across the Archean-Proterozoic transition and a giant meteorite impact in Scotland. In addition, Anthropogenic data question for a new geological epoch is reviewed.

Monazite in a granite pegmatite from Yamanoo, Sakuragawa, Ibaraki Prefecture, Japan has been examined in terms of U, Th, Pb, and rare earth element (REE) chemistry by electron microprobe analysis. This pegmatite occurs as a lenticular vein in the Kabasan granitic body. Oscillatory and sector zoning were observed by back-scattered electron images. An averaged empirical formula was (Ce_0.286Nd_0.180Sm_0.104La_0.077Y_0.069Pr_0.041Gd_0.069Dy_0.020Er_0.004Th_0.069U_0.011Ca_0.052)_Σ0.982(P_0.983Si_0.027)_Σ1.010O₄. Compositional variations suggest that the (Th + U) contents are controlled mainly by the coupled substitution (Th,U) + Ca = 2REE. Calculated U-Th-Pb chemical ages based on the assumption that the initial Pb is negligible are 61 ± 11-69 ± 10 Ma (2σ) and consistent with previously reported radiometric ages for the Kabasan granitic body.

Granitoids cover approximately 30% surface area of the Japanese island. Intense magma activities of the granitoids occurred during the Paleogene to Cretaceous (50-130 Ma). The Cretaceous granitoid batholith is exposed over an area of about 100 km from east to west and 50 km from north to south in northern Kyushu. Small mafic bodies occur sporadically in the batholith. Some of these mafic bodies represent high-Mg diorite (HMD) compositions derived from high-Mg andesite (HMA) magma, and their petrogenesis has been discussed by several researchers. The coexistence of both mafic and felsic rocks has been reported from volcanic-plutonic sites of the active continental margin in the Cretaceous granitoid batholith of southwest Japan and the Cordillera Mountains in North America. In addition, mantle-derived mafic magma is strongly involved in the origin of granitic magma as a parent magma and a heat source of crustal melting. Therefore, clarification of the behavior of the HMA-derived magma in the crust play an important role in examining the genetic relationship with granitoids. Furthermore, plutonic rocks are often called “fossils of magma reservoirs” as they preserve magmatic or emplacement processes and the time scale of plutonic activity. Investigation of internal structures may help to clarify the lithological and chemical variation in plutonic rocks as well as their growth process, which is crucial for understanding the relationship between volcanic and plutonic activity. These studies will contribute to the understanding of crustal growth and evolution. Thus, in this study, the magmatic process, lithofacies variations and the growth process of a newly finding HMA-derived rock, the Shaku-dake body (a high-Mg diorite body), are discussed as a member of the Cretaceous granitoids batholith of northern Kyushu.

Ultramafic rocks, i.e., peridotites and pyroxenites, occur in a variety of tectonic settings on Earth. Ultramafic rocks can form as accumulation of mafic minerals from basaltic to komatiitic melts and be a major component of the Earth's mantle. The origin and history of ultramafic rocks are expected to provide information on the processes of partial melting and melt migration/extraction in the mantle and on the tectonic evolution of geologic units containing ultramafic rocks. I study ultramafic rocks in metamorphic belts, ocean floor, and mantle sections of ophiolites. My career began with a study of the Horoman Peridotite Complex in the Hidaka metamorphic belt in Japan. The ultramafic rocks and associated mafic rocks in the Horoman body record a very complex evolutionary history from the mantle conditions to crustal conditions. It is difficult to constrain the tectonic setting affecting events in the Horoman Peridotite Complex. On the other hand, ultramafic rocks in the mantle section of ophiolites and abyssal peridotites directly recovered from ocean floor to study melting processes and melt-rock interactions in the mantle can be used to constrain their tectonic setting, or at least as analogs to these tectonic settings. Studies on the Oman ophiolite by Japanese groups and literature studies of other ophiolites suggest that many ophiolites are later modified by subduction-related magmatism. Several ophiolites are being studied to elucidate the maturing process by subduction-related magmatism. Simple partial melting and melt extraction is expected in the adiabatically upwelling mantle beneath the mid-ocean ridge. In fact, abyssal peridotites directly recovered from mid-ocean ridges provided a unique opportunity to elucidate these processes. Comparison of abyssal peridotites recovered from the mid-ocean ridges and arc regions (fore arc and back arc) is key to understand the differences in magmatic processes in the two regions. Ocean science with research vessels has a well-defined working hypothesis that can only be addressed by direct sampling from the seafloor. To understand a crucial issue in Earth science as to why plate tectonics occurs on Earth, it is essential to elucidate the life of the oceanic lithosphere from its birth to its subduction into the mantle. Direct sampling of oceanic lithosphere by drilling is the key to solving this issue. I would like to emphasize that members of the Japan Association of Mineralogical science can play an essential role in leading analyses of rock samples directly recovered from seafloor. Rock samples recovered from seafloor by drilling and any methodology, as well as samples from anywhere on Earth, should be published in as papers, and these data would help integrate knowledge about the history of the Earth and planet and its future.

The identification of light elements in the Earth's core has been an open question for more than 50 years. From seismic observation, the Earth’s core is believed to contain some light elements because it is about 10% less dense than pure Fe at the corresponding pressure and temperature conditions. Among the candidates of light elements in the core, hydrogen is one of the promising light elements. Neutron diffraction is very useful in systems when detecting light atoms surrounded by heavy atoms because its scattering intensity is independent of atomic number. Hence, many neutron diffraction studies have been performed on the iron-hydrogen binary system. On the other hand, it is likely that there is more than one type of light elements dissolved in the core, and interactions between light elements are also involved. Here, we focused on an Fe-Si-H system, and we investigated the sites occupied by deuterium and its site occupancy of hcp Fe_0.95Si_0.05Hx at 14.7 GPa and 800 K by in-situ neutron diffraction experiments. So far, this pressure condition is the highest for neutron diffraction experiments at high pressure and high temperature, where crystal structure analysis has been conducted.

The life cycle of the oceanic lithosphere commences in the spreading axis and ends in the subduction zone. To trace the cooling and evolutional history of the Earth, the change in thermochemical state during the life cycle of present‒day oceanic lithosphere is desired to be elucidated. In terms of the material science, spatial limitation of human‒accessible Earth interior is a bottleneck in reconstructing the thermochemical state of the oceanic lithosphere. Yet, by combining active sampling methods using ocean research vessels (ocean drilling, ocean bottom dredging, submersible survey, etc.) and passive sampling methods using Earth’s deep materials exposed to the surface owing to tectonic forces and volcanoes, we can collect samples that cover a considerable dimension. Here, I present efforts toward the elucidation of the thermochemical state of the oceanic lithosphere during its life cycle from the spreading axis to the subduction zone. The Oman ophiolite is presented as an analogue of oceanic lithosphere formed in the vicinity of a fast‒spreading axis, whereas the peridotite xenoliths from Tahiti Island are treated as an analogue of thermochemically disturbed oceanic lithosphere by a mantle plume, and those from petit‒spots are considered as an analogue less affected by thermochemical disturbance considering the lack of mantle plume beneath the petit‒spots. A heterogeneous thermal state corresponding to the segment structure is inferred in the fast‒spreading axis. The thermochemical state of the aging oceanic lithosphere is modified by mantle plume and petit‒spot magmatism, but pristine state can be reconstructed by using suitable peridotite xenoliths whose heating‒cooling and melting history is well characterized. The peridotite xenoliths from the petit‒spots can enhance a step toward reconstructing the thermochemical state of the deep oceanic lithosphere because deep‒rooted garnet‒stable peridotite xenoliths can be found.

Gemology is one of the mineralogical science and is facing a great challenge to identify various natural, synthetic and simulant gem materials, especially the geographic origin determination of high value gemstones is strongly required by gem trade and consumer. However, gemologist and research scientist in gemological laboratories must improve on their experience in identification skills, geological field studies, collecting available samples and establish an informative data base by combination of traditional gemological observations and advanced analytical instrumentations. It is very important to understand the new finding gemstone deposits and gemstone treatment procedures and development of advance sophisticated instrumentation to support the difficulties of gemological identification.

Mineral Industry Museum, Akita University —Highlights in Mineral Science—

Spectroscopy has been widely used in geology since the 1990s because it is non-destructive and easy to analyze. Raman spectroscopy has generally been used to identify mineral phases in geology, but recent studies have proposed new methods to quantitatively estimate the metamorphic pressure (quartz Raman barometry) and peak temperature (Raman carbonaceous material geothermometry). Studies using infrared spectroscopy are also underway to advance our understanding of hydration processes in subduction zones and mantle through the analysis of water in rocks. In addition to the development of new methods using spectroscopy, new tools are also being developed to analyze huge amounts of data through iterative processing, which will enable the extraction of more informative and quantitative results in a shorter time. This paper introduces examples of spectroscopy applications in geology and examines future developments.

The usage of “potassium feldspar” and “alkali feldspar” has been confused in earth sciences for a long time. The term “alkali feldspar” is for the solid solution series between the two components of KAlSi₃O₈ (potassium feldspar) and NaAlSi₃O₈ (sodium feldspar), including more or less CaAl₂Si₂O₈ (lime feldspar) as the third component. The term “potassium feldspar” is in practice for KAlSi₃O₈ as an end member of the feldspar solid solution. At present, the scientific or appropriate usage of the two terms without confusion is needed in earth sciences. This review outlines the history of hitherto confused usage of the two terms, and interprets how important the appropriate usage of the two terms is in earth sciences, especially in mineralogical and petrological sciences. We recommend that perthite should be termed not “potassium feldspar” but alkali feldspar, and that the term potassium feldspar should be applied to alkali feldspar at least with KAlSi₃O₈ content ≥ 90 mol%.