Momoiite, a Mn²⁺– and V³⁺–dominant member of the garnet group, has been reported from metamorphosed manganese deposits in Japan. Near end–member momoiite (Mn₃V₂Si₃O₁₂) has not yet been found in nature, and the chemical composition of natural momoiite is written as (Mn,Ca)₃(V,Al)₂Si₃O₁₂, which represents intermediate compositions in solid–solution series between spessartine and Mn–rich goldmanite. Single–crystal X–ray diffraction data were obtained for an optically isotropic momoiite crystal from the Fujii mine, Fukui Prefecture, Japan. Structural refinement in the Ia3d space group converged to R₁ = 1.45%, and the lattice constant is a = 11.8745(7) Å (Z = 8). The refined structural formula is ^X(Mn_1.763Ca_1.237)_Σ3.000^Y(V_1.461Al_0.539)_Σ2.000^ZSi₃O₁₂, which is consistent with electron microprobe analysis. Raman spectra of the solid–solution series between spessartine and Mn–rich goldmanite show continuous change in peak positions. The Si–O stretching and O–Si–O bending vibrations show downward frequency shifts with decreasing Mn/(Mn + Ca) and increasing V/(V + Al), which are associated with increase in mean ionic radii in the X and Y sites. The Al–V substitution controls the frequency shifts of the O–Si–O bending vibrations much more effectively than the Mn–Ca substitution. In contrast, the frequency shift of the SiO₄ rotation mode shows strong correlation with the Mn–Ca substitution and is apparently independent on the Al–V substitution. This behavior suggests that the rotation of the SiO₄ tetrahedron is strongly influenced by sharing two edges with XO₈–dodecahedra. Crystal–chemical and Raman spectroscopic data of natural momoiite are consistent with those expected as an intermediate composition in the solid–solution series.

The Nishisonogi unit of the Nagasaki Metamorphic Rocks represents a part of a Late Cretaceous subduction complex exposed in western Kyushu, Japan. We estimate peak metamorphic temperatures using a Raman carbonaceous material (CM) geothermometer on 60 pelitic schists. No systematic regional changes were observed in the mineral assemblage of samples collected over a large area (about 30 × 15 km), which include chlorite ± garnet + white micas + albite + quartz + titanite + CM. However, the estimated peak metamorphic temperature increases structurally upward from 440 to 524 °C, suggesting an inverted thermal gradient.

The crystal structures of skorpionite from the Skorpion zinc deposit in Namibia [Ca₃Zn₂(PO₄)₂CO₃(OH)₂·H₂O; monoclinic; a = 19.0715(8), b = 9.3321(3), c = 6.5338(3) Å, β = 92.6773(12)°; space group C2/c] and [a = 19.0570(14), b = 9.3346(5), c = 6.5322(4) Å, β = 92.752(2)°; space group Cc] are analyzed using single–crystal X–ray diffraction and refined to yield R values of 0.0253 and 0.0272 for 1576 and 2446 unique reflections with F_o > 4σ(F_o), respectively. Hydrogen atoms in the structure determined by the difference Fourier method. Although two space groups, C2/c and Cc, are possible, the Cc space group without center of symmetry is more likely the structure of skorpionite, which shows that skorpionite is a ferroelectric mineral. The disordered structure is induced in skorpionite by twinning and/or domain structures because of the relaxation of the natural polarization caused by the arrangement of polarized water molecules. The space group Cc model without the center of symmetry eliminates the need for statistical distribution. Bond valence sum calculations and hydrogen bond networks can be explained in detail by the model. In the complicated structure caused by the chemical composition, the local structure with a non–ideal coordination environment is observed near the Zn sites. Hydrogen atoms are continuously arranged with regular arrangements of water molecules in the tunnel structure.

Single crystals of katoite hydrothermally synthesized were examined by single–crystal X–ray diffraction, EPMA, and Raman spectroscopic techniques. The chemical formulas of the katoite fell inside the miscibility gap proposed by Kyritsis et al. (2009). The systematic absences observed through single–crystal X–ray diffraction were completely consistent with the cubic space group Ia3d. In two kinds of katoite with chemical formulas Ca₃Al₂(SiO₄)_0.57(H₄O₄)_2.43 and Ca₃Al₂(SiO₄)_0.69(H₄O₄)_2.31, the O atom position was split into two independent crystallographic sites, O1 and O2; the O1 is coordinated with Si, whereas the O2 forms a tetrahedral interstice. The a lattice parameter monotonically decreased as increasing Si content. The variation lay along a straight line between grossular and Si–free katoite solid solution. The coordination volume of the T site decreased with Si incorporation into the T site. The coordination volume of CaO₈ dodecahedra also decreased with the Si incorporation into the T site because the edges of the CaO₈ dodecahedron are shared with the adjacent TO₄ tetrahedra. These contractions lead to a monotonous decrease of the a lattice parameter. The volume of the AlO₆ octahedra, on the other hand, increased with the Si incorporation. There were no clear structural constraints resulting in a miscibility gap in the solid solution. A Raman band corresponding to the OH stretching vibration was observed at 3650 cm⁻¹, but with the substitution of Si for H, a new Raman peak appeared at 3580 cm⁻¹. The two Raman band positions remained unchanged with increasing Si content. These results strongly suggest that there are two types of OH stretching vibration in siliceous katoite. We therefore conclude that with Si substitution for H the O position is split into two inequivalent sites that correspond to the SiO₄ and H₄O₄ tetrahedra. The oxygen position splitting in katoite results in the emergence of two Raman bands at 3580 and 3650 cm⁻¹.

Calcic plagioclase with anorthite (An) content of up to 91% commonly occurs in coexistence with the sodic phase (An_<40–50) in the Cretaceous Kaizuki–yama (Mt. Kaizuki) granitoid body that intrudes into the Mino belt of the Jurassic accretionary sediment complex in central Japan. The Kaizuki–yama granitoids are mainly composed of plagioclase, K–feldspar, quartz, and biotite with subordinate amounts of amphibole, ilmenite, and apatite. The sodic plagioclase grains show common igneous–type normal zoning with decreasing anorthite content toward the rim. In most cases, the calcic plagioclase occurs as a crystal core, which is discontinuously surrounded by a sodic mantle. The calcic parts rarely show fine and nebular texture with sodic domains; however, a thin calcic zone (100–150 µm in width) develops along the boundary between the nebular core and the sodic mantle. In the nebular core, calcic plagioclase sometimes rims the fine–grained calcite, prehnite, and zoisite/epidote (less than 10–20 µm in size). Most amphibole grains have ferropargasite/ferrohornblende compositions and are usually rimmed by secondary ferro–actinolite. Although the pressure/temperature conditions estimated based on amphibole–plagioclase equilibria vary widely between samples, they show systematic decreases in pressure with decreasing temperature from 0.5 GPa/800 °C to approximately 0.2 GPa/700 °C. Some anorthite–rich plagioclase grains may have been xenocrystic and/or antecrystic in origins, and others were crystallized from calcic part of locally heterogeneous magma. The incorporation of calcic plagioclase into the felsic magma and the locally calcic environment probably resulted from the assimilation of skarn and related calcareous rocks consisting of the wall rocks of the Kaizuki–yama body during magmatic intrusion and solidification.

In order to facilitate identification of tridymite modifications using micro–Raman spectroscopy, the Raman spectra of synthetic and natural tridymite modifications (MC, MX–1, and PO–10) including low frequency region (ν = 15–200 cm⁻¹) are measured. The Raman spectrum of MX–1 is reported for the first time. All modifications revealed the characteristic peaks below 150 cm⁻¹. Using the compiled Raman spectra, tridymite modifications of the reported Raman spectra from meteorites in the literature were identified. It revealed that not only MC, but PO–10 modification exists in the meteorites.