Structural changes and transformation of nanocrystalline mackinawite to greigite were investigated using powder X–ray diffraction measurements, Raman spectroscopy, X–ray absorption fine structure, and ab initio quantum chemical calculations. We clarify the formation conditions for greigite. Nanocrystalline mackinawite utilized by Sano et al. (2020) was used as a starting material. By hydrothermal heating at 140 °C for 12 h, nanocrystalline mackinawite was transformed into greigite without other iron phases. During the transformation, the crystallite size increased from 5 to 12 nm. After 3 h of hydrothermal heating, the Fe³⁺/ΣFe ratio markedly increased, but the a and c lattice parameters of nanocrystalline mackinawite were almost unchanged. Immediately after the transformation, greigite exhibited a larger lattice parameter, which suggests that excess Fe²⁺ was incorporated into the octahedral site. Thereafter, the greigite gradually shrunk and Fe³⁺/ΣFe progressively approached the ideal value. The changes in the unit cell volume and Fe valence state implied that when greigite initially occurred, the nonstoichiometry allowed to accommodate more Fe²⁺ in greigite to the octahedral site. The hydrothermal heating of nanocrystalline mackinawite is potentially applicable to the synthesis of pure nanocrystalline greigite.

Pressure–induced phase transitions of tridymite modifications (MC, MX–1, and PO–10) were investigated by in–situ Raman spectroscopy. Starting from MC, a transition to PO–10 was observed at 0.4 GPa. At about 1.6 GPa, new Raman peaks appeared and were observed up to 8.7 GPa. This revealed a new phase, and it reverted to PO–10 at around 0.4 GPa during decompression. Contrary to previous study, PO–10 was recovered to ambient pressure. MX–1 also transformed to PO–10, and PO–10 was recovered. Starting from PO–10, the transition to the new phase was also observed but was gradual and the phase reverted to PO–10 during decompression. Present study revealed a new route to form PO–10 from MC and suggests rare PO–10 found in meteorites could be formed through this route.

In situ high–pressure single–crystal X–ray diffraction experiments on natural hemimorphite, ideally Zn₄[Si₂O₇](OH)₂·H₂O, up to 4.7 GPa were conducted to investigate its pressure response property associated with pressure–induced phase transition. After the phase transition confirmed between 2.46 and 3.01 GPa, pairs of satellite reflections were newly found at certain Bragg reflections. The modulation vector q of the satellites was approximately [0, 1/8.4, 0]. The results of the refinements on the averaged structure indicated that the modulation arose from displacements of atomic sites associated with the mechanism of the phase transition, i.e., the rotation of the secondary building unit (SBU). The lower rotation angle of the SBU (Φ) than the value estimated from the non–modulated structure meant that the high–pressure phase contained anti–phase boundaries (APBs) resulting from the opposite direction of the SBU rotation and that the coherency was held across the APBs. Within the coherent domain, the APB’s intervals were distributed along the b–axis with a mean value of 8.4b ≈ 90 Å, where the displacement of each site η (y ) was approximated as the first–harmonic. The distribution of the direction of SBU rotation was initially considered to be inhomogeneous, but the elimination of the APBs had proceeded anisotropically and had been aborted below 3.01 GPa.

Au(Ag)–Sn–Sb–Pb minerals occurring in association with gold, rumoiite (AuSn₂), shosanbetsuite (Ag₃Sn), yuanjiangite (AuSn), aurostibite (AuSb₂), and anyuiite (AuPb₂), were found from the Shosanbetsu River (the former three), Shosanbetsu village and the Ainusawa River (the latter two), Haboro town, Rumoi province, Hokkaido, Japan. Rumoiite (IMA No. 2018–161) and shosanbetsuite (IMA No. 2018–162) have been approved as new minerals by the International Mineralogical Association, the Commission on New Minerals, Nomenclature and Classification (IMA–CNMNC) and named after the locality. Both minerals show anhedral shape at less than 5 µm and occur in close association with one another, yuanjiangite, and native lead in spherical aggregates in placer gold. The densities of rumoiite and shosanbetsuite based on their empirical formulae and powder diffraction data were calculated to be 10.1 and 11.1 g/cm³, respectively. The empirical formulae of rumoiite and shosanbetsuite were (Au_0.95Ag_<0.01)_Σ0.96(Sn_1.93Sb_0.08Pb_0.02Bi_0.01)_Σ2.04 (basis of 3 apfu) and (Ag_2.46Au_0.54)_Σ2.99(Sn_0.97Sb_0.01Pb_0.01Bi_0.01)_Σ1.01 (basis of 4 apfu), respectively. Rumoiite is orthorhombic, Pbca, with lattice parameters a = 6.9088(7) Å, b = 7.0135(17) Å, c = 11.7979(19) Å and V = 571.6(2) ų (Z = 8). Shosanbetsuite is orthorhombic, Pmmn, with lattice parameters a = 5.986(8) Å, b = 4.779(3) Å, c = 5.156(6) Å and V = 147.5(3) ų (Z = 2). Rumoiite and shosanbetsuite correspond to the synthetic AuSn₂ and Ag₃Sn phases, respectively. The chemical compositions for aurostibite, anyuiite, yuanjiangite, and native lead, and the unit cell parameters for yuanjiangite and native lead are also reported in this paper. Hydrothermal activity in ultramafic rocks after the formation of gold (electrum) grains may have been involved in the occurrence of Au(Ag)–Sn–Sb–Pb minerals.

The crystal structure of prehnite Ca₂(Al, Fe)(Si₃AlO₁₀)(OH)₂ from Upper New Street Quarry, Paterson, Passaic County, New Jersey, USA was determined by single–crystal X–ray diffraction. The converged structural model in the space group P 2cm [a = 4.63309(9) Å, b = 5.4839(1) Å, c = 18.5100(3) Å, Z = 2] allowed for the investigation of an ordered Si/Al distribution together with the feasible hydrogen positions inferred from the difference Fourier maps.