Dehydration–induced structural change in ikaite, CaCO₃·6H₂O, is investigated using a low–temperature single–crystal X–ray diffraction study. At −50 °C, the crystal structure of ikaite is monoclinic, of space group C2/c with the unit cell parameters a = 8.8134 (1), b = 8.3108 (1), c = 11.0183 (1) Å, and β = 110.418 (1)°. The measurements were performed in 10 °C steps, revealing a monotonous increase of unit cell volume from 756.3 to 758.0 ų, up to −20 °C. The unit cell volume then jumps to 771.0 ų at −10 °C. The unit cell expands anisotropically along the a–axis followed by the c–axis. The ikaite structure is finally lost at 0 °C, which is a much lower temperature for decomposition than previously reported values. The low temperature decomposition is attributable to the aridity of the sample. The elongation of the O1–O4 intermolecular distance parallel to the (101) plane engenders the substantial increase in the a–axis and c–axis. The two–dimensional molecular sheets composed of the CaCO₃·6H₂O molecules are stacked with hydrogen bondings along the c–axis. The expansion of the c–axis is affected by variations in the hydrogen bondings between the sheets. The intramolecular Ca–O2 and Ca–O5 bond lengths and the intermolecular O1–O5 distance are greatly elongated immediately before the decomposition of ikaite structure. These expansions along the b–axis, however, are offset by the increase in the O2–C–O2 bond angle in the CO₃ geometry, aligned perfectly parallel to the b–axis. The intermolecular angles are maintained as almost constant until the ikaite structure is lost. It can be concluded therefore that the movement of H₂O molecules from the crystal lattice occurs simultaneously because the CaCO₃·6H₂O molecules are stabilized by the hydrogen–bonding network immediately before dehydration.

抄録全体を表示

Coexistence of jadeite (Jd₉₂) and quartz was newly found in an inner segment of a composite–zoned garnet from a metapelite in the southern albite–biotite zone of the Asemi–gawa region, Sanbagawa belt in central Shikoku, Japan. The assemblage of jadeite + quartz in garnet gives a minimum metamorphic pressure of 1.4–1.9 GPa at 500–700 °C, which was significantly higher than that previously proposed for epidote–amphibolite facies conditions in the albite–biotite zone. Garnet includes quartz retaining high residual pressure (Δω₁ values up to 11.7 cm⁻¹), which is comparable to the residual pressure reported in eclogite and metapelite within the eclogite unit in central Shikoku. These results imply that (1) the inner segment of the composite–zoned garnet records prograde recrystallization under high–pressure blueschist to eclogite facies conditions and (2) at least a part of the southern albite–biotite zone of the Asemi–gawa region recrystallized under higher pressure conditions prior to the regional metamorphism from the greenschist to epidote–amphibolite facies that formed the regional thermal structure of the Sanbagawa belt of central Shikoku.

抄録全体を表示

Cuprobismutite group minerals have been discovered for the first time in Japan from the Obari mine, Yamagata Prefecture. Furthermore, many kinds of Bi–sulfosalts and Bi–tellurides have been found from this mine. Cuprobismutite occurs as irregular particles which are intergrown with makovickyite. The empirical formula of cuprobismutite is (Cu_7.71Fe_0.41)_∑8.12(Bi_12.17Ag_1.57Pb_0.07Sb_0.07)_∑13.88(S_23.95Se_0.06)_∑24.01 (based on total atoms = 46). Hodrušhite, kupčíkite and padĕraite occur as euhedral to subhedral blade–shaped crystals with parallel intergrowth. The empirical formulae of these minerals are (Cu_7.64Fe_0.36)_∑8.00(Bi_11.18Cu_0.35Ag_0.26Pb_0.07Sb_0.07)_∑11.93(S_21.98Se_0.09)_∑22.07 (based on total atoms = 42), (Cu_3.56Fe_0.50Zn_0.04)_∑4.10(Bi_4.84Sb_0.03)_∑4.87(S_10.00Se_0.03)_∑10.03 (based on total atoms = 19) and Cu_7.00[(Cu_0.50Ag_0.17)_∑0.67Pb_1.27Bi_11.01Sb_0.05]_∑13.00(S_21.91Se_0.09)_∑22.00 (based on total atoms = 42) for hodrušhite, kupčíkite and padĕraite, respectively. The content of trace elements (Ag, Fe and Pb) in each cuprobismutite group mineral is consistent with the characteristics shown by previous works.

抄録全体を表示

Ferropicrite (FeO* = 14.0 wt% and MgO = 13.9 wt%) and picritic ferrobasalt (FeO* = 14.7 wt% and MgO = 10.8 wt%) lava flows are found near Lalibela in the Oligocene (∼ 30 Ma) Ethiopian large igneous province (LIP) in association with ultratitaniferous transitional basalt and picrite of the second high–Ti (HT2) series. The dominant phenocryst in the studied samples is Mg–rich olivine (up to Fo_88.9) with high CaO contents (to 0.42 wt%) without any kink band structure, indicating that the olivines are crystallized from a magnesian melt. Spinel microphenocrysts and inclusions in olivine are characterized by extremely high Cr# (79–84), moderate Mg# (18–51), moderate Fe³⁺# (11–39) and high TiO₂ (3.6–14.8 wt%). The clinopyroxene phenocrysts are Mg#=74–88, TiO₂ = 0.84–1.82 wt%, and Al₂O₃ = 1.2–3.2 wt% in the cores. The REE contents of clinopyroxenes display enrichment in LREE (La_N/Yb_N = 1.2–1.9) and MREE (Eu_N/Yb_N = 3.3–4.3) relative to HREE. Relative depletion of HFSE (e.g., Nb and Zr) is also observed. In general, these trace element characteristics of clinopyroxene are similar to those of the HT2 basalts. Some of the clinopyroxene crystals show strong reverse zoning with abrupt increase in Cr, Ni and Mg# from core to rim, which may be resulted from reaction of melt with mantle peridotite during magma ascent. Bulk rock chemistry of the studied samples exhibits very low Al₂O₃/TiO₂ (1.8–2.2) and high Zr/Y (8.2–10.2) ratios. These may indicate the important role for garnet during melting and that the lavas were formed by a small degree of partial melting which apparently contradicts to the high Cr# of spinel. In view of low Cr/Al ratio of the bulk rock, the high Cr# of spinel suggests very high temperature of the magma (and the mantle plume). The origin of these ferropicrite and picritic ferrobasalt could be attributed to high pressure partial melting of peridotite–eclogite (pyroxenite) mixture that possibly incorporated recycled oceanic crust components.

抄録全体を表示