Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 103 , Issue 6
December
Showing 1-7 articles out of 7 articles from the selected issue
ORIGINAL ARTICLES
  • Hidehiko SHIMAZAKI, Ritsuro MIYAWAKI, Kazumi YOKOYAMA, Satoshi MATSUBA ...
    2008 Volume 103 Issue 6 Pages 385-389
    Published: 2008
    Released: January 10, 2009
    JOURNALS FREE ACCESS
    A new occurrence of dellaite, a very rare mineral, at the Akagane mine, Japan, was confirmed and its chemical composition and XRD data for the natural material are reported for the first time in this paper. The mineral occurs in the high-temperature skarn of bicchulite and vesuvianite as a veinlet or interstitial constituent. Results of electron microprobe analysis indicated that the chemical composition of dellaite is almost identical to Ca6Si3O11(OH)2, and only a small amount of chlorine substitutes (OH). The six strongest lines in the powder XRD pattern are as follows: 3.07 (53) 0-22, 2.99 (38) 022, 2.86 (44) -2-11, 2.82 (100) 20-3, 2.55 (51) 005, and 2.29 (38) 030. The lattice parameters refined from the powder XRD data are as follows: a = 6.815(4), b = 6.937(3), and c = 12.890(6) Å; α = 90.71(4), β = 97.68(4), γ = 98.20(4)°, and V = 597.4(5) Å3. The indices are α = 1.655 and γ = 1.664, and optically biaxial; negative with 2V = large (>70°). Dellaite appears to be a later-stage hydration product of pre-existing skarn mineral(s) under a silica-deficient environment.
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  • Maki HAMADA, Shizue SETO, Masahide AKASAKA, Akira TAKASU
    2008 Volume 103 Issue 6 Pages 390-399
    Published: 2008
    Released: January 10, 2009
    JOURNALS FREE ACCESS
    Chromian pumpellyite and associated chromian minerals occur in basic schists in the Osayama ultramafic body in Okayama Prefecture, southwest Japan. The minerals were investigated to examine their chromium contents and mode of occurrence. The chromian minerals occur in basic schist blocks within the ultramafic body, but have not been found in basic schists adjacent to the ultramafic body. Three mineral assemblages were recognized in the basic schist blocks from the ultramafic body: (1) tremolite + chromian pumpellyite + chromian phengite + chromian chlorite + relic chromite; (2) amphibole + chromian chlorite + chromian omphacite + chromian pumpellyite + titanite; (3) diopside + chromian chlorite + chromian vesuvianite + chromian garnet + stilpnomelane + titanite + ilmenite ± relic chromite. Chromian pumpellyite, chromian phengite, chromian chlorite and relic chromite in assemblage (1) form domains in a matrix consisting of fine prismatic tremolite crystals. In assemblage (2), chromian pumpellyite occurs within chromian chlorite domains, and chromian omphacite is distributed along their margins. In assemblage (3), chromian vesuvianite, chromian garnet and stilpnomelane form veinlets along fine fractures within chromian chlorite domains.
         Chromian pumpellyite is reddish gray and twinned. The Cr distribution is extremely heterogeneous, and Cr2O3 content is highest around chromite, reaching 13.3 wt%. However, Fe distribution is relatively uniform, and is not related to the distribution of chromite. On the basis of the distribution coefficient of Cr between the X and Y sites determined by Nagashima and Akasaka (2007), the chemical formula of the chromian pumpellyite with greatest Cr2O3 content is calculated to be (K0.02Na0.19Ca7.86)Σ8.07(Mn2+0.05Fe2+0.35Ni0.01Mg1.71Cr1.13Al0.76)Σ4.01(Cr2.34V0.01Al5.65)Σ8.00(Al0.16Si11.84)Σ12.00O41.66(OH)14.34.
         The chromian mineral domains often contain relic chromite, and regions rich in Cr are confirmed to the vicinity of these chromite grains. These features suggest that chromite is the source of the Cr in the chromian metamorphic minerals, and that metamorphic fluid caused dissolution of chromite and diffusion of Cr into the surrounding metamorphic minerals. The bulk compositions of the basic schist blocks in the ultramafic body suggest their protoliths were ultrabasic rocks of picrobasaltic composition.
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  • Tetsuo MINAKAWA, Hiroyuki FUKUSHIMA, Daisuke NISHIO-HAMANE, Hiroyuki M ...
    2008 Volume 103 Issue 6 Pages 400-406
    Published: 2008
    Released: January 10, 2009
    JOURNALS FREE ACCESS
    Epidote-(Sr), CaSrAl2Fe3+(Si2O7)(SiO4)(OH), the Sr-analog of epidote, was found in the Nagakawara and Hohnomori deposits at the Ananai mine, Kochi Prefecture, Japan. It occurs in the form of prismatic crystals up to 1 cm in length in the tinzenite veins or the fine crystal aggregates in piemontite breccia. Epidote-(Sr) is optically biaxial negative, α = 1.737(2), β = 1.780 (2), γ = 1.792 (2), and 2Vcalc = 62° and has perfect cleavage parallel to {001}. It exhibits pleochroism—X: pale greenish yellow, and Y and Z: pale reddish brown to brownish pink. Its calculated density is 3.74 g/cm3, and it has a Mohs' hardness of 6.5. The representative empirical formula of epidote-(Sr) from the Nagakawara deposit is (Ca1.10Sr0.90)Σ2.00 (Al1.92Fe3+0.87Mn3+0.20)Σ2.99Si3.01O12(OH) on the basis of OH = 1 and O = 12 per formula unit. The mineral is monoclinic with a space group of P21/m, a = 8.928 (5), b = 5.652 (1), c = 10.244 (5) Å, β = 114.46 (4)°, V = 470.5 (3) Å3, and Z = 2. The strongest seven lines in the X-ray powder diffraction pattern [d in Å (I/I0) (hkl)] were 2.92 (100) (11-3), 2.58 (49) (202), 3.50 (42) (21-1), 2.61 (42) (31-1), 2.72 (41) (013), 2.83 (32) (020), and 3.26 (23) (201). Epidote-(Sr) from the Nagakawara deposit may have been formed from the residual hydrothermal fluid after the crystallization of tinzenite.
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  • Hiroki MASHIMA, Junji AKAI
    2008 Volume 103 Issue 6 Pages 407-411
    Published: 2008
    Released: January 10, 2009
    JOURNALS FREE ACCESS
    Monoclinic (1M) and orthorhombic (2O) polytypes are known to exist in joaquinite group minerals. Strontio-orthojoaquinite [Sr2Ba2(Na,Fe2+)2Ti2Si8O24(O,OH)H2O], a member of the joaquinite group, has been reported previously in the Ohmi-Itoigawa district, Japan. A new polytype (4O), mainly comprising a 2O polytype, was found in mineral grains by transmission electron microscopy (TEM). This four-layer orthorhombic polytype (4O) was also found in mineral grains with the 1M polytype. The TEM observation revealed that the 4O polytype appeared as interstratified bands having a width of ∼ 8 μm that were embedded parallel to the (001) plane in the 2O crystal grains. The unit cell parameters of the 4O polytype are approximately a = 10.6 Å, b = 9.8 Å, c = 44.2 Å, V = 4591.5 Å3, and Z = 8. High-resolution TEM (HRTEM) imaging showed that a unit cell of the 4O polytype can be interpreted as being identical to that of a superstructure having a repeated twinning of two times the size of the 1M unit cell on the (001) plane. The formation conditions of the polytypes were discussed.
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  • Yuka HIRAHARA, Kenji SHUTO
    2008 Volume 103 Issue 6 Pages 412-426
    Published: 2008
    Released: January 10, 2009
    JOURNALS FREE ACCESS
    We identify four stages in the Miocene magmatic activity on Rebun Island, northern Hokkaido, Japan: Stage I (18 Ma), Stage II (13 Ma), Stage III (10 Ma), and Stage IV (<10 Ma). Stages I and II produced small intrusions of dolerite and dacite, respectively. During Stage III, basalt to andesite lavas erupted, while the magmatism of Stage IV formed large intrusions of dolerite, porphyrite, and andesite with dacitic veins. The large variations of major and trace element compositions of Stage III and IV rocks can be explained by considering the fractional crystallization of a basaltic melt; however, the proportion of phases fractionating is different for the two stages. Further, in comparison to Stage IV rocks, Stage III rocks have lower concentrations of incompatible elements and higher Zr/Y ratios with similar Zr/Nb ratios, suggesting higher degree of partial melting of the source material. The initial values of 87Sr/86Sr (SrI) and 143Nd/144Nd (NdI) indicate that the rocks of all stages were derived from a common source material, although Stage IV andesites were apparently formed by assimilation fractional crystallization (AFC) during their intrusion into sedimentary rock.
         Most of the rocks have SrI and NdI values similar to those of basaltic rocks derived from the upwelling of depleted asthenospheric mantle during the spreading of the Japan Sea back-arc basin. In addition, incompatible element signatures obtained for Stage I rock are similar to those obtained for the basalts in the Japan Sea back-arc basin. In contrast, Stage II-IV rocks are similar to the basaltic rocks that originated from subduction-related magmatism in northern Hokkaido.
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  • Masami KANZAKI
    2008 Volume 103 Issue 6 Pages 427-428
    Published: 2008
    Released: January 10, 2009
    JOURNALS FREE ACCESS
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  • Yoshio KONO, Hiroaki OHFUJI, Yuji HIGO, Akihiro YAMADA, Toru INOUE, Te ...
    2008 Volume 103 Issue 6 Pages 429-431
    Published: 2008
    Released: January 10, 2009
    JOURNALS FREE ACCESS
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