Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 101 , Issue 6
December
Showing 1-8 articles out of 8 articles from the selected issue
ORIGINAL ARTICLES
  • Masayuki TAKADA, Isao KUSACHI, Shigetomo KISHI, Mitsuo TANABE, Takashi ...
    2006 Volume 101 Issue 6 Pages 285-288
    Published: 2006
    Released: January 08, 2007
    [Advance publication] Released: November 24, 2006
    JOURNALS FREE ACCESS
    Euhedral crystals of borcarite were found in vugs or fissures of crystal aggregates of nifontovite or pentahydroborite, which occur as irregularly shaped bodies in the crystalline limestone near the gehlenite-spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Interfacial angles of the reference faces of borcarite crystals were measured for each zone using a one-circle goniometer. The crystal basically comprises the {110}, {230}, {201}, {124} and {201} faces, but some crystals also have the {100}, {102}, {101}, {130}, {140}, {180}, {311} and {511} faces. Almost all the mineral is thin plate crystals along the zone [010]. The cleavage is perfect on {100}. Striation can be observed in the direction of [001]. This is the first description of a crystalline form of borcarite.
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  • Yukiyasu TSUTSUMI, Kazumi YOKOYAMA, Kenji HORIE, Kentaro TERADA, Hiros ...
    2006 Volume 101 Issue 6 Pages 289-298
    Published: 2006
    Released: January 08, 2007
    [Advance publication] Released: November 24, 2006
    JOURNALS FREE ACCESS
    We obtained the radiometric ages of detrital zircons from two samples of paragneiss from Oki-Dogo Island, Japan, from the 238U/206Pb ratio and isotopic composition of Pb determined using a Sensitive High-Resolution Ion MicroProbe (SHRIMP II). The zircons show two main age clusters around 2200 Ma and 1800 Ma, with some discordant ages. One zircon grain has a rim with a concordant age of 236 ± 3 Ma, which is consistent with the age of peak metamorphism in the Hida Metamorphic Belt. The modal proportions of zircon ages from Oki-Dogo Island are clearly different from those for paragneiss of the Hida Terrane. This indicates that the Oki Gneiss is derived from an Early Proterozoic rock and is different from the Hida Gneiss in terms of provenance and/or the depositional age of the parent sediment.
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  • Yohey SUZUKI, Takeshi SUKO
    2006 Volume 101 Issue 6 Pages 299-307
    Published: 2006
    Released: January 08, 2007
    [Advance publication] Released: November 24, 2006
    JOURNALS FREE ACCESS
    Understanding the behavior of uranium (U) in the environment is essential not only for the protection of aquifers from U contamination but also for predicting the fate of U and other actinides disposed of in deep geological settings. It has long been believed that the redox chemistry of U can be simply predicted by thermodynamics and that the development of a low redox potential is a sufficient condition for U reduction. However, recent studies have demonstrated that redox transformations of U are controlled by kinetic factors that are strongly influenced by microbial activity. Although abiological U oxidation proceeds efficiently under oxygenic conditions, abiological reduction of U is inhibited by the formation of negatively charged U(VI)-CO3 complexes that prevail in nature. Phylogenetically diverse microorganisms are capable of enzymatically reducing U(VI)-CO3 complexes to form U(IV)-bearing minerals such as uraninite (UO2+x). The only abiological pathway currently known for the reduction of U(VI)-CO3 complexes involves the Fe(II) monohydroxo surface complex ≡FeIIIOFeIIOH0. This complex is mainly produced by the microbial reduction of Fe(III) in natural systems. Thus, U(VI) reduction is controlled both directly and indirectly, at least in part, by microbial activity. Several mechanisms of U oxidation under anoxic conditions have been revealed recently by laboratory and field studies. U(IV) is abiologically oxidized by Fe(III) and Mn(IV) oxides. Microbial reduction of nitrate to molecular nitrogen, which occurs following the depletion of O2, produces nitrogen intermediates including nitrite (NO2), nitrous oxide (NO), and nitric oxide (N2O). Although the nitrogen intermediates oxidize U(IV), poorly crystalline Fe(III)-oxide minerals resulting from the oxidation of aqueous Fe(II) species by the nitrogen intermediates oxidize U(IV) more efficiently than the nitrogen intermediates alone. Remarkably, the formation of Ca-U(VI)-CO3 complexes resulting from increased levels of Ca2+ and/or HCO3 leads to the reoxidation of bioreduced U(IV) under reducing conditions. These geomicrobiological factors pose challenges in manipulating and/or predicting the mobility and fate of U in complex and heterogeneous environmental settings.
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  • Yoshihiro NAKAMUTA, Tomoki NAKAMURA, Noboru NAKAMURA
    2006 Volume 101 Issue 6 Pages 308-318
    Published: 2006
    Released: January 08, 2007
    [Advance publication] Released: November 24, 2006
    JOURNALS FREE ACCESS
    We determined the structural state of six plagioclase crystals from the Kobe meteorite by measuring the separation between the 131 and 131 X-ray diffraction peaks (Δ131 parameter) using a Gandolfi camera. The formation temperature of each plagioclase crystal was then estimated by plotting the Δ131 parameter on the relation diagram between the Δ131 parameters and temperatures of plagioclase synthesis, as proposed by Smith (1972). Three of the crystals record temperatures of just under 700 °C, while the other three crystals record much lower temperatures. The temperature estimates can be regarded as representing the crystallization temperature of plagioclase, and the highest recorded temperature is thought to correspond to the peak temperature of the thermal metamorphism, close to but not exceeding 700 °C. A thermal model of the CK chondrites is proposed on the assumption that thermal metamorphism, with 26Al decay as a heat source, began prior to accretion of the CK parent body. The model explains the homogeneous compositions of olivine and pyroxene in CK chondrites, which are inconsistent with the petrographic type suggested by their grain sizes, and explains the occurrence of silicate darkening in almost all CK chondrites.
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LETTERS
  • Norio YANAGISAWA, Takahiro MATSUMURA
    2006 Volume 101 Issue 6 Pages 319-323
    Published: 2006
    Released: January 08, 2007
    [Advance publication] Released: November 24, 2006
    JOURNALS FREE ACCESS
    A high-frequency electrolysis method was used to reduce the calcium in circulating water by precipitating CaCO3 on the electrodes. The oxidation-reduction potential, pH, and chemical composition were measured during the precipitation of the CaCO3. The calcite precipitated on the electrode cover. The concentration of calcium ions in the solution decreased under reducing conditions, and the precipitation rate of calcite depended on the pH. For initial Ca and HCO3 concentrations of 200 ppm, the maximum precipitation rate of CaCO3 was 2.4 mg/m2/s at pH = 6.8, and the precipitation rate decreased linearly as the pH decreased to pH = 4.6. The precipitation rate depended on the initial concentrations of calcium and HCO3, rather than being regulated by the initial pH using citric acid.
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  • Masaaki OWADA, Yasuhito OSANAI, Tomokazu HOKADA, Nobuhiko NAKANO
    2006 Volume 101 Issue 6 Pages 324-328
    Published: 2006
    Released: January 08, 2007
    [Advance publication] Released: November 24, 2006
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    Garnet-bearing granite (Grt-granite) and high-grade pelitic gneisses occur in the Kannak Complex, southern Kontum Massif, central Vietnam. Electron microprobe (EMP) dating of monazite in Grt-granite and garnet-orthopyroxene gneiss (Grt-Opx gneiss) gave ages of ca. 260 Ma from cores and ca. 230 Ma from thin rims. Since most monazite grains are predominately of core composition, the ca. 260 Ma age indicates the timing of high-temperature metamorphism and the formation of Grt-granite. Grt-granite is locally accompanied by coeval intrusions of fine-grained gabbro, which is a candidate for the heat source leading to high-grade metamorphism and partial melting of the crust during the Late Permian.
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  • Tomoaki MATSUI
    2006 Volume 101 Issue 6 Pages 329-333
    Published: 2006
    Released: January 08, 2007
    [Advance publication] Released: November 24, 2006
    JOURNALS FREE ACCESS
    Plagioclase crystals up to 5 mm long partially replaced by pink-colored smectite, from the Takakumayama Granite, Kagoshima, Japan, were examined using optical microscopy, X-ray diffraction, electron probe microanalysis, and X-ray analytical microscopy. A map of Ca concentration reveals distinct zoning that is probably due to abrupt changes in growth conditions. In terms of zoning, there are three distinct regions within the plagioclase crystal: (1) an unzoned core, (2) an intermediate region between the core and the rim that corresponds to the pink alteration mineral visible in hand specimens, and (3) oscillatory zoning in the rim. The pink alteration mineral was identified as smectite, with a characteristic basal spacing of 1.58 nm under air-dried and untreated conditions. The alteration is associated with defects and dendritic textures that are abundant in the intermediate regions of the crystals. It is postulated that the alteration resulted from the action of externally derived hydrothermal fluids that accessed the defects and dendritic texture in the plagioclase crystals via microfractures that subsequently sealed during cooling of the granite.
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  • Takashi HOSHIDE, Masaaki OBATA, Takashi AKATSUKA
    2006 Volume 101 Issue 6 Pages 334-339
    Published: 2006
    Released: January 08, 2007
    [Advance publication] Released: November 24, 2006
    JOURNALS FREE ACCESS
    The Murotomisaki Gabbro is a sill-like layered igneous complex that contains several layers of olivine enrichment. In our previous paper (Hoshide et al., 2006), we have identified two zones of olivine enrichment: ‘the crystal accumulation zone (AC zone)’, formed by gravity settling and accumulation of olivine crystals, and ‘the crystal growth zone (GR zone)’, in which increase of modal olivine was caused by crystal growth of olivine and not by crystal accumulation.
         Based on whole rock compositional data, we have found that the AC zone rocks define a linear compositional trend (termed as ‘AC trend’) which is consistent with the crystal settling and accumulation hypothesis. However the GR zone data define another linear trend with a slope different from that of the AC trend. Moreover, the compositions of the coarse gabbros and the upper olivine gabbros that occur above the GR zone and an anorthosite vein from the GR zone roughly lie on the same trend, but on the opposite side of the GR zone composition, defining the ‘GR trend’ as a whole. Some anorthositic veins and wavy pegmatitic veins have plume-like structures, suggesting that these veins are remnant of crystal mushes that have been mobilized and ascended diapirically during magmatic differentiation.
         Considering the observed compositional relationships and the mode of occurrences of the anorthosite and wavy pegmatitic veins, we conclude that the segregation and separation of anorthositic material out of semi-solidified crystallization boundary layers was responsible for the formation of the GR zone and the GR trend. Phase equilibrium calculations reveal that the hypothetical anorthosite material was a mixture of fractionated melt and plagioclase crystals that precipitated from the melt. The GR zone represents a residue from the separation of anorthositic crystal mushes and the coarse gabbros and the upper olivine gabbro parts represent mixtures of the crystal mush and the initial melt.
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