Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 95 , Issue 6
August
Showing 1-4 articles out of 4 articles from the selected issue
ORIGINAL ARTICLES
  • Mohammad ZAFAR, Mamoru MURATA, Tahseenullah KHAN, Hiroaki OZAWA, Hiros ...
    2000 Volume 95 Issue 6 Pages 101-106
    Published: 2000
    Released: December 13, 2005
    JOURNALS FREE ACCESS
    Garam Chashma granitic rocks intrude discordantly into the Hindukush metamorphic complex on the southern margin of the Asian plate, Trans-Himalayas, northwestern Pakistan. The rocks are fine to coarse-grained, two-mica leucogranite. K-Ar biotite analyses of four samples from the Garam Chashma pluton yield 18-20 Ma ages. These ages are younger than those of the other post-collisional granitic rocks in Trans-Himalayas of Pakistan and resemble those of the Higher Himalayan granitic rocks.
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  • Takao HIRAJIMA, Daisuke NAKAMURA, Katsushi SHIRAHATA
    2000 Volume 95 Issue 6 Pages 107-112
    Published: 2000
    Released: December 13, 2005
    JOURNALS FREE ACCESS
    Sodic-calcic amphibole closely associated with aegirine-augite was newly found from a sandy hornfels at Mt. Hiei contact aureole, Kyoto, Japan. The amphibole shows a pleochroism of X=colorless, Y=blue and Z=light blue and is winchite with significant amounts of TiO2 (1.5-2.5 wt.%) and of Mg-Fe-Mn-Li amphibole component at B site (ca. 0.1-0.4 cations for O=23 basis). The chemical composition and the coexisting phases of the winchite in this study are similar to those in the alkaline igneous complex.
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  • Puniti SINGH, Makoto ARIMA, Alok K. GUPTA
    2000 Volume 95 Issue 6 Pages 113-124
    Published: 2000
    Released: December 13, 2005
    JOURNALS FREE ACCESS
    The join diopside (Di)-nepheline (Ne) was studied to determine the upper stability limit of melilite (Mel) in nephelinitic magmas under both dry and wet conditions. Experiments at 28 kbar under dry condition show that the join Di-Ne has a pseudoeutectic at 1420°C and Ne70Di30. The solidus is attained at 1220±10°C in the compositional range of Di100-Di40Ne60, but beyond the composition Di40Ne60 toward the nepheline-rich side, the solidus is reached at 1200±10°C. Melilite is absent in the 28 kbar polythermal join. In order to explore the upper stability of melilite in the join Di-Ne under dry condition, subsolidus runs were performed between 15 and 25 kbar at 1000°C. In the run at 15 kbar, melilite coexists with nepheline and diopside but this phase disappears in the runs above 20 kbar. Investigation at 10 kbar in the presence H2O [P(H2O)=P(Total)] shows that the join Di-Ne cuts through the primary phase volumes of diopsidess, nepheline and amphiboless (Amph) with a pseudoeutectic at Di5Ne95 and 680°C. Melilite is not stable in the 10 kbar isobaric-polythermal H2O-bearing join. Variation of phase assemblages between 1 to 10 kbars and under the presence of excess water for an arbitrarily chosen composition Di40Ne60 was also described. The study shows that rocks having melilite nephelinite compositions are represented at pressures above about 18 kbar (under dry condition) by nephelinite, but in the presence of excess water by an amphibole-bearing nephelinites. The study also demonstrates that an olivine-melilite nephelinite and a melilite nephelinite crystallize from an aqueous magma only under volcanic to subvolcanic conditions.
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  • Akira TSUCHIYAMA, Takashi HANAMOTO, Yoshito NAKASHIMA, Tsukasa NAKANO
    2000 Volume 95 Issue 6 Pages 125-137
    Published: 2000
    Released: December 13, 2005
    JOURNALS FREE ACCESS
    CT values (numerical expression for contrast of CT images) of 66 minerals, were measured by using a medical X-ray CT (computed tomography) scanner. These values were compared with the calculated X-ray linear attenuation coefficients of the minerals. The comparison permitted to evaluate the attenuation contrast resolution by polychromatic X-ray beam of the scanner and to estimate the practical restrictions to use the method for mineralogical purposes. Plots of the CT values vs. monochromatic linear attenuation coefficients calculated for different photon energies revealed the existence of the most efficient photon energy giving the best linear correlation. The energies are about 70 and 80 keV for accelerating voltages of 120 and 130 kV, respectively. These values roughly coincide with the peaks of the photon energy spectra of the incoming X-ray beams. The existence of the most efficient energies (hitherto called “effective energies”) shows that CT values can be evaluated quantitatively for minerals in a medical CT scanner with polychromatic X-ray beams. Resolution of the attenuation contrast is about 0.05 cm−1 for these two effective energies in the scanner used. It is expected from the contrast resolution that the degree of Mg-Fe substitution in ferromagnesian silicate minerals is easily estimated if the Fe contents are sufficiently different. However, in the Mg-rich compositional range of ultramafic rocks (XMg≈0.9), olivine and Ca pyroxene may not be discriminated due to the overlap of the CT values. The CaAl-NaSi substitution in plagioclase solid solution is also detectable whereas the CT values of quartz and alkali feldspar are within the plagioclase range.
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