Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 95 , Issue 5
Showing 1-4 articles out of 4 articles from the selected issue
  • Keiji SHINODA, Takaya NAGAI, Nobuyuki AIKAWA
    2000 Volume 95 Issue 5 Pages 65-70
    Published: 2000
    Released: December 13, 2005
    NIR and IR absorption spectra of portlandite were measured to investigate the behavior of hydroxyls in portlandite under pressure with DAC. The IR absorption peak of fundamental OH stretching motion of portlandite at 3645 cm−1 shifted to the lower wavenumber at the rate of −2.1 cm−1/GPa, and disappeared at 16.6 GPa. The secondary broad IR absorption peak at 3400 cm−1 appeared over 8.3 GPa and its absorption was enhanced with applying pressure. The peak broadening of OH together with the absence of H2O molecules (5200 cm−1) during compression suggests pressure-induced amorphization of portlandite. The NIR absorption peak due to overtone of OH stretching motion of portlandite at 7075 cm−1 shifted to the lower wavenumber at the rate of −7.5 cm−1/GPa, which indicates more clearly increasing hydrogen bonding between layers in portlandite than the red shift of the IR peak. Assuming the Morse function as anharmonic potential energy of OH vibration, anharmonic coefficient of OH bond of portlandite against pressure was obtained from the pressure-dependent peak shift of the fundamental and overtone modes. The anharmonicity of OH bond in portlandite is enhanced with increasing pressure.
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  • Masahiko YAMAMOTO, Shinko IMAYOSHI, Hirotsugu NISHIDO
    2000 Volume 95 Issue 5 Pages 71-78
    Published: 2000
    Released: December 13, 2005
    The Tsubashiki diorite body in the western part of Yamaguchi Prefecture, Southwest Japan, is a small stock, with comagmatic dikes of various rocks. It is composed of melanocratic hornblende diorite and mesocratic hornblende quartz diorite, with hornblende-pyroxene diorite intruding them. Comagmatic mafic and felsic dikes are accompanied with the diorite stock. The mafic dikes are composed of pyroxene andesite, hornblende andesite and hornblende porphyrite. The felsic dikes are mainly composed of hornblende-bearing rocks of tonalite, granodiorite and granite, with minor quartz porphyry. Enclaves of andesitic and dioritic rocks occur in some felsic dikes. The SiO2 content ranges from 52 to 59 wt.% in the diorite stock, from 55 to 56 wt.% in the mafic dikes, from 60 to 74 wt.% in the felsic dikes and from 55 to 59 wt.% in the andesitic enclaves. The diorite stock, mafic dikes and the andesitic enclaves are rich in K2O, while the felsic dikes are poor in K2O and rich in Na2O. Whole rock K-Ar ages are 82.8±1.8 Ma for the hornblende-pyroxene diorite in the diorite stock and 77.3±1.7 Ma for the biotite-hornblende granodiorite in the felsic dike, indicating the last stage in the Sanyo-type plutonic activities during Cretaceous in the western Chugoku district.
        From field relations, modes of occurrence, petrographical features, K-Ar ages and chemical compositions, it is suggested that the Tsubashiki diorite stock was formed by intrusion of a basic andesitic magma under the condition of increase of water and by slight fractional crystallization of ferromagnesian minerals in the hydrous magma. It is also suggested that the mafic dikes were derived from the undifferentiated basic andesitic magma, and that the felsic dikes were formed through filter-pressing differentiation of the residual liquids with magma mixing during the process of cooling of the stock.
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  • Satoshi MATSUBARA, Ritsuro MIYAWAKI, Tokiko TIBA, Hiroyuki IMAI
    2000 Volume 95 Issue 5 Pages 79-83
    Published: 2000
    Released: December 13, 2005
    Tamaite, (Ca, K, Ba, Na)3−4Mn24(Si, Al)40(O, OH)112·21H2O, the Ca-analogue of ganophyllite, is found from the Shiromaru mine, Okutama, Tokyo, Japan. It is monoclinic, P21/a, a=16.64(1), b=27.11(2), c=25.35(2)A, β=98.74(7)°, Z=4. The strongest lines in the X-ray powder diffraction pattern are 12.6 (vvs) (002), 3.46 (m) (027), 3.13 (s) (008), 2.84 (s) (38 -2), 2.69 (vs) (38 -4), 2.60 (s) (602), 2.46 (s) (60 -6), 2.38 (m) (604), 1.623 (m) (98 -2), 1.609 (m) (3 10 11). Electron microprobe and Karl-Fischer analyses gave SiO2 41.23, Al2O3 7.79, FeO 0.16, MnO 35.17, MgO 0.23, CaO 1.94, BaO 2.03, Na2O 0.34, K2O 0.82, H2O 11.07, total 100.78 wt%, and lead to the empirical formula, (Ca1.65K0.83Ba0.63Na0.53)Σ3.64(Mn23.71Mg0.27Fe0.11Al0.12)Σ24.21(Si32.81Al7.19)Σ40.00 O95.27(OH)16.73·21H2O on the basis of O=133. It is transparent and colorless to pale yellowish brown in color with vitreous to pearly luster. The streak is white and cleavage perfect on {001}. The Mohs' hardness is about 4. The measured and calculated densities are 2.85(5) g/cm3 and 2.83 g/cm3, respectively. It is colorless in thin section and optically biaxial negative with β=1.612(2) and 2 V<15°. It occurs as veinlets up to 1.5 mm thick composed of micaceous platy crystals under 0.5 mm in diameter and as spotted crystals in celsian-barian orthoclase veinlets cutting host rocks of metamorphosed manganese ore deposit. The mineral is considered to be formed during later stage activity of low grade metamorphism.
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  • Masahide AKASAKA, Yuanyuan ZHENG, Yayoi SUZUKI
    2000 Volume 95 Issue 5 Pages 84-94
    Published: 2000
    Released: December 13, 2005
    In order to determine the maximum solubility of strontium in piemontite, hydrothermal syntheses of piemontite in the join Ca2Al2Mn3+Si3O12(OH)-Sr2Al2Mn3+Si3O12(OH) were attempted at pressures of 200-300 MPa, temperatures of 500-600°C, and fO2 defined by Mn2O3-MnO2 and CuO-Cu2O buffers. Ca-piemontite crystallized as almost a single phase from a starting material of Ca2Al2Mn3+Si3O12.5-oxide mixture with excess H2O. Sr-bearing piemontites crystallized from starting materials containing both Ca2Al2Mn3+Si3O12(OH)- and Sr2Al2Mn3+Si3O12(OH)-components. They were accompanied by slawsonite (SrAl2Si2O8), bixbyite (Mn2O3) and other minor phases. Piemontite did not crystallize from Sr2Al2Mn3+Si3O12.5-oxide mixture with excess H2O at 300 MPa and 550°C; slawsonite+SrSiO3+SrSi2O5·H2O+Al2O3+bixbyite assemblage was formed instead.
        Synthesized piemontites had variable Sr contents between grains even in the same run product. But average Sr content of the synthesized piemontite increases with increasing Sr content in the starting material from Ca : Sr=2 : 0 to Ca : Sr=1 : 1. The Sr content of piemontite crystallized from starting material with Ca : Sr=1 : 1 reaches 19.65 wt% SrO, which corresponds to about one Sr2+ atom per formula unit (O12(OH)), but piemontite containing more than one Sr atom per formula unit did not crystallize from starting material with Ca : Sr=0.8 : 1.2. The a- and c-dimensions and cell volume increase with increasing Sr content of the starting material from Ca : Sr=2 : 0 to 1 : 1, and reach the upper limit at Ca : Sr=1 : 1. These changes in cell parameters and chemical compositions of the synthesized Sr-bearing piemontite indicate that Sr substitutes for Ca up to one atom per formula unit.
        The results show that the maximum content of Sr in piemontite is limited to one atom per formula unit, and are in agreement with the published view that Sr2+-ions occupy only the ten-coordinated A(2) site in natural Sr-bearing piemontites.
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