Journal of Mineralogical and Petrological Sciences
Online ISSN : 1349-3825
Print ISSN : 1345-6296
ISSN-L : 1345-6296
Volume 105, Issue 2
April
Displaying 1-8 of 8 articles from this issue
ORIGINAL ARTICLES
  • Satoshi NAKANO, Kuniaki MAKINO
    2010 Volume 105 Issue 2 Pages 45-56
    Published: 2010
    Released on J-STAGE: May 09, 2010
    Advance online publication: February 02, 2010
    JOURNAL FREE ACCESS
    This is the first detailed description of amazonitic alkali feldspar in Japan. The amazonitic feldspar is from pegmatite in the Tanakami Granite, southwest Japan. Macroscopically, the feldspar is characterized by the coexistence of pale blue and white parts. The two parts of microperthite are distinctly different in terms of their microscopic texture. The pale blue part has clear crosshatched twin patterns, albite laths, and few albite patches with relatively few micropores. In contrast, the white part has unclear twin patterns, few albite laths, and many albite patches with abundant micropores. The pale blue color is due to the host microcline containing 103 ppm of PbO at the maximum, and the white color is due to both the two albite phases and turbid microcline causing strong diffuse reflection. It is deduced that plagioclase laths were first crystallized and then enclosed by pale blue microcline that crystallized later. Crosshatched twins were formed in the microcline during cooling, and albite replaced the microcline forming perthitic patches with many micropores. The composition of plagioclase laths is very close to that of pure albite, which is same as that of albite patches. The microcline part having undergone extensive albitization appear white to the naked eye; the part that did not undergo albitization has retained its pale blue color. The amazonitic microcline crystallization and albitization are estimated to have occurred at very low temperatures around 200 °C.
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  • Kyoko N. MATSUKAGE, Mami OYA
    2010 Volume 105 Issue 2 Pages 57-69
    Published: 2010
    Released on J-STAGE: May 09, 2010
    Advance online publication: March 04, 2010
    JOURNAL FREE ACCESS
    Supplementary material
    Peridotite xenoliths from the Cameroon volcanic line (CVL), which is a 1600-km-long volcanic chain on the continental margin of West Africa and the Gulf of Guinea, exhibit regional variations in petrography, mineral chemistry, and equilibrium temperature. The xenoliths are mainly composed of spinel lherzolite, spinel harzburgite, dunite, and small amounts of wehrlite and websterite. Lherzolite and harzburgite are similar in texture, and the MgO and Cr2O3 components in these minerals increase with a decrease in the Al2O3 and Na2O components and the modal abundance of clinopyroxene. This feature can be explained by a process of the partial melting of pyrolitic lherzolite and subsequent melt extraction in shallow mantle at a depth of 30-80 km. The Bioko, which is located near the center of the CVL, is distinctive from other regions (Annobon, Principe, Mt. Cameroon, Oku, and Ngaoundere) because of the higher degree of partial melting of residual peridotites. The estimated degree of partial melting in Bioko was about 25-30%, while in other areas it was less than 25%. Bioko was also characterized by the amount of cumulus dunite and wehrlite. The equilibrium temperatures of the xenoliths, as determined by a two-pyroxene geothermometer, were higher at the center of the CVL (e.g., about 1000 °C in Bioko) and gradually decreased toward the margins of the CVL (e.g., they were about 850 °C in Annobon, the margin of the oceanic area). These observations indicate that extensive igneous activity occurred at the uppermost mantle under the Bioko area, near the center of the CVL. Based on our study and previous isotope studies of volcanic rocks, we conclude that the regional variations in the CVL mantle were formed by the upwelling and emplacement of a deep hot plume about 120 Ma, which caused the opening of the Atlantic Ocean.
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LETTERS
  • Ryosuke TAKAHASHI, Isao KUSACHI, Hiroyuki MIURA
    2010 Volume 105 Issue 2 Pages 70-73
    Published: 2010
    Released on J-STAGE: May 09, 2010
    Advance online publication: March 13, 2010
    JOURNAL FREE ACCESS
    The crystal structure of parasibirskite (CaHBO3) obtained at Fuka, Okayama Prefecture, Japan, was determined by Monte Carlo simulation using powder X-ray diffraction data and confirmed by Rietveld refinement. Parasibirskite is monoclinic with space group P21/m and cell constants a = 6.722(4), b = 5.437(2), c = 3.555(2) Å, and β = 93.00(5)°. The calcium atom is surrounded by seven oxygen atoms with a monocapped distorted trigonal prism arrangement. The edge-shared CaO7 polyhedra form a layer structure elongated along the crystallographic b- and c-axes. These CaO7 layers are connected to each other by BO3 triangles, thus forming the parasibirskite structure. The hydrogen atom was presumed to connect to the O1 atom from the bond-valence calculation. Parasibirskite is a dimorph of sibirskite (CaHBO3), which consists of CaO6 double chains connected by BO3 triangles.
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  • Tahmineh PIRNIA, Shoji ARAI, Ghodrat TORABI
    2010 Volume 105 Issue 2 Pages 74-79
    Published: 2010
    Released on J-STAGE: May 09, 2010
    Advance online publication: March 04, 2010
    JOURNAL FREE ACCESS
    Plagioclase lherzolites of Nain mélange, Iran, show peculiar textures that indicate melt impregnation: (1) droplet or bleb-like grains of plagioclase distributed in the peridotite matrix, (2) plagioclase-bearing clinopyroxenite seams, and (3) trails of plagioclase crosscutting pyroxene porphyroclasts. The textural characteristics show post-deformational igneous formation of plagioclase, and possibly, associated clinopyroxene, from the impregnating melt. The melt has precipitated the clinopyroxenite seams and chemically modified all the peridotite minerals. Highly refractory compositions of the precipitated minerals suggest involvement of a highly depleted MORB-like melt. The melt was an increment of partial melt produced by 8% to 10% fractional melting from the MORB source. This is in contrast to the involvement of ordinary MORB in melt impregnation in abyssal plagioclase peridotites. Integration of increments of mantle partial melts to form MORB was possibly incomplete in the very incipient mid-ocean ridge as in the short-lived Nain back-arc basin.
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  • Asel TOGONBAEVA, Akira TAKASU, Michio TAGIRI, Apas B. BAKIROV, Azamat ...
    2010 Volume 105 Issue 2 Pages 80-85
    Published: 2010
    Released on J-STAGE: May 09, 2010
    Advance online publication: February 19, 2010
    JOURNAL FREE ACCESS
    Supplementary material
    Eclogites occurring in the Neldy Formation of the Makbal district in the Kyrgyz Northern Tien-Shan are newly described. The metamorphism of the eclogites is divided into two events, a precursor metamorphic event and a subsequent eclogitic metamorphic event. The metamorphic conditions of the precursor event are represented by the amphibolite facies. The peak metamorphic conditions of the eclogitic event are estimated to be T = 550-610 °C and P = 22-25 kbars. The prograde, peak and retrograde metamorphic conditions define a hairpin-type clockwise P-T path for the Neldy eclogites.
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  • Weimin LI, Akira TAKASU, Yongjiang LIU, Xinzhuan GUO
    2010 Volume 105 Issue 2 Pages 86-91
    Published: 2010
    Released on J-STAGE: May 09, 2010
    Advance online publication: March 15, 2010
    JOURNAL FREE ACCESS
    Supplementary material
    Newly found garnet-barroisite schists from the Heilongjiang Complex in the Yilan area, northeastern China, are petrographically described. They are composed mainly of albite, amphibole, phengite, epidote, garnet, quartz, titanite and magnetite, with minor amounts of stilpnomelane, apatite, hematite, rutile, and ilmenite. The metamorphism of the garnet-barroisite schists are divided into three stages: pre-peak stage of the epidote-blueschist facies; peak stage of the epidote-amphibolite facies (T = 500-540 °C and P = 10-12 kbar); and retrograde stage of the greenschist facies. These results reveal a clockwise high-P/T path for the garnet-barroisite schists in the Heilongjiang Complex.
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  • Hidekazu TANAKA, Shunsuke ENDO, Tetsuo MINAKAWA, Masaki ENAMI, Daisuke ...
    2010 Volume 105 Issue 2 Pages 92-96
    Published: 2010
    Released on J-STAGE: May 09, 2010
    Advance online publication: March 15, 2010
    JOURNAL FREE ACCESS
    Momoiite, (Mn2+,Ca)3(V3+,Al)2Si3O12, an Mn2+- and V3+-rich analog of silicate garnet, was found in three localities: the Kurase (type locality), Hokkejino, and Fujii mines that are respectively located in Ehime, Kyoto, and Fukui Prefectures, Japan. Momoiite is greenish-yellow to emerald-green in color, and it occurs in the form of an aggregate having a size of up to 1 mm in a vein having a thickness of several millimeters with calcite and tephroite. Momoiite is optically isotropic, with n = 1.86(1). Its calculated density is 4.01 g/cm3, and its hardness on the Mohs scale is 6.5. The representative empirical formula of momoiite from the Kurase mine is (Mn1.80Ca1.12Mg0.04)Σ2.96(V1.51Al0.52)Σ2.03Si3.00O12 on the basis of O = 12 per formula unit. The mineral is cubic with a space group of Ia3d, a = 11.9242(7) Å, V = 1695.5(3) Å3, and Z =8. The strongest seven lines in the X-ray powder diffraction pattern [d in Å (I/I0) (hkl)] are 2.982 (78) (400), 2.668 (100) (420), 2.435 (42) (510), 2.178 (20) (521), 1.935 (21) (611), 1.654 (20) (640), and 1.594 (30) (642). A momoiite solid solution substantially exists at relatively low pressure; however, extremely high momoiite content may not be formed in the pressure conditions of ore deposits in the Kurase, Hokkejino, and Fujii mines even if V and Mn are highly concentrated in the ore.
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Mineralogical abstracts from scientific papers published in Japan
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