The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists

Brucite serpentinite in Yamabe district, Hokkaido

Jun Suzuki, Yoshio Suzuki

1970 Volume 63 Issue 4 Pages 123-145
Published: April 05, 1970
Released on J-STAGE: August 07, 2008

DOIhttps://doi.org/10.2465/ganko1941.63.123

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The serpentinite body in Yamabe district in central Hokkaido intruded into Jurassic formation. This plutonic body crops out about 2.5km length by 600m maximum width, in which good chrysotile asbestos has been well mined since 1940.
The serpentinite body is traversed by numerous hypabyssal rocks having leucocractic and sometimes melanocratic natures. The representative type of which is trondhjemite.
The hard specimens of serpentinite were collected from fresh outcrops in this body, and they are exmained by optical microscope, X-ray and electron diffraction methods. The most of the rocks are composed of mixture of minor flakes of antigorite, chrysotile, and lizardite. It should be noticed that these serpentinite specimens frequently contain a pretty amount of brucite, and sometimes the brucite occupies about a half amount of the rock. Minute crystals of olivine, pyroxene and opaque minerals are contained in very small quantity. The mineralogical composition varies gradually at every placs within the serpentinite body.
Brucite can be extracted by acetic acid from serpentinite. One of the serpentinite samples contains about 54 percent of brucite by the method. This specimen shows only 18.15 percent of SiO₂ content by chemical analysis, whcih is extremely basic nature. This rock can be call as a brucite serpentinite.
Serpentinite has been generally considered to be formed by addition of water to peridotite. However, this brucite serpentinite cannot be explained as a direct product by serpentinization from original peridotite without any large migration of chemical components. Partial concentration of MgO within serpentinite body may result the localization of large amount of brucite mineral in the body.
Serpentinization to form the serpentinite body in Yamabe district was performed at the depth beneath the present surface. Then the solidifies serpentinite intruded during an orogenic period.

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A consideration on the crystallization process of diopside

Isamu Shinno

1970 Volume 63 Issue 4 Pages 146-159
Published: April 05, 1970
Released on J-STAGE: August 07, 2008

DOIhttps://doi.org/10.2465/ganko1941.63.146

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In the crystallization process of diopside from glass or chemical mixture, a new phase of diopside composition with wollastonite structure, called here Mg-wollastonite, has been found to appear firstly and to transform to diopside. The chemical composition of the new phase was confirmed by the electron microprobe analyses (Fig. 1) and the X-ray powder data (Fig. 4). The unit cell dimensions, calculated density, optical properties and infra-red absorption spectra indicate that the Mg-wollastonite has a chain structure identical with wollastonite.
Crystallization processes of diopside have been examined by DTA and X-ray diffraction methods. In the DTA curve of the glass, we observe an endothermic peak caused probably by glass transition and an exothermic peak by crystallization (Fig. 6). For these facts and microscopic texture, the crystallization from the glass is considered to proceed in the state of super cooled liquid at the glass surface. It is also deduced that asymmetrical shape of the exothermic peak may indicate the existence of a polymorph in the process of crystallization. In the case of crystallization of diopside from the chemical mxiture, a diffusion speed of each component oxide seems play an important role.

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Clinopyroxene megacrysts from Itino-megata volcano, northeastern Japan

Ken-ichiro Aoki, Etsuko Kameyama

1970 Volume 63 Issue 4 Pages 160-165
Published: April 05, 1970
Released on J-STAGE: August 07, 2008

DOIhttps://doi.org/10.2465/ganko1941.63.160

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Clinopyroxene and olivine megacrysts with welldeveloped crystal faces up to 2cm and 6mm in size, respectively occur sporadically in air-fall lapilli tuff and explosion breccia of olivine-bearing titanaugite alkali basalt from Itino-megata volcano. They are accompanied by various types of accidental ultramafic and mafic inclusions derived from upper mantle and lower crust. Two clinopyroxenes have been chemically analysed.
The clinopyroxene megacrysts contain considerable amounts of Ca-Tschermak's component (7.6 and 8.9 per cent, respectively). The clinopyroxene and olivine megacrysts are thought to crystallized from enclosing alkali basalt magma as liquidus phases at a depth of about 20 to 30km in the lower part of the crust in this region, from their mineralogy and chemistry and the experimental work at high pressures and temperatures on natural rock systems.

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