TrekGEO > Mineral Trail > Kanto region > Gunma pref. > Magnetite

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Sanbagawa, Fujioka city, Gunma pref., Japan

Source for Iron


Field of view: 20 mm. Magnetite associated with serpentinite. The black grains with a metallic luster are magnetite. The surrounding white to pale green parts are the host serpentinite. The green serpentinite turned white by weathering.

Recovered from serpentinite in an accretionary complex subjected to Greenschist facies high-pressure metamorphism.

Magnetite from this location was described in 1982.

It is considered that magnetite in serpentinite was formed by reacting Fayalite component with SiO2 which released during bastite formation by serpentinization of enstatite in Harzburgite. For example,

3 MgSiO3 (Enstatite) + 2H2O (Water) => Mg3Si2O5(OH)4 (Bastite) + SiO2

24 Fe2SiO4 (Fayalite) + 30 H2O (Water) + 4SiO2 => 14 Fe3Si2O5(OH)4 (Serpentine) + 2 Fe3O4 (Magnetite) + 2 H2

Other localities

  • Chichibu Mine (Hydrothermal Skarn, Granular)
  • Asemi River (High-P metamorphic, Octahedral)
  • Hase Mine (High-P metamorphic, Octahedral)
  • Onara (Serpentinite, Granular)
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