Note on rock-forming minerals in the Joetsu district, Niigata Prefecture, Japan (8) : Chromite in serpentinite from the Kotaki-Omi district

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The serpentinite melange including crystalline schist is distributed in the Omi-Kotaki district, Niigata Prefecture. Chromite and chromian spinel occur in the serpentinite. Three generations of chromite and chromian spinel were distinguished petrographically and chemically. Type I chromian spinel appears as xenolith, Type II chromite with low Al content occurs as a fine grained rectangle and square crystal in the serpentinite. Type III chromite and magnetite are as aggregates and a fine grained crystal. The Type I chromian spinel of the xenolith (CX-1) has the formula (Mg_<0.75>Fe^<2+>_<0.25> Mn_<0.01>) <1.01> (Ti<0.01>Al_<0.96>Cr_<0.94>Fe^<3+>_<0.08>)_<1.99>O_4 and unit cell parameter a = 8.240 (1) A^^o. The structure of the fine grained chromite (OM-01) in serpentinite is (Mg_<0.19>Fe^<2+>_<0.80>Mn_<0.03>)_<1.02> (Ti_<0.02>Al_<0.25>Cr_<1.51>Fe^<3+>_<0.20>)_<1.98>O_4 with a = 8.379 (2) A^^o.
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