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

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_Fe^_ Mn_) (TiAl_Cr_Fe^_)_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_Fe^_Mn_)_ (Ti_Al_Cr_Fe^_)_O_4 with a = 8.379 (2) A^^o