Chemistry of chromian spinel in picrite sill from Ogi, Sado island, Niigata Prefecture, Japan

概要

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Chromian spinels occur in two textural forms in picrite sill intruded in Neogene volcanic rocks, from Ogi at the southwestern end of the Sado island, Niigata Prefecture. Small (<40μm) euhedral chromian spinels occur as inclusions in altered and fresh olivines throughout the sill. Some chromian spinels and magnetites occur as small phenocrysts and separate grains. There is no difference in composition between chromian spinels occurring as inclusion and phenocryst. Cr/ (Al +Cr) ratio is from 0.38 to 0.57. Fe^<3+>/ (Cr+Al+Fe^<3+>) atomic ratio of chromian spinel is lower than 0.2. The molecule proportion of spinel is more than about 40 %. The sum of the molecule proportions of spinel, chromite and magnesio-chromite is beyond 90 %. The formula of the common chromian spinel was given as (Mg_<0.65>Fe^<2+>_<0.37>Mn_<0.01>)_<1.03> (Ti_<0.03>Al_<1.08>Cr_<0.70>Fe^<3+>_<0.16>)_<1.97>O_<4>.
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