Abstract
Three clinopyroxenes, separated from main dolerite and dolerite pegmatites of the Otaki sill (tholeiite series), Yamagata Prefecture, have been chemically analysed and optically determined. These pyroxenes are an augite (Wo36 En41 Fs23) and two subcalcic ferroaugites (Wo26 En36 Fs38 and Wo18 En34 Fs48). With crystal fractionation Ca+2 and Mg+2 of the augite are progressively replaced by Fe+2, the ratio of Fe+2, distributed between Ca+2 and Mg+2 being 70:30, consegzently the crystallization of the clinopyroxenes takes a course from augite, passing through subcalcic augite, to subcalcic ferroaugite.
