A jadeitite block probably derived from serpentines of the Osayama ultramafic body in the Renge metamorphic belt is described petrographically．The jadeitite（Osa-1）consists mostly of jadeite，with subordinate omphacite，diopside，wollastonite，pectolite，prehnite，grossular，calcite，vesuvianite，Naamphibole，Na-Caamphibole，phlogopite，serpentine，chlorite and chromite．The clinopyroxenes in Osa-1 are classified into jadeite，omphacite and diopside．The chemical compositions of clinopyroxenes depend on the mode of their occurrence．Clinopyroxenes forming columnar crystals in the massive jadeitite portions consist of jadeite（Jd_<84-100>），and columnar or acicular crystals in fracture zones are omphacite（Jd_<32-45>）．Clinopyroxenes occurring in veins are classified into jadeite-omphacite（Jd_<74-83>），omphacite（Jd_<41-54>）and diopside（Jd_<0-9>）．Clinopyroxenes occurring as aggregates of acicular crystals are diopside（Jd_<0-2>）．
Ca-rich minerals such as diopside，prehnite，vesuvianite，calcite，wollastonite，pectolite and grossular occur within veins or along cracks in a matrix consisting mostly of jadeite．This suggests that infiltration of Ca-rich fruid occurred at the later stage of a formation of the jadeitites within these serpentinite body．