Chromian diopside with high K content up to 0.56 wt% was discovered from the Osayama serpentinite melange, SW Japan (Tsujimori and Liou, 2005). K-bearing clinopyroxene (K-Cpx) fills hydrothermal micro-veins together with uvarovite within albite vein of a tremolite rock. K-Cpx consists mainly of kosmochlor + augite (92-98%; Ko19-38Aug56-76) components and minor amounts of jadeite (0-6%), aegirine (0-5%), Ca-Tschermak (0-3%), and K-kosmochlor (0-2%). K correlates with Na and Cr, indicating a simultaneous enrichment of K for Na and Cr during pyroxene growth. The incorporation of K into the kosmochlor_-_diopside series solid solution with at least 0.2 Cr cation p.f.u. is possible even at low P conditions.