AbstractDiopsidite, mainly composed of diopsidic clinopyroxene (diopside for simplicity), was precipitated from a high‐temperature hydrothermal solution, rich in silicate components mainly in the mantle peridotite. We present here a new type of diopsidite in the uppermost mantle section, which contains Cr‐rich minerals such as chromite, Cr‐rich diopside (<2.5 wt.% Cr2O3) and Cr‐rich grossular (<7 wt.% Cr2O3). The chromites are varied in shape, subhedral to anhedral, and sometimes contain globular inclusions of grossular and chlorite. Film‐like chromite branches off from coarse partially dissolved chromite grains, filling diopside grain boundaries. The diopside is Cr‐rich solely around the chromite seam or chromite concentrate, but is almost free from Cr elsewhere. The diopsidite is surrounded by a whitish rock, which is composed of tremolite with a trace amount of chromite, diopside and grossular. The coarse chromite grain (<1 mm) in the whitish rock is highly vermicular in shape. The petrological and chemical characteristics of the mantle diopsidite suggest that a hydrothermal solution had collected Cr by partial to complete dissolution of chromite when replacing the mantle peridotite, and possibly transported Cr upward to the crust. Abundances of trace elements (rare earth elements(REE), Sr, Zr, Ti and Y) in the mantle diopsidite indicate their mobility in a hydrothermal solution together with Cr.