The structure analysis of so-called 9CaO·4CrO(3)·Cr(2)O(3) proved it to be the title compound, decacalcium hexakis[chromate(V)] chromate(VI), with the simultaneous presence of unusual chromium oxidation states. The structure determination was carried out on a crystal that had inversion twinning. The Cr(VI)O(4) tetrahedron is situated on a threefold axis and is disordered over two possible orientations that share three O atoms, while the Cr(V)O(4) tetrahedra are in general positions and are ordered. The charge is balanced by Ca(2+) cations, one of which is located on a threefold axis. The Ca(2+) ions are coordinated by six, seven or eight O atoms. The compound is a significant phase in the CaO-CrO(x) system and its formation reduces the refractoriness of calcium-rich compositions in an oxidizing atmosphere.