Solid-state reaction of CoO with GeO2 has yielded two cobalt metagermanate (CoGeO3) phases. X-ray diffraction shows an orthorhombic form analogous to the orthopyroxene enstalite: a=18.77Å, b=8.99, c=5.35, Pbca; and a monoclinic phase analogous to the diopside-augite clinopyroxenes: a =9.64Å, b = 8.99, c=5.15, β= 101°10’, C2/c. Monoclinic single crystals were grown from the melt. The orthorhombic structure is stable below 1351° C. and the monoclinic form is stable above this temperature. A minor amount of the monoclinic phase coexists metastably down to 1241° C. The orthorhombic form transforms to the monoclinic rapidly and completely above 1351° C. Lengthy runs were unsuccessful in bringing about the reverse transition. CoGeO3 melts congruently at 1377° C.

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