Magnesites obtained at six most important mines near Daisekkyo, South Manchuria, were ground and pressed into tablettes. They were heated for an hour at seven constant temperatures ranging from 1200° to 1800°C in a Tamman kiln, and then were examined for microstructures: (1) The chief phases observed were periclase, amorphous magnesia, forsterite, and clinoenstatite. The identification of spinel, fayalite, monticellite, diopside, ceylonite, and chlorospinel which must or may occur in them as crystals or solid solutions was pretty difficult. (2) The volume percentages of olivine (Ma₂SiO₄, Fe₂SiO₄ and CaMgSiO₄), spinel, and periclase calculated from the chemical compositions of the magnesites agreeded well with the results of the microscopic observations. (3) The magnesites underwent very marked changes at the intervals of 1200-1300° and 1600-1700°C. At 1200-1300°, clinoenstatite combinded with magnesia and changed almost completely to forsterite, and the amorphous magnesia changed to periclase crystals. At 1600-1700°, forsterite melted and filled the voids of the periclase crystals, developing the so-called poikilitic structure. (4) The more silica contained in the magnesites, the greater the rate of recrystallization.
Six powdered magnesites and twelve mixtures consisting of 100 parts of the powdered Seizanhai magnesite and 1 to 4 parts of silica, alumina, ferric oxide, chromic oxide, manganese dioxide, or oxide of nickel or cobalt were calcined at 1000°, formed into tablettes and fired to cones 26, 28 and 30 in a gas-fired kiln; Then their thin sections were observed under a microscope: (1) The microstructures of the magnesites without admixture resembled to those fired at 1500° in the electric kiln. (2) The introduction of silica favored the development of forsterite- and periclase crystals. (3) The addition of ferric oxide aided the development of periclase, and caused the formation of minute crystals of magnesioferrite. (4) The chromic oxide seemed to dissolve in forsterite. (5) The alumina favored the development of periclase, and produced a poikilitic structure, probably due to the formation of cordierite which melts at a comparatively low temperature.

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