Transformations of Ice Vi and Ice Vii at Atmospheric Pressure

The irreversible transformations that occur when ices VI and VII are heated at atmospheric pressure from liquid-nitrogen temperature have been examined by simple thermal analysis and by X-ray diffraction. They transform first to cubic ice I (ice Ic) which subsequently transforms to hexagonal ice I (ice Ih). The rates of transformation depend on the temperature. The heat evolved is 16 ± ~2 cal/g for ice VI and 31 ± ~4 cal/g for ice VII.