Transformation of ice VIII to amorphous ice by ‘‘melting’’ at low temperature

The melting curve of ice VIII near 25 kbar, which has a positive slope, has been estimated from various thermodynamic data and extrapolated approximately to 60 K at zero pressure. When ice VIII is heated from 77 K and ambient pressure it should, therefore, ‘‘melt,’’ presumably below the glass transition. It has been shown to do so, and transforms to low-density amorphous ice when heated to about 125 K at ambient pressure. Ice I, whose melting curve has a negative slope, is already known to transform to a high-density amorphous ice at 77 K and 10 kbar, and so this and the present transformation are symmetrical equivalents.