- © The Mineralogical Society Of America
Ice has a very rich phase diagram (Petrenko and Whitworth 1999) as shown in Figure 1⇓. Up to now, sixteen crystalline phases have been identified experimentally. All crystalline phases except ice X consist of water molecules connected to four adjacent ones by hydrogen bonds. Broadly speaking, the crystalline phases of ice can be described as follows: the low-pressure forms (P < ~1 GPa) consist of a unique hydrogen-bond network (1HBN). The high-pressure forms (~1 GPa < P < ~80 GPa) are made up of two interpenetrating networks (2HBN); they are often referred to as self-clathrates. Ice Ih and XI are prototypical forms of low-pressure ice with low density, and ice VII and VIII are those of high-pressure ices with high density. Figure 2⇓ shows atomic structures of ice XI and VIII, hydrogen-ordered phases. Between these prototypical phases, there are many complex phases.