Ice-two (ice II) may be synthesized from hexagonal ice at 198 K
and 300 MPa or by decompressing ice-five (ice V) at 238 K but is
not easily formed by cooling ice-three (ice III) (see Phase
Diagram). It may form a major proportion of icy moons such as
Jupiter's Gannymede Its unit cell, which forms rhombohedral
crystals, (Space group ) is shown opposite. In the crystal, all
water molecules are hydrogen bonded to four others, two as donor
and two as acceptor. Ice-two may exist metastably below ~100 K
between ambient pressure and ~5 GPa. At ambient pressure it
irreversibly transforms into ice Ic above 160 K. As the H-O-H
angle does not vary much from that of the isolated molecule, the
hydrogen bonds are not straight (although shown so in the
figures). Half the open hexagonal channels of ice Ih have
collapsed in ice II. The relationship of the ice II structure to
ice Ih can be visualized by detaching the columns of hexameric
rings, moving them relatively up or down at right angles to
their plane, rotating them abo