FLOW IN POLYCRYSTALLINE ICE

Part 2 - Background information

By Chris Wilson and Brett Marmo

2.7 Ice

Two solid phases of water are known to occur at the earth's surface. Water molecules that freeze at normal atmospheric condition and between 0°C and -80°C form a crystalline solid, with hexagonal symmetry, referred to as hexagonal ice or Ice Ih (Petrenko & Whitworth, 1999). Ice below -80°C and above -130°C has a cubic symmetry, and so is referred to as cubic ice or Ice Ic. There are a further eight high pressure polymorphs of ice that have been recognised at pressures greater that 200 MPa (Bridgman 1937). Ice within outlet glaciers at the fringes of the Antarctic ice sheet is exclusively hexagonal ice.

Ice Ih has a wertzite type structure with layers of oxygen atoms arrayed in puckered hexagonal rings that are stacked in a ABAB sequence (Fig. 2.6.1). The concentration of oxygen atoms in these puckered planes forms the basal plane within the crystal. The c-axis of the crystal is perpendicular to the basal plane.

Each oxygen atom must have two hydrogen atoms associated with it. The angle between oxygen atoms within Ice Ih is 109°, thus only a small variation of the H-O-H angle in the gas phase is necessary to accommodate hydrogen in the Ice Ih lattice. The hydrogen atoms lie 1Å from their associated oxygen atoms and 1.76Å from the closest neighbouring oxygen atom. There are six possible configurations of hydrogen atoms around oxygen atoms that satisfy this arrangement.