Published online by Cambridge University Press: 01 February 2010
Introduction
There are 12 crystalline forms of ice. At ordinary pressures the stable phase is termed ice I, terminology that followed Tammann's (1900) discovery of high-pressure phases. There are two closely related low-pressure variants: hexagonal ice, denoted Ih, and cubic ice, Ic. Ice Ih is termed ordinary ice whose hexagonal crystal symmetry is reflected in the shape of snow-flakes. Ice Ic is made by depositing water vapor at temperatures lower than about −130 ℃. High-pressure ices are of little interest in relation to geophysical processes on Earth, but constitute the primary materials from which many extra-terrestrial bodies are made. We describe their structure and creep properties in Chapter 8.
In addition to the 12 crystalline forms, there are two amorphous forms. One is termed low-density ice (940 kg m−3 at −196 ℃ at 1 atmosphere) and the other, high-density ice (1170 kg m−3, same conditions). The density of ice Ih is 933 kg m−3 at the same temperature and pressure (Hobbs, 1974). Amorphous ices can be made at low temperatures in five ways (see review by Mishima and Stanley, 1998): by condensing vapor below −160 ℃; by quenching liquid; by compressing ice Ih at −196 ℃; by electron irradiation; and through transformation upon warming from one amorphous state to another. Although once thought to be a nanocrystalline material, amorphous ice is now considered to be truly glassy water. Its mechanical behavior remains to be explored.
In this chapter we address the structure of ice Ih.
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