INTRODUCTION

With the exception of volcanic glasses and a few rare rock types, all igneous and metamorphic rocks are composed of crystalline mineral grains. The compositions of these grains are readily explained in terms of thermodynamic phase equilibria, but their growth is controlled by kinetic factors that are still poorly understood. The texture of a rock, which bears testimony to the rock's origin, is determined largely by crystal growth. Grain size, which is also determined by crystal growth, affects the rheology of rocks and, thus, plays important roles in determining flow rates in the mantle, which in turn, affect globally so many other geological processes through plate tectonics. Investigating crystal growth in anything other than a vapor or low-temperature aqueous solution at atmospheric pressure is experimentally difficult. A few studies, however, have shed significant light on high-temperature crystal growth of silicates that operate on the timescale typical of cooling lavas (see reviews by Kirkpatrick, 1975; Lofgren, 1980; Cashman, 1990; and Sunagawa, 1992). Growth rates in plutonic igneous rocks and metamorphic rocks, however, are so slow that they will probably always remain outside the reach of the experimentalist. Only through understanding the principles are we likely to gain insight into these slower growth processes (Dowty, 1980; Brandeis et al., 1984). Considerably more is now being learned about the growth of crystals through the use of a variety of tools including, in addition to the optical microscope, the electron microscope, X-ray and cathode luminescence topography, and electron and ion beam microanalysis.