Introduction

Although quantum mechanics laid a solid foundation for condensed-matter physics earlier this century, we are now witnessing a new era in that branch of physics in which we begin to understand crystal structures in nonempirical ways. As stressed in the Preface of this volume, this has been particularly fruitful in understanding rock-forming minerals. It is indeed fascinating if we can predict crystal structures of minerals or other materials in general from a knowledge of the chemical composition alone. This also opens up a way to search for novel crystal structures, either in denser or open-structured forms.

The purpose of this chapter is twofold. One is to take silica as an example for elaborating on various crystal structures, obtained from a nonempirical computer-simulation study. Silica comprises the two most abundant elements on the Earth, i.e., silicon and oxygen. Silica, along with silicates, is an important ingredient in rock-forming minerals. Crystal-to-crystal phase transformations, as well as pressure-induced amorphisation, are described.