Silicon is, next to oxygen, the most abundant clement in the earth's crust which occurs mostly in the form of oxides. Silicon dioxide (silica), Si02, occurs in nature in three different crystal forms: as the minerals quartz (hexagonal), cristobalite (cubic), and tridymitc (hexagonal). Quartz is the most widespread of these minerals; it occurs in many deposits as well-formed crystals, and also as a crystalline constituent of many rocks, such as granite. It is a hard, colourless substance. Its crystals arc identified as right-handed or left-handed by their face development
and also by the direction in which they rotate the plane of polarization of polarized light.
The structure of quartz is closcly related to that of silicic acid, H4Si04. In this acid silicon has ligancy 4, and is surrounded by a tetrahedron or four oxygen atoms, with one hydrogen atom attached to each oxygen atom. Silicic acid, which is a very weak acid, has the property of undergoing condensation very readily, with elimination of water. If each of the four hydroxyl groups of a silicic acid molcculc condenses with a similar hydroxyl group of an adjaccnt molecule, eliminating water, a structure is obtained in which the silicon atom is bonded to four surrounding silicon atoms by silicon-oxygcn-silicon bonds. This proccss leads to a condensation product with formula Si02, sincc each silicon atom is surrounded by four oxygen atoms, and each oxygen atom serves as a neighbour to two silicon atoms. The structure of quartz and of the other forms of silica is described as consisting of Si04 tetrahedra, and cach oxygen atom is a corncr of two of these tetrahedra. In order to break a crystal of quartz it is ncccssary to break some silicon-oxygcn bonds. In this way the structure of quartz accounts for the hardness of the mineral.
Cristobalitc and tridymitc are similarly made from Si04 tetrahedra fused together by sharing oxygen atoms, with, however, different arrangements of the tetrahedra in spacc from that of quartz. Tridymitc resembles ordinary icc in stmcturc, with silicon atoms in the oxygen-atom positions; cristobalitc similarly resembles cubic icc.
~~
