Comparison of crystalline SiO2 and amorphous SiO2. Quartz is crystalline due to
orderedness in bonding and repeating pattern whereas glass is amorphous lacking a well ordered structure and repetition of bonds.
A solid substance with its atoms held apart at equilibrium spacing, but with no long–range periodicity in atom location in its structure is an amorphous solid.
Examples of amorphous solids are glass and some types of plastic. They are sometimes described as super cooled liquids because their molecules are arranged in a random manner somewhat as in the liquid state. For example, glass is commonly made from silicon dioxide or quartz sand, which has a crystalline structure. When the sand is melted and the liquid is cooled rapidly enough to avoid crystallization, an amorphous solid called a glass is formed. Amorphous solids do not show a sharp phase change from solid to liquid at a definite melting point, but rather soften gradually when they are heated. The physical properties of amorphous solids are identical in all directions along any axis so they are said to have isotropic properties.
Lack of long range translational periodicity makes glass (SiO2) an amorphous solid.
An amorphous substance is something that has a random molecular formation in it's natural form (when solidified). Some materials are amorphous and do not form crystals. One characteristic of a non–crystalline substance is that it does not have a distinct, sharp melting point. Rather, these materials soften gradually over a large temperature range. Ordinary glass is an example. Although glass is composed mainly of SiO2, the atoms are not arranged in a network crystal. Glass has often been described as a super cooled liquid. Many plastics (polymers) have combined characteristics; they are partially crystalline and partially amorphous.
In a single crystal, the physical and mechanical properties often differ with orientation. It can be seen from looking at our models of crystalline structure that atoms should be able to slip over one another or distort in relation to one another easier in some directions than others. When the properties of a material are the same in all directions, the material is said to be isotropic. When a material is formed, the grains are usually distorted and elongated in one or more directions which makes the material anisotropic.