Structural Types 

A number of common minerals, called orthosilicates, contain isolated 
silica tetrahedra. Examples are the OLIVINES, which include forsterite 
and the GARNET femily, of which the most common is almandine. A few 
minerals are made up of double silicate groups (the commonest, 
EPIDOTE, also has single tetrahedra), and a few of silicate rings. 
Among the latter, the best-known is BERYL, which has six-sided rings 
of six tetrahedra. The gems EMERALD and aquamarine are varieties of 
beryl. 

Chain silicates, an important and diverse group, include the PYROXENES 
and pyroxenoids, the AMPHIBOLES, and band silicates. These minerals 
contain other elements as well; almost all include magnesium and iron. 
They are common and characteristic minerals of many igneous and 
metamorphic rocks. 

Sheet silicates are widespread, for they include kaolin and other 
clays (see CLAY MINERALS), MICAS, and TALC. Some, like the clays, 
occur in microscopic crystals. Those that occur in large crystals, 
such as the micas, have perfect cleavage, or splitting, parallel to 
the sheets of silica tetrahedra. 

In framework silicates, every oxygen is shared between two tetrahedra, 
forming three-dimensional networks. The commonest of these, and 
perhaps the most abundant of all minerals, is quartz. Most other 
framework silicates contain some aluminum substituting for silicon, 
with interstitial ions commonly of calcium, sodium, or potassium. 
These minerals include the FELDSPARS (the most widespread of all 
mineral families), the FELDSPATHOIDS (found in silica-poor and 
alkali-rich rocks), and the ZEOLITES, which occur in pockets in lava 
flows or form by the alteration of volcanic ash in arid regions. PETER 
B. LEAVENS 

Bibliography: Ahrens, L. H., et al., eds., Physics and Chemistry of 
the Earth, vol. 3 (1959); Bloss, F. Donald, Crystal Chemistry (1971); 
Deer, W. A., et al., Rock-Forming Minerals, vol. 2A, 2d ed. (1979); 
Eitel, Wilhelm, ed., Silicate Science, 4 vols. (1965, 1966); Iler, 
Ralph K., The Chemistry of Silica (1979).