Aluminum Silicates

While alumina, Al₂O₃, and silica, SiO₂, are two most abundant minerals of the earth crust, a the class of minerals containing both silicon and aluminum oxides, aluminum silicates, exist. Many minerals contain aluminum and silicon oxides. For example, three minerals andalusite, sillimanite, and kayanite all have the same chemical composition of Al₂O(SIO₄). Topaz is closely related in chemical composition, Al₂O(SIO₄)(OH,F)₂.

Beryl, Be₃Al₂(Si₆O₁₈, contains rings of (SiO₃)₆ type. It is usually found in cavities of granite. This mineral is also known as aquamarine, a precious stone. This is the main source of Be metal. A closely related mineral is cordierite, Al₃(Mg, Fe)₂(SI₅AlO₁₈).

Layer aluminum silicates such as kaolinite goup of minerals of Al₄(Si₄O₁₀)(OH)₈ is resulted from weathering of feldspar group of minerals, (K, Na)(AlSi₃O₈, or Ca(Al₂Si₂O₈. The koalinite group is an important clay mineral. The picture shown here is the structure of kaolinite. Two layers are shown here, the bottom octahedral layer represent the oxygen atoms and the 6-coordinated Al atoms or ions. Some of these oxygen atoms are shared with 4-coordionated silicon on the top layer, each tetrahedron represent a SiO₄ group. Zeolites are the most important group of aluminum silicates. About 40 natural zeolites are known and 150 zeolites have been synthesized. Jadeite is often confused with nephrite. These are aluminum silicates.