Silicates

The most abundant elements in the Earth's crust are oxygen (46.6%) and silicon (27.7%). Minerals which combine these two elements are called silicates, and combined they are the most abundant minerals on the Earth. The silicates can be organized in terms of their chemical compositions and their crystal structures (indicated by the existance of cleavage planes). The table below is an example of such organization from Lutgens and Tarbuck.

Mineral
Idealized Formula
Cleavage
Olivine
(Mg,Fe)2SiO4
None
Pyroxene group
(Augite)
(Mg,Fe)SiO3
Two planes at
right angles
Amphibole group
(Hornblende)
Ca2(Fe,Mg)5Si8O22(OH)2
Two planes at
60° and 120°
Micas
Biotite
K(Mg,Fe)3AlSi3O10(OH)2
One plane
Muscovite
KAl2(AlSi3O10)(OH)2
Feldspars
Orthoclase
KAlSi3O8
Two planes at 90°
Plagioclase
(Ca,Na)AlSi3O8
Quartz
SiO2
None

Shipman, et al. comment that about 95% of the continental crust rocks are composed of the two types of feldspar or quartz.

Selection of common minerals
 Index

References
Lutgens & Tarbuck
Ch 2

Shipman, et al.
Ch 21
 
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