The Silicates
Building Blocks of The Earth’s Crust

Silicates are the most widespread of the minerals. They are made up of oxygen and silicon the number one and number two most abundant elements in the earth's crust. By themselves they make up over 90% of the weight of the earth’s crust. Most rocks are composed mainly of this class of minerals.

There are two forms of silicate when looking at their chemistry:

  • felsic-The fel stands for feldspar while the sic represents silica. They form in granites and are lighter in weight and color than other silicates because they have less iron and magnesium. Quartz, micas, and the K-feldspars are noteable members of this group.
  • mafic- Ma stands for magnesium and fic is for iron (ferric). This group of silicates usually form in magmas moving up to fill the gap left when tectonic plates are moving away from each other in the sea floor. Basalt and gabbro are of this type. Olivine and pyroxene are also in this group. They are relatively dense and dark They are called ultra mafic. Plagioclase feldspars are mafic silicates that have calcium and sodium as part of their chemical composition.
A more modern approach to classifying silicates is by their structure. This class of minerals uses SiO4 molecules connected as tetrahedrons. A tetrahedron is a triangular based pyramid. The oxygen atoms occupy the corners of the tetrahedron with the silicon atom in the center.The arrangement of this basic shape is the basis for classification. There are six subclasses. They are:
  • Nesosilicates (single tetrahedrons)
    • olivene
    • topaz
    • garnets
    • howelite
    • kayanite
  • Sorosilicates (double tetrahedrons)
    • epidot
    • hemimorphite
  • Inosilicates (single and double chains)
    • Single chain
      • rhodonite
      • jadeite
    • Double chain
      • actinolite
  • Cyclosilicates (rings)
    • Beryl
    • tourmaline
  • Phyllosilicates (sheets)
    • mica
    • biotite
    • talc
    • chrysocolla
  • Tectosilicates (frameworks)
    • quartz
    • feldspars- labradorite, microcline, amazonite

How many of these can you pick out of the chart below?

Silicate Minerals
Felsic Silicate Minerals
Mica quartz Amazonite
Mica
Mica is a metamorphic mineral. The many variations come from the diverse ways it formed. Mica formations are associated with volcanoes and hydrothermal vents.
Quartz
Quartz is one of the most common of all minerals that make up the continental crust. It is found in igneous, metamorphic, and sedimentary rocks.
Amazonite
Amazonite is a beautiful green variety of microcline feldspar. Its chemical formula is KAlSi3O8 , potassium aluminum silicate.
Mafic Silicate Minerals
Olivine Labradorite Biotite Mica
Olivine
Labradorite
Biotite


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