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Silicates

Silicates are some of the most abundant minerals on Earth. They are some of the most common raw material that takes over 75% of the Earth's crust. A majority of the igneous rocks and sedimentary rocks are made of silicate minerals. The most common type of silicate is (SiO4)4-.

There are many different types of silicates. Most of them have a general chemical formula of XxYy(ZzOo)Ww.

  • X = +1 or +2 cations
  • Y = +2, +3, or +4 cations
  • Z =  + 3 or +4 cations
  • O = oxygen
  • W = usually OH-, F-, or Cl-
  • x, y, z, o, w = subscript numbers

Some of the subcategories of silicates are the following:

  • Nesosilicates 
  • Sorosilicates
  • Cyclosilicates
  • Inosilicates 
  • Phyllosilicates
  • Tectosilicates

Nesosilicates

Nesosilicates are made up of units of independent tetrahedral. Some of the minerals that contain nesosilicates are olivine, garnet, zircon, kyanite, topaz, and staurolite. Olivine is important in the processes of igneous rock forming. It has a general formula of (Mg, Fe)2SiO4.  As for garnet, it belongs to the isomorphic group, where it often occurs as dodecahedron crystals, such as pyrope, almandine, and grossularite. It is usually found in metamorphic rocks, and is known for being the January birthstone. Zircon, on the other hand, is marketed as gemstone and is oxidized to produce gemstones that are similar to diamonds known as cubic zirconia. Kyanite is a part of a polymorphic group (Al2OSiO4).                            

Ex. (SiO4)-4 or (Si3O12)-12                                                                                     

Sorosilicates

Sorosilicate is made up of two tetrahedrals shared by an oxygen. Some of the minerals that are classified as sorosilicates are hemimorphite, epidote, and allanite. Hemimorphite is usually found as bladed crystals. Epidote belongs to the isomorphic group, which is important in forming mineral. Lastly, allanite have metamict structure that is usually black with no cleavage.   

ex. (Si2O7)-6

Cyclosilicates

Cyslosilicates are made up of closed ring units of tetrahedral sharing two oxygen atoms. They are known for their hardness and consists a variety of gemstones. They also have poor cleavage. Some minerals that are classified as cyclosilicates are beryl, cordierite, and troumaline. The gemstones that are classified as beryl include emerald (deep green), aquamarine (greenish-blue), and morganite (red). Tourmalines also have a variety of gemstones, which include rubellite (red-pink) and indicolite (dark blue). As for cordierite, it often show dichroism, meaning that it shows different colors  different concentrations.

Ex. (Si6O18)-12                                                                                                                                                                  

Inosilicates                                                        

Inosilicates are made up of continuous double chain units of tetrahedral, each sharing 2 and 3 oxygen. They include the pyroxene group, which are single chain minerals without hydroxide, and the amphibole group, which are double chain with hydroxide. The pyroxene group has two directional 90 degree cleavages. Some examples are enstatite-ferrosilite, diopside-hedenbergite, augite, and spodumene. As for amphibole group, it has two directional cleavages at 124-56 degrees. Some examples are tremolite-actinolite and hornblende. Both of these groups are rock-forming minerals.  

Ex. (SiO3)-2 or (Si2O6)-4

Phyllosilicates

Phyllosilicates comprise continuous sheet units of tetrahedral, each sharing 3 oxygen atoms. They include the clay and mica minerals, which are rock-forming minerals. The clay group is made of hydrous aluminum layered silicates. Some examples are kaolinite and talc. On the other hand, the mica group consists of thin sheets and a multitude of ionic substitutions of Al3+ and Si4+. Some examples are muscovite (light color), biotite (black or dark colored), and lepidolite (pink colored and a source of lithium). There is also the serpentine group that belongs to the phyllosilicates. Some examples are serpentine and crysotile.   

Ex. (Si2O5)-2 or (Al Si3O10)-5

Tectosilicates

Tectosilicates consists of continuous framework of tetrahedrals, each sharing all 4 oxygen atoms. Its structure has a great amount of Al-Si substitution. Some of the groups that are classified as tectosilicates are SiO2 polymorphic group, K-feldspar polymorphic group, feldspathoid group and zeolite group. SiO2 polymorphic group has a variety of quartz, such as smoky quartz, amethyst, and jasper. Some minerals in the K-felspar polymorphic group include orthoclase and microcline. Microcline has 1 Pb2+ ion replaced for every 2 K1+ ion, showing an omission solid solution and causing a blue green color in the mineral. The felspathoid group minerals are similar to feldspars but only have two-thirds of the amount of silica; they form a silica deficient magma. Some examples of it are leucite and sodalite. Lastly, the zeolite group has hydrous silicates with ionic exchange and absorption properties that can act as water softeners by exchanging Na1+ ion for Ca2+ ion in solution. An example of it is this:

\[ Na_2Al_2Si_3O_{10}-2H_2O \rightarrow CaAl_2Si_3O_{10}-2H_2O. \]

Ex. (SiO2), (AlSiO4)-1, (Al2Si2O8)-2, or (Al2Si4O12)-2

References

  1. "Garnet." USGS. N.p., 17 July 2002. Web. 28 May 2012. <http://minerals.usgs.gov/minerals/pu...95/garnet.html>.
  2. "Olivine." UND:The University of North Dakota. N.p., n.d. Web. 28 May 2012. <http://www.und.nodak.edu/instruct/mi...in/olivine.htm>.
  3. "Silicate Mineral Class." Missouri State University. N.p., 11 October 2011 . Web. 28 May 2012. <http://courses.missouristate.edu/EMa...silicates.html>.
  4. "Zirconium and Hafnium." USGS. N.p., 24 January 2012. Web. 28 May 2012. <http://minerals.usgs.gov/minerals/pu...ity/zirconium/>.