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Silicates

  • Page ID
    164528
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    Silicates are some of the most abundant minerals on Earth. They are some of the most common of the raw material that takes over 75% of the Earth's crust. A majority of 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 tetrahedrals. 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. Garnet 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 minerals. Lastly, allanite has a metamict structure that is usually black with no cleavage.

    ex. (Si2O7)-6

    Cyclosilicates

    Cyclosilicates are made up of closed ring units of tetrahedrals sharing two oxygen atoms. They are known for their hardness and consist of a variety of gemstones. They also have poor cleavage. Some minerals that are classified as cyclosilicates are beryl, cordierite, and tourmaline. 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 at different concentrations.

    Ex. (Si6O18)-12

    Inosilicates

    Inosilicates are made up of continuous double chain units of tetrahedrals, each sharing 2 and 3 oxygens. They include the pyroxene group, which are single chain minerals without hydroxides, and the amphibole group, which are double chains with hydroxides. The pyroxene group has two directional 90 degree cleavages. Some examples are enstatite-ferrosilite, diopside-hedenbergite, augite, and spodumene. As for the 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 tetrahedrals, 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). The serpentine group also belongs to the phyllosilicates. Some examples are serpentine and crysotile.

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

    Tectosilicates

    Tectosilicates consist of a continuous framework of tetrahedrals, each sharing all 4 oxygen atoms. Their structure has a great amount of Al-Si substitution. Some of the groups that are classified as tectosilicates are the SiO2 polymorphic group, the K-feldspar polymorphic group, the feldspathoid group, and the zeolite group. The 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 replacing every 2 K1+ ions, 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 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+ ions for Ca2+ ions in solution. For example:

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

    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. <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/>.

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