Silicates

Introduction

  • In chemistry, a silicate is any member of a family of anions consisting of silicon and oxygen.
  •  The family includes orthosilicate, metasilicate, and pyrosilicate.
  • Chain silicate network: (a) silicate tetrahedron, (b) single-row silicate chain, (c) two tetrahedra join together corner-to-corner, (d) & (e) larger structures of silicate tetrahedra. 

Types of Silicates

  • Jade: sodium aluminum silicate, NaAl(SiO3)2 (metasilicate, SiO32−)   * Example of Jade in art: The Jadeite Cabbage or Jadeite Cabbage with Insects, 19th century, jade, 18.7 cm (7.4 in), National Palace Museum, Taipei, Taiwan
  • The formation of Agate is most often from deposition of layers of silica, SiO2, filling voids in volcanic vesicles or other cavities. The layers form in stages with some of new layers providing an alternating color.   * Example of Agate in art: The Meat-shaped Stone, a piece of jasper agate carved into the shape of a Dongpo pork. 19th-century, 7 cm (2.8 in), National Palace Museum, Taipei, Taiwan
  • Carnelian: silica, SiO2   * Example of Carnelian in art: Carnelian scarab, ca. 450 B.C. , Etruscan, carnelian, Metropolitan Museum of Art, New York City
  • Olivine: (Mg2+,Fe2+)2SiO4, Mg2SiO4 and Fe2SiO4, magnesium and iron(II) orthosilicate   * Olivine gemstones are called peridot and chrysolite.
  • Garnet (deep red): Fe3Al2Si3O12 (Fe3Al2(SiO4)3, iron(II)aluminum orthosilicate    * Pyrope: Mg3Al2Si3O12   * Almandine: Fe3Al2Si3O12   * Spessartine: Mn3Al2Si3O12   * Andradite: Ca3Fe2Si3O12   * Grossular: Ca3Al2Si3O12   * Uvarovite: Ca3Cr2Si3O12
  • Topaz: Al₂SiO₄(F,OH)₂, orthosilicate mineral of aluminum and fluorine \n Most topaz is naturally colorless or white but can come in a number of tints due to impurities, usually chromium   * The rarest, and thus most expensive, colors of topaz range from golden yellow to pink-orange and is known as Imperial Topaz or sometimes Precious Topaz.
  • Zircon: ZrSiO₄, zirconium silicate (source of zirconium)   * Zircon occurs in many colors, including reddish brown, yellow, green, blue, gray and colorless.   * The color of zircons can sometimes be changed by heat treatment.   * Common brown zircons can be transformed into colorless and blue zircons by heating to 800 to 1000 °C.   * Pink, red, and purple zircon occurs after hundreds of millions of years, if the crystal has sufficient trace elements to produce color centers.
  • Beryl: Be3Al2Si6O18 (Be3Al2(SiO3)6, metasilicate SiO32−)   * Well-known varieties of beryl include morganite, emerald and aquamarine.   * Three varieties of beryl: morganite, aquamarine and emerald.      * The pink/golden color of morganite is attributed to trace quantities of manganese.      * The pale blue color of aquamarine is attributed to Fe2+.      * Emerald is green beryl is colored by around 2% chromium and sometimes vanadium.
  • Tanzanite: Ca₂Al₃(SiO₄)₃(OH)   * Blue and violet color is caused by small amounts of vanadium

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