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Silicates
The silicate group is one of the most diverse groups
of minerals, and provides more gemstones than any other group. Gems of this
group include Emerald, Garnet, Topaz, Aquamarine, Jade, Peridot, Opal,
Zircon, Tourmaline and others. They are usually light, not completely opaque,
and can be dissolved in acids. The group is made up of minerals composed of
varying amounts of silicon and oxygen. In the strictest sense, silicates are
an Oxide because they contain oxygen atoms, but mineralogists normally group
silicates on their own for simplicity. They are based on a tetrahedron
construct using one silicon atom and four oxygen atoms, giving them a base
chemical value of SiO4. The diversity of the group is based on how these
tetrahedrons combine with each other and with other elements.
There are six groups of silicates:
- Tectosilicates [SiO2 - oxygen atoms shared between two
molecules]
- This group is never soft, but always light. It is sometimes
called the "Framework Silicate" because its structure is composed of
interconnected tetrahedrons going outward in all directions forming
an intricate framework like the skeleton of a large building. Members
of this group include Quartz (Amethyst, Citrine, Smokey Quartz, etc),
Sodalite, and Labradorite. The group is broken down further into the
Feldspar, Feldspathoid, Quarts, and Zeolite groups with many other
individual minerals not in the subgroups
- Phyllosilicates [Si2O5 - Oxygen atoms only
partially shared - also known as Disilicates]
- Members of this group are sometimes referred to as sheet
silicates because the silicon and oxygen molecules form sheets held
together by other elements. This loose bonding causes the minerals to
usually be soft and flaky or powdery. Members include Chrysocola, and
Talc. Subgroups include the Clay, Mica and Serpentine groups. Although
Mica is a commonly known mineral in the group of silicates, it is
actually a group of minerals including Muscovite, Biotite, Lepidolite
and others.
- Inosilicates [SiO3, Si3O8,
Si4O11 also known as Metasilicate]
- Inosilicates are sometimes referred to as chain silicates because
of the way the oxygen atoms are shared between molecules to form long
slender chains. These single or double chains form hard and compact
minerals that may be fibrous or prismatic. Inosilicates are grouped as
single-chain (including the Pyroxene subgroup) and double-chain
(including the Amphibole subgroup). Minerals include Rhodonite, Diopside
and Charoite.
- Cyclosilicates [ SiO3 also known as Metasilicate]
- Cyclosilicates, or ring silicates, share the oxygen atoms between
molecules in such a way that the molecules are linked in rings of three,
four, or six tetrahedrons. This formation produces minerals that are
hard, usually with a high luster. They are often formed in elongated
crystals. Major subgroups include Benitoite, Milarite-Osumilite, and
Tourmaline. This group contains a wide assortment of gemstones including
Aquamarine, Emerald, Sugalite, and Elbaite, the members of the Tourmaline
subgroup known commonly by the name of Tourmaline
- Sorosilicates[Si2O7 also known as Pyrosilicates]
- Sorosilicates form unusual hour-glass shaped structures and may
contain chains of other minerals such as aluminum oxide tetrahedrons.
Most minerals of this class are rare and mostly unknown by the public
with the exception of one member of the Epidote subgroup; Tanzanite
(Zoisite).
- Nesosilicates [SiO4 also known as Orthosilicates]
- The Nesosilicate group contains only single, non-combining groups
of tetrahedrons. These minerals are dense, causing them to be hard with
high specific gravity. Subgroups include Datolite, Euclase, Garnet,
Humite, Olivine, Phenakite, Trimerite, Uranophane, and Zircon. Garnet,
Peridot, Sphene, Zircon, and Topaz are all Nesosilicates.
images 1-zircon, 2-amethyst, 3- chrysocola,
4-tourmaline, 5-tanzanite, 6-garnet - - -
images 3 © casual designs / 1,2,4,5&6 © www.theimage.com
Copyright © 2005 by Gem And Mineral Info.com.
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