The silicates are by far the largest group of minerals. Minerals composed of molecules containing varying amounts of silicon and oxygen are silicates. It is easy to distinguish silicate minerals as a group, although individual minerals are hard to determine. None are completely opaque, none are fully soluble in acid, and most are light in weight. Because of the great amount of silicates, they are divided based on their atomic structure. The construction component of all silicates is the tetrahedron, which is one silicon atom equidimensionally placed around four oxygen atoms. These tetrahedrons, when combined with other molecules or tetrahedrons form the silicate minerals.
The types of silicates are classified based on the interactive formation of the tetrahedrons. Each group of silicates has a correct ratio of silicon and oxygen, but in some minerals the silicon atoms are replaced by other atoms. An example of this is Orthoclase Feldspar (KAlSi3O8), in the tectosilicate group, which has a ratio of silicon to oxygen of 1:2. The ratio of 1:2 seems incorrect since there are only three silicon atoms for eight oxygen atoms, when in fact there should be four silicon atoms. This is because an aluminum atom takes the place of a silicon atom, thus leaving only three silicon atoms.
The groups of silicates are:
Tectosilicates (framework silicates) [Silicon:Oxygen = 1:2]
In this group of silicates, all four oxygen atoms from each
tetrahedron shares itself with an adjacent tetrahedron. The tectosilicates are compact
minerals and are light in weight. None are soft.
Phyllosilicates
(sheet silicates)
or Disilicates [Silicon:Oxygen = 2:5]
The phyllosilicates have each set of
tetrahedrons surrounded by three oxygen atoms, forming a sheet like structure. The
phyllosilicates have quality basal cleavage, and usually occur in a flaky habit.
Inosilicates
(chain silicates)
or Metasilicate chains [Silicon:Oxygen = 1:3, 3:8, 4:11]
The tetrahedrons in this group form single or multiple chains,
with two oxygen atoms of each tetrahedron part of its neighboring tetrahedron forming
long, thin, chains. These minerals are tough, compact, and hard. They form prismatic and
fibrous crystals.
Cyclosilicates
(ring silicates)
or Metasilicate rings [Silicon:Oxygen = n:3n]
In this group of silicates, the tetrahedrons are linked into
rings. Each silicon atom is bound by two oxygen atoms that are part of another
tetrahedron. Each ring consists of three, four, or six linked tetrahedrons. None of these
minerals are soft; most are rather hard. They frequently occur in elongated crystals.
Sorosilicates (double-linked silicates)
or Pyrosilicates [Silicon:Oxygen = 2:7]
The sorosilicates are composed of groups of two tetrahedrons
where one of the oxygen atoms is shared by both tetrahedrons; the other three oxygen atoms
are not shared by another tetrahedron. Most of the sorosilicates are hard and tabular.
Nesosilicates (single-linked silicates)
or Orthosilicates [Silicon:Oxygen = 1:4]
The orthosilicates contain only single, non-combining groups of
tetrahedrons. These minerals are dense, causing them to be hard and providing reason for
some of to be high in specific gravity.
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