–185°C; b.p. –112°C. Silane is produced by reduction of silicon with
lithium tetrahydridoaluminate(III). It
is also formed by the reaction of
magnesium silicide (Mg 2 Si) with
acids, although other silicon hydrides
are also produced at the same time.
Silane itself is stable in the absence
of air but is spontaneously Ûammable, even at low temperatures. It is a
reducing agent and has been used for
the removal of corrosion in inaccessible plants (e.g. pipes in nuclear reactors). 2. (or silicon hydride) Any of
a class of compounds of silicon and
hydrogen. They have the general formula Si n H 2n+2 . The Ürst three in the
series are silane itself (SiH 4 ), disilane
(Si 2 H 6 ), and trisilane (Si 3 H 8 ). The compounds are analogous to the alkanes
but are much less stable and only
the lower members of the series can
be prepared in any quantity (up to
Si 6 H 14 ). No silicon hydrides containing double or triple bonds exist (i.e.
there are no analogues of the alkenes
and alkynes).
silica See silicon(iv) oxide.
silica gel A rigid gel made by coagulating a sol of sodium silicate and
heating to drive off water. It is used
as a support for catalysts and also as
a drying agent because it readily absorbs moisture from the air. The gel
itself is colourless but, when used in
desiccators, etc., a blue cobalt salt is
added. As moisture is taken up, the
salt turns pink, indicating that the
gel needs to be regenerated (by heating).
silicane See silane.
silicate Any of a group of substances containing negative ions
composed of silicon and oxygen. The
silicates are a very extensive group
and natural silicates form the major
component of most rocks (see silicate minerals). The basic structural
unit is the tetrahedral SiO 4 group.
This may occur as a simple discrete
SiO 4
4– anion as in the orthosilicates,
e.g. phenacite (Be 2 SiO 4 ) and willemite
(Zn 2 SiO 4 ). Many larger silicate species
are also found (see illustration).
These are composed of SiO 4 tetrahedra linked by sharing oxygen atoms
as in the pyrosilicates, Si 2 O 7
6–
, e.g.
Sc 2 Si 2 O 7 . The linking can extend to
such forms as bentonite, BaTiSi 3 O 9 ,
or alternatively inÜnite chain anions,
which are single strand (*pyroxenes)
or double strand (*amphiboles). Spodumene, LiAl(SiO 3 ) 2 , is a pyroxene
and the asbestos minerals are amphiboles. Large two-dimensional sheets
are also possible, as in the various
*micas (see illustration), and the
linking can extend to full threedimensional framework structures,
often with substituted trivalent
atoms in the lattice. The *zeolites
are examples of this.
silicate minerals A group of rockforming minerals that make up the
bulk of the earth’s outer crust (about
90%) and constitute one-third of all
minerals. All silicate minerals are
based on a fundamental structural
unit – the SiO 4 tetrahedron (see silicate). They consist of a metal (e.g.
calcium, magnesium, aluminium)
combined with silicon and oxygen.
The silicate minerals are classiÜed on
a structural basis according to how
the tetrahedra are linked together.
The six groups are: nesosilicates (e.g.
olivine and *garnet); sorosilicates
(e.g. hemimorphite); cyclosilicates
(e.g. axinite, *beryl, and *tourmaline); inosilicates (e.g. *amphiboles
and *pyroxenes); phyllosilicates (e.g.
*micas, *clay minerals, and *talc);
and tektosilicates (e.g. *feldspars and
*feldspathoids). Many silicate minerals are of economic importance.
silicide A compound of silicon with
a more electropositive element. The
481
silicide
s
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lithium tetrahydridoaluminate(III). It
is also formed by the reaction of
magnesium silicide (Mg 2 Si) with
acids, although other silicon hydrides
are also produced at the same time.
Silane itself is stable in the absence
of air but is spontaneously Ûammable, even at low temperatures. It is a
reducing agent and has been used for
the removal of corrosion in inaccessible plants (e.g. pipes in nuclear reactors). 2. (or silicon hydride) Any of
a class of compounds of silicon and
hydrogen. They have the general formula Si n H 2n+2 . The Ürst three in the
series are silane itself (SiH 4 ), disilane
(Si 2 H 6 ), and trisilane (Si 3 H 8 ). The compounds are analogous to the alkanes
but are much less stable and only
the lower members of the series can
be prepared in any quantity (up to
Si 6 H 14 ). No silicon hydrides containing double or triple bonds exist (i.e.
there are no analogues of the alkenes
and alkynes).
silica See silicon(iv) oxide.
silica gel A rigid gel made by coagulating a sol of sodium silicate and
heating to drive off water. It is used
as a support for catalysts and also as
a drying agent because it readily absorbs moisture from the air. The gel
itself is colourless but, when used in
desiccators, etc., a blue cobalt salt is
added. As moisture is taken up, the
salt turns pink, indicating that the
gel needs to be regenerated (by heating).
silicane See silane.
silicate Any of a group of substances containing negative ions
composed of silicon and oxygen. The
silicates are a very extensive group
and natural silicates form the major
component of most rocks (see silicate minerals). The basic structural
unit is the tetrahedral SiO 4 group.
This may occur as a simple discrete
SiO 4
4– anion as in the orthosilicates,
e.g. phenacite (Be 2 SiO 4 ) and willemite
(Zn 2 SiO 4 ). Many larger silicate species
are also found (see illustration).
These are composed of SiO 4 tetrahedra linked by sharing oxygen atoms
as in the pyrosilicates, Si 2 O 7
6–
, e.g.
Sc 2 Si 2 O 7 . The linking can extend to
such forms as bentonite, BaTiSi 3 O 9 ,
or alternatively inÜnite chain anions,
which are single strand (*pyroxenes)
or double strand (*amphiboles). Spodumene, LiAl(SiO 3 ) 2 , is a pyroxene
and the asbestos minerals are amphiboles. Large two-dimensional sheets
are also possible, as in the various
*micas (see illustration), and the
linking can extend to full threedimensional framework structures,
often with substituted trivalent
atoms in the lattice. The *zeolites
are examples of this.
silicate minerals A group of rockforming minerals that make up the
bulk of the earth’s outer crust (about
90%) and constitute one-third of all
minerals. All silicate minerals are
based on a fundamental structural
unit – the SiO 4 tetrahedron (see silicate). They consist of a metal (e.g.
calcium, magnesium, aluminium)
combined with silicon and oxygen.
The silicate minerals are classiÜed on
a structural basis according to how
the tetrahedra are linked together.
The six groups are: nesosilicates (e.g.
olivine and *garnet); sorosilicates
(e.g. hemimorphite); cyclosilicates
(e.g. axinite, *beryl, and *tourmaline); inosilicates (e.g. *amphiboles
and *pyroxenes); phyllosilicates (e.g.
*micas, *clay minerals, and *talc);
and tektosilicates (e.g. *feldspars and
*feldspathoids). Many silicate minerals are of economic importance.
silicide A compound of silicon with
a more electropositive element. The
481
silicide
s
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