potassium is dominant with a
smaller proportion of sodium and
negligible calcium; and the plagioclase feldspars, which vary in composition in a series that ranges from
pure sodium feldspar (albite) through
to pure calcium feldspar (anorthite)
with negligible potassium. Feldspars
form colourless, white, or pink crystals with a hardness of 6 on the
Mohs’ scale.
feldspathoids A group of alkali
aluminosilicate minerals that are
similar in chemical composition to
the *feldspars but are relatively deÜcient in silica and richer in alkalis.
The structure consists of a framework of (Si,Al)O 4 tetrahedra with aluminium and silicon atoms at their
centres. The feldspathoids occur
chieÛy with feldspars but do not coexist with free quartz (SiO 2 ) as they
react with silica to yield feldspars.
The chief varieties of feldspathoids
are: nepheline, KNa 3 (AlSiO 4 ) 4 ; leucite,
KAlSi 2 O 6 ; analcime, NaAlSi 2 O 6 .H 2 O;
cancrinite, Na 8 (AlSiO 4 ) 6 (HCO 3 ) 2 ; and
the sodalite subgroup comprising:
sodalite, 3(NaAlSiO 4 ).NaCl; nosean,
3(NaAlSiO 4 ).Na 2 SO 4 ; haüyne,
3(NaAlSiO 4 ).CaSO 4 ; lazurite
(Na,Ca) 8 (Al,Si) 12 O 24 (S,SO 4 ) (see
lapis lazuli).
FEM See field-emission
microscope.
femto- Symbol f. A preÜx used in
the metric system to denote 10
–15
.
For example, 10
–15 second = 1 femtosecond (fs).
femtochemistry The investigation
of chemical processes that occur on
the timescale of a femtosecond (10
–15
s). Femtochemistry has become possible as a result of the development of
*lasers capable of being pulsed in
femtoseconds. This has enabled observations to be made on very shortlived species, such as activated
complexes, which only exist for
about a picosecond (10
–12 s). In a femtochemical experiment a femtosecond pulse causes dissociation of a
molecule. A series of femtosecond
pulses is then released, the frequency
of the pulses being that of an absorption of one of the products of the dissociation. The absorption can be used
as a measure of the abundance of the
product of the dissociation. This type
of study enables the course of the
mechanism of a chemical reaction to
be studied in detail.
Fenton’s reagent A mixture of hydrogen peroxide and iron(II) sulphate
used to produce free radicals by reactions of the type
Fe
2+ + H 2 O 2 → Fe
3+ + . OH + OH
-
Fe
3+ + H 2 O 2 → Fe
2+ + . OOH + H
+
It is used in water treatment and as a
reagent in organic synthesis to introduce a OH group into an aromatic
drug.
fermentation A form of anaerobic
respiration occurring in certain microorganisms, e.g. yeasts. Alcoholic
fermentation comprises a series of
biochemical reactions by which pyruvate (the end product of *glycolysis)
is converted to ethanol and carbon
dioxide. Fermentation is the basis of
the baking, wine, and beer industries.
fermi A unit of length formerly
used in nuclear physics. It is equal to
10
–15 metre. In SI units this is equal
to 1 femtometre (fm). It was named
after Enrico *Fermi.
Fermi–Dirac statistics See quantum statistics.
Fermi level The energy in a solid
at which the average number of particles per quantum state is ½; i.e. one
half of the quantum states are occupied. The Fermi level in conductors
lies in the conduction band (see enfeldspathoids
222
f
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smaller proportion of sodium and
negligible calcium; and the plagioclase feldspars, which vary in composition in a series that ranges from
pure sodium feldspar (albite) through
to pure calcium feldspar (anorthite)
with negligible potassium. Feldspars
form colourless, white, or pink crystals with a hardness of 6 on the
Mohs’ scale.
feldspathoids A group of alkali
aluminosilicate minerals that are
similar in chemical composition to
the *feldspars but are relatively deÜcient in silica and richer in alkalis.
The structure consists of a framework of (Si,Al)O 4 tetrahedra with aluminium and silicon atoms at their
centres. The feldspathoids occur
chieÛy with feldspars but do not coexist with free quartz (SiO 2 ) as they
react with silica to yield feldspars.
The chief varieties of feldspathoids
are: nepheline, KNa 3 (AlSiO 4 ) 4 ; leucite,
KAlSi 2 O 6 ; analcime, NaAlSi 2 O 6 .H 2 O;
cancrinite, Na 8 (AlSiO 4 ) 6 (HCO 3 ) 2 ; and
the sodalite subgroup comprising:
sodalite, 3(NaAlSiO 4 ).NaCl; nosean,
3(NaAlSiO 4 ).Na 2 SO 4 ; haüyne,
3(NaAlSiO 4 ).CaSO 4 ; lazurite
(Na,Ca) 8 (Al,Si) 12 O 24 (S,SO 4 ) (see
lapis lazuli).
FEM See field-emission
microscope.
femto- Symbol f. A preÜx used in
the metric system to denote 10
–15
.
For example, 10
–15 second = 1 femtosecond (fs).
femtochemistry The investigation
of chemical processes that occur on
the timescale of a femtosecond (10
–15
s). Femtochemistry has become possible as a result of the development of
*lasers capable of being pulsed in
femtoseconds. This has enabled observations to be made on very shortlived species, such as activated
complexes, which only exist for
about a picosecond (10
–12 s). In a femtochemical experiment a femtosecond pulse causes dissociation of a
molecule. A series of femtosecond
pulses is then released, the frequency
of the pulses being that of an absorption of one of the products of the dissociation. The absorption can be used
as a measure of the abundance of the
product of the dissociation. This type
of study enables the course of the
mechanism of a chemical reaction to
be studied in detail.
Fenton’s reagent A mixture of hydrogen peroxide and iron(II) sulphate
used to produce free radicals by reactions of the type
Fe
2+ + H 2 O 2 → Fe
3+ + . OH + OH
-
Fe
3+ + H 2 O 2 → Fe
2+ + . OOH + H
+
It is used in water treatment and as a
reagent in organic synthesis to introduce a OH group into an aromatic
drug.
fermentation A form of anaerobic
respiration occurring in certain microorganisms, e.g. yeasts. Alcoholic
fermentation comprises a series of
biochemical reactions by which pyruvate (the end product of *glycolysis)
is converted to ethanol and carbon
dioxide. Fermentation is the basis of
the baking, wine, and beer industries.
fermi A unit of length formerly
used in nuclear physics. It is equal to
10
–15 metre. In SI units this is equal
to 1 femtometre (fm). It was named
after Enrico *Fermi.
Fermi–Dirac statistics See quantum statistics.
Fermi level The energy in a solid
at which the average number of particles per quantum state is ½; i.e. one
half of the quantum states are occupied. The Fermi level in conductors
lies in the conduction band (see enfeldspathoids
222
f
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