potassium is dominant with a
smaller proportion of sodium and
negligible calcium; and the plagio-
clase feldspars, which vary in compo-
sition 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 crys-
tals with a hardness of 6 on the
Mohs’ scale.feldspathoidsA group of alkali
aluminosilicate minerals that are
similar in chemical composition to
the *feldspars but are relatively deÜ-
cient in silica and richer in alkalis.
The structure consists of a frame-
work of (Si,Al)O 4 tetrahedra with alu-
minium and silicon atoms at their
centres. The feldspathoids occur
chieÛy with feldspars but do not co-
exist 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).
FEMSee field-emission
microscope.femto-Symbol f. A preÜx used in
the metric system to denote 10–15.
For example, 10–15second = 1 femto-
second (fs).femtochemistryThe investigation
of chemical processes that occur on
the timescale of a femtosecond (10–15
s). Femtochemistry has become possi-
ble as a result of the development of
*lasers capable of being pulsed in
femtoseconds. This has enabled ob-
servations to be made on very short-
lived species, such as activatedcomplexes, which only exist for
about a picosecond (10–12s). In a fem-
tochemical experiment a femtosec-
ond pulse causes dissociation of a
molecule. A series of femtosecond
pulses is then released, the frequency
of the pulses being that of an absorp-
tion of one of the products of the dis-
sociation. 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 hy-
drogen peroxide and iron(II) sulphate
used to produce free radicals by reac-
tions of the type
Fe2++ H 2 O 2 →Fe3++ .OH + OH-Fe3++ H 2 O 2 →Fe2++ .OOH + H+
It is used in water treatment and as a
reagent in organic synthesis to intro-
duce a OH group into an aromatic
drug.fermentationA form of anaerobic
respiration occurring in certain mi-
croorganisms, e.g. yeasts. Alcoholic
fermentationcomprises a series of
biochemical reactions by which pyru-
vate (the end product of *glycolysis)
is converted to ethanol and carbon
dioxide. Fermentation is the basis of
the baking, wine, and beer indus-
tries.fermiA unit of length formerly
used in nuclear physics. It is equal to
10 –15metre. In SI units this is equal
to 1 femtometre (fm). It was named
after Enrico *Fermi.Fermi–Dirac statisticsSee quan-
tum statistics.
Fermi levelThe energy in a solid
at which the average number of par-
ticles per quantum state is ½; i.e. one
half of the quantum states are occu-
pied. The Fermi level in conductors
lies in the conduction band (see en-feldspathoids 222f