ciated with anti-Stokes radiation are
called anti-Stokes lines. The quantum
theory of the Raman effect provides
an explanation for the intensity of
the anti-Stokes line being much less
than the intensity of the Stokes line.apAntiperiplanar. See torsion
angle.apatiteA complex mineral form of
*calcium phosphate, Ca 5 (PO 4 ) 3 -
(OH,F,Cl); the commonest of the
phosphate minerals. It has a hexago-
nal structure and occurs widely as an
accessory mineral in igneous rocks
(e.g. pegmatite) and often in regional
and contact metamorphic rocks, es-
pecially limestone. Large deposits
occur in the Kola Peninsula, Russia. It
is used in the production of fertiliz-
ers and is a major source of phospho-
rus. The enamel of teeth is composed
chieÛy of apatite.apicalDesignating certain bonds
and positions in a bipyramidal or
pyramidal structure. For example, in
a trigonal bipyramid the two bonds
aligned through the central atom are
the apical bonds and the groups at-
tached to these bonds are in apical
positions. The three other bonds and
positions are equatorial. Similarly, in
a square pyramidal structure the
bond perpendicular to the base of
the pyramid is the apical bond. The
four other coplanar bonds (and posi-
tions) are said to be basal.aprotic See solvent.
aqua acidA type of acid in which
the acidic hydrogen is on a water
molecule coordinated to a metal ion.
For example,
Al(OH 2 ) 6 3++ H 2 O →
Al(OH 2 ) 5 (OH)2++ H 3 O+aqua regia A mixture of concen-
trated nitric acid and concentrated
hydrochloric acid in the ratio 1:3 re-
spectively. It is a very powerful oxi-dizing mixture and will dissolve all
metals (except silver, which forms an
insoluble chloride) including such
noble metals as gold and platinum,
hence its name (‘royal water’). Nitro-
syl chloride (NOCl) is believed to be
one of the active constituents.aquationThe process in which
water molecules solvate or form co-
ordination complexes with ions.aqueousDescribing a solution in
water.aquo ionA hydrated positive ion
present in a crystal or in solution.arachidonic acidAn unsaturated
fatty acid, CH 3 (CH 2 ) 3 (CH 2 CH:CH) 4 -
(CH 2 ) 3 COOH, that is essential for
growth in mammals. It can be syn-
thesized from *linoleic acid. Arachi-
donic acid acts as a precursor to
several biologically active com-
pounds, including prostaglandins,
and plays an important role in mem-
brane production and fat metabo-
lism. The release of arachidonic acid
from membrane phospholipids is
triggered by certain hormones.arachno structure See borane.aragoniteA rock-forming anhy-
drous mineral form of calcium car-
bonate, CaCO 3. It is much less stable
than *calcite, the commoner form of
calcium carbonate, from which it
may be distinguished by its greater
hardness and speciÜc gravity. Over
time aragonite undergoes recrystal-
lization to calcite. Aragonite occurs
in cavities in limestone, as a deposit
in limestone caverns, as a precipitate
around hot springs and geysers, and
in high-pressure low-temperature
metamorphic rocks; it is also found
in the shells of a number of molluscs
and corals and is the main con-
stituent of pearls. It is white or
colourless when pure but the pres-ap 42a