crystalline structure. The term is also
often applied generally to organic
substances that are obtained by min-
ing (e.g. coal, petroleum, and natural
gas) but strictly speaking these are
not minerals, being complex mix-
tures without deÜnite chemical for-
mulas. Rocks are composed of
mixtures of minerals. Minerals may
be identiÜed by the properties of
their crystal system, hardness (meas-
ured on the Mohs’ scale), relative
density, lustre, colour, cleavage, and
fracture. Many names of minerals
end in -ite.mineral acidA common inorganic
acid, such as hydrochloric acid, sul-
phuric acid, or nitric acid.
mirabiliteA mineral form of
*sodium sulphate, Na 2 SO 4 .10H 2 O.misch metalAn alloy of cerium
(50%), lanthanum (25%), neodymium
(18%), praseodymium (5%), and other
rare earths. It is used alloyed with
iron (up to 30%) in lighterÛints, and
in small quantities to improve the
malleability of iron. It is also added
to copper alloys to make them
harder, to aluminium alloys to make
them stronger, to magnesium alloys
to reduce creep, and to nickel alloys
to reduce oxidation.
Mitscherlich’s law (law of isomor-
phism)Substances that have the
same crystal structure have similar
chemical formulae. The law can be
used to determine the formula of an
unknown compound if it is isomor-
phous with a compound of known
formula. It is named after Eilhard
Mitscherlich (1794–1863).
mixture A system of two or more
distinct chemical substances. Homo-
geneous mixtures are those in which
the atoms or molecules are inter-
spersed, as in a mixture of gases or in
a solution. Heterogeneous mixtures
have distinguishable phases, e.g. amixture of ironÜlings and sulphur.
In a mixture there is no redistribu-
tion of valence electrons, and the
components retain their individual
chemical properties. Unlike com-
pounds, mixtures can be separated
by physical means (distillation, crys-
tallization, etc.).m.k.s. unitsA *metric system of
units devised by A. Giorgi (and some-
times known as Giorgi units) in 1901.
It is based on the metre, kilogram,
and second and grew from the ear-
lier *c.g.s. units. The electrical unit
chosen to augment these three basic
units was the ampere and the perme-
ability of space (magnetic constant)
was taken as 10–7Hm–1. To simplify
electromagnetic calculations the
magnetic constant was later changed
to 4π× 10 –7Hm–1to give the rational-
ized MKSA system. This system, with
some modiÜcations, formed the basis
of *SI units, now used in all scientiÜc
work.mmHgA unit of pressure equal to
that exerted under standard gravity
by a height of one millimetre of mer-
cury, or 133.322 pascals.
mobility(of an ion)Symbol u. The
terminal speed of an ion in an elec-
tricÜeld divided by theÜeld strength.modeThe pattern of motion in a vi-
brating body. If the body has several
component particles, such as a mol-
ecule consisting of several atoms, the
modes of vibration are the different
types of molecular vibrations possi-
ble.moietyA characteristic part of a
molecule. For example, in the ester
C 2 H 5 COOCH 3 , the OCH 3 can be re-
garded as the alcohol (methanol)
moiety.moissaniteA clear form of silicon
carbide used as an inexpensive sub-
stitute for diamond. It is named aftermineral acid 356m