magnesiaSee magnesium oxide.
magnesiteA white, colourless, or
grey mineral form of *magnesium
carbonate, MgCO 3 , crystallizing in
the trigonal system. It is formed as a
replacement mineral of magnesium-
rich rocks when carbon dioxide is
available. Magnesite is mined both as
an ore for magnesium and as a
source of magnesium carbonate. It
occurs in Austria, USA, Greece, Nor-
way, India, Australia, and South
Africa.
magnesiumSymbol Mg. A silvery
metallic element belonging to group
2 (formerly IIA) of the periodic table
(see alkaline-earth metals); a.n. 12;
r.a.m. 24.305; r.d. 1.74; m.p. 648.8°C;
b.p. 1090°C. The element is found in
a number of minerals, including
magnesite (MgCO 3 ), dolomite
(MgCO 3 .CaCO 3 ), and carnallite
(MgCl 2 .KCl.6H 2 O). It is also present in
sea water, and it is an *essential el-
ement for living organisms. Extrac-
tion is by electrolysis of the fused
chloride. The element is used in a
number of light alloys (e.g. for air-
craft). Chemically, it is very reactive.
In air it forms a protective oxide coat-
ing but when ignited it burns with
an intense whiteÛame. It also reacts
with the halogens, sulphur, and ni-
trogen. Magnesium wasÜrst isolated
by Bussy in 1828.
A- Information from the WebElements site
magnesium bicarbonateSee
magnesium hydrogencarbonate.
magnesium carbonateA white
compound, MgCO 3 , existing in anhy-
drous and hydrated forms. The an-
hydrous material (trigonal; r.d.
2.96) is found in the mineral *mag-
nesite. There is also a trihydrate,
MgCO 3 .3H 2 O (rhombic; r.d. 1.85),
which occurs naturally as nesque-
honite, and a pentahydrate,
MgCO 3 .5H 2 O (monoclinic; r.d. 1.73),
which occurs as lansfordite. Magne-
sium carbonate also occurs in the
mixed salt *dolomite (CaCO 3 .MgCO 3 )
and as basic magnesium carbon-
atein the two minerals artinite
(MgCO 3 .Mg(OH) 2 .3H 2 O) and hydro-
magnesite(3MgCO 3 .Mg(OH) 2 .3H 2 O).
The anhydrous salt can be formed by
heating magnesium oxide in a
stream of carbon dioxide:
MgO(s) + CO 2 (g) →MgCO 3 (s)
Above 350°C, the reverse reaction
predominates and the carbonate de-
composes. Magnesium carbonate is
used in making magnesium oxide
and is a drying agent (e.g. in table
salt). It is also used as a medical
antacid and laxative (the basic car-
bonate is used) and is a component
of certain inks and glasses.magnesium chlorideA white
solid compound, MgCl 2. The anhy-
drous salt (hexagonal; r.d. 2.32; m.p.
714 °C; b.p. 1412°C) can be prepared
by the direct combination of dry
chlorine with magnesium:
Mg(s) + Cl 2 (g) →MgCl 2 (s)
The compound also occurs naturally
as a constituent of carnallite
(KCl.MgCl 2 ). It is a deliquescent com-
pound that commonly forms the
hexahydrate, MgCl 2 .6H 2 O (mono-
clinic; r.d. 1.57). When heated, this
hydrolyses to give magnesium oxide
and hydrogen chloride gas. The fused
chloride is electrolysed to produce
magnesium and it is also used for
ÜreprooÜng wood, in magnesia ce-
ments and artiÜcial leather, and as a
laxative.magnesium hydrogencarbonate
(magnesium bicarbonate)A com-
pound, Mg(HCO 3 ) 2 , that is stable only
in solution. It is formed by the action
of carbon dioxide on a suspension of
magnesium carbonate in water:magnesia 336m