Modern inorganic chemistry

THE ELEMENTS OF GROUP III 141

PROPERTIES OF THE ELEMENTS

OCCURRENCE AND EXTRACTION

Boron

Boron does not occur free in nature; in minerals, it occurs as

borates, for example, kernite. Na 2 B 4 O 7. 4H 2 O. and borax.

Na 2 B 4 O 7 .10H 2 O; there are extensive deposits of these in the USA.

Boron can be obtained by heating boron trioxide with magnesium:

B 2 O 3 + 3Mg -> 2B + 3MgO

The boron so obtained is an amorphous powder. It can be obtained

in the crystalline state by reducing the vapour of boron tribromide

with hydrogen, either in an electric arc or in contact with an elec-

trically-heated tungsten filament:

2BBr 3 + 3H 2 - 2B + 6HBr|

Pure boron in the form of a thin film can also be obtained by

heating diborane to 1000 K:

B 2 H 6 -> 2B + 3H 2 t

Amorphous boron has not been obtained in the pure state.

Crystalline boron is a black powder, extremely hard, with a metallic

appearance but with very low electrical conductivity.

Aluminium

Aluminium is not found free but its compounds are so widespread
that it is the most abundant metal in the earth's crust. Alumino-
silicates such as clay, kaolin (or china clay), mica and feldspar are
well known and widely distributed. The oxide. A1 2 O 3. occurs
(anhydrous) as corundum and emery, and (hydrated) as bauxite.
Cryolite. Na 3 AlF 6. (sodium hexafluoroaluminate). is found exten-
sively in Greenland.
Aluminium is obtained on a large scale by the electrolysis of the
oxide, dissolved in fused cryolite The oxide, occurring naturally as
bauxite, A1 2 O 3 .2H 2 O, usually contains silica and iron(III) oxide as
impurities. These must be removed first, since aluminium, once
prepared, cannot be freed of other metals (which will be deposited
on electrolysis) by refining it. The crude oxide is dissolved under
pressure in caustic soda solution; the aluminium oxide and silica
dissolve and the iron(III) oxide is left:
A1 2 O 3 + 2OH~ + 7H 2 O ^ 2[Ai(OH) 4 (H 2 O) 2 ]~