188 GROUP IV
The zeolites are aluminatesilicates, having large, open-structured
anions and balancing cations. Because of the open structure,
zeolites can take up water molecules reversibly into the interstices
of their structures. More importantly, they may be able to act as
molecular sieves, by taking up from a gas mixture only molecules
in a certain size range; the zeolite can then be taken out of the gas
and the absorbed species pumped off. Thus the zeolite mordenite
will occlude small molecules, e.g. nitrogen, argon*, but not, for
example, methane or ethane. Synthetic zeolites possess the property
of ion-exchange. The cations in a zeolite may move freely through
the open structure, and hence replacement of one cation by another
can occur without affecting the rest of the lattice. Many artificial
ion-exchange zeolites have been made, and used to remove cations
from water, e.g. the 'permutits', and more recently, ion-exchange
materials with a framework of an organic polymer have been made
and used extensively (e.g. in the purification of water, p. 275).
Clay and kaolin describe groups of substances with compositions
which are similar chemically (they contain aluminium, silicon,
oxygen and water) but with many different kinds of structure, the
nature of which has been established by X-ray diffraction studies.
The clays all possess a layer-like structure. When water is added to
clay it enters between the layers and the clay swells and acquires
plasticity thus enabling it to be moulded into bricks, pottery, and so
on. On ignition or Tiring', these lose plasticity permanently acquiring
thereby a fixed shape, hardness and strength. Kaolin is rather less
'plastic' than clay but can be moulded and then fired to give
porcelain or kchina'.
Glass is the name given to any amorphous solid produced when a
liquid solidifies. Glasses are non-crystalline and isotropic, i.e. their
physical properties are independent of the direction in which they
are measured. When a glass is heated, it does not melt at a fixed
temperature but gradually softens until a liquid is obtained.
The word 'glass' commonly means the transparent substance
obtained when white sand is fused with metal oxides or carbonates
to give a mixture of silicates. Ordinary or 'soda-glass' has the
approximate composition Na 2 O. CaO. 6SiO 2. (This is the com-
position obtained by analysis: it does not represent the compounds
present.) If sodium is replaced by potassium the melting point is
- Traces of oxygen can be removed from argon (required for an inert atmosphere
in certain metallurgical processes). Oxygen molecules can pass through the spaces or
windows 'end-ways' while the larger argon atoms are kept out.
