Aluminosilicic acids, aluminosilicates

It is usual to regard the numerous known silicates, including those described in the preceding section, as the salts of various hypothetical silicic acids, or as isomorphous mixtures of such salts. There are, however, good reasons for supposing that many of the compounds usually classed as aluminium silicates are no more related to the silicates than the ferrocyanides, for instance, are related to the cyanides. Numerous reasons can be adduced in favour of the view that the alumina in numerous silicates plays the rdle of an acid and not a base, and that, as was originally proposed by Vernadski, the silicates themselves are the salts of a number of complex aluminosilicic acids, comparable with the phosphomolybdic, silicomolybdic, and silicotungstic acids, etc. For the nature of the evidence, however, the reader must be referred to the literature.

Many silicates are decomposed by the prolonged action of water and carbon dioxide, furnishing colloidal silicic acid or opal. In a similar manner, the aluminosilicates give rise to colloidal aluminosilicic acids or clays. The empirical formulae of these acids may be tabulated as follows: -

Alumino-monosilicic acid (allophanic type)	Al2O3.SiO2.nH2O
Alumino-disilicic acid (kaolinic type)	Al2O3.2SiO2.nH2O
Alumino-trisilicic acid (natrolitic type)	Al2O3.3SiO2.nH2O
Alumino-tetrasilicic acid (pyrophyllitic type)	Al2O3.4SiO2.nH2O
Alumino-pentasilicic acid (chabazitic type)	Al2O3.5SiO2.nH2O
Alumino-hexasilicic acid (felspathic type)	Al2O3.6SiO2.nH2O

The constitution of these acids at present can only be eonjectured. The following groupings of important compounds are due to Mellor and Holdcroft:

Allophane is Al₂O₃.SiO₂.5H₂O. Salts of this type are: augite, MgO.Al₂O₃.SiO₂; chlorite, 2MgO.Al₂O₃.SiO₂.2H₂O.

Various alumino disilicic acids are: -

Rectorite – Al₂O₃.2SiO₂.H₂O;
Halloysite – Al₂O₃.2SiO₂.3H₂O;
Kaolinite – Al₂O₃.2SiO₂.2H₂O;
Newtonite – Al₂O₃.2SiO₂.5H₂O;
Among the aluminosilicates that may be looked upon as salts of the preceding acids are nephelite, K₂O.Al₂O₃.2SiO₂, and anorthite, CaO.Al₂O₃.2SiO₂.

Salts are represented by natrolite, Na₂O.Al₂O₃.3SiO₂; garnet, 3CaO.Al₂O₃. 3SiO₂; scolecite, CaO.Al₂O₃.3SiO₂.3H₂O; lepidolite, KLiO.Al₂O₃.3SiO₂; etc.

Pyrophyllite is Al₂O₃.4SiO₂.H₂O. Salts are leucite, K₂O.Al₂O₃.4SiO₂; analcite, Na₂O.Al₂O₃.4SiO₂.2H₂O; spodumene, Li₂O.Al₂O₃.4SiO₂; glaucophane, Na₂O.Al₂O₃.4SiO₂; etc.

The group is represented by chabazite, CaO.Al₂O₃.5SiO₂.7H₂O, and harmotine, BaO.Al₂O₈.5SiO₂.6H₂O.

This type is represented by orthoclase, K₂O.Al₂O₈.6SiO₂.

For further information concerning attempts to derive the constitutions of the aluminium silicates, the reader is referred to the literature.