Kaolinite, H₄Al₂Si₂O₉

Kaolinite, H₄Al₂Si₂O₉, is a rare, white mineral. It crystallises in the monoclinic system (a:b:c = 0.5748:1:0.5997; β = 96°19'); density, 2.62; hardness, 2.5. The best crystals are found near Amlwch in the island of Anglesey; they are six-sided plates or pyramids with a perfect basal cleavage. The mean refractive index is 1.563; the birefringence (0.004) is much lower than that of mica, which resembles kaolinite in appearance. The water is only expelled from kaolinite at high temperatures (above 330°); hence, when the formula is written in the usual way, Al₂O₃.2SiO₂.2H₂O, it must be remembered that the water is "water of constitution" and not water of crystallisation. Chemically, kaolinite is best regarded as an acid.

The silicates described above are naturally occurring substances. A number of artificial aluminium silicates, however, are of considerable industrial importance and may therefore be mentioned. These are the substances prepared by Gans and known technically as permutits. Sodium permutit is practically an artificial zeolite of the composition 2SiO₂Al₂O₃.Na₂O.6H₂O. It is prepared by heating together china-clay, sodium carbonate and quartz in the requisite proportions and treating the mass with water. Other substances may be added in the preparation; a mixture of 2.5 to 3 parts of china-clay, 5 to 6 of alkali carbonate, and 1.5 to 2.4 parts of borax, for instance, has been particularly specified. The permutit is of a granular or leafy, porous character.

Sodium permutit, like certain natural zeolites, is able to exchange its sodium for other metals with remarkable ease. If, for instance, a solution of calcium or magnesium chloride is allowed to percolate through a column of the permutit, a solution of sodium chloride, free from calcium or magnesium, is obtained. If now, a concentrated solution of sodium chloride is percolated through the permutit, the calcium of magnesium is expelled from it and replaced by sodium. The application of sodium permutit to the softening of boiler water will therefore be obvious without further explanation.

The sodium or calcium permutits can readily be made to exchange sodium or calcium for manganese, by treatment with a manganous salt solution. When this product is acted upon by potassium permanganate solution, the latter is reduced, and a manganese-potassium permutit produced which is covered with very finely divided higher oxides of manganese. This material is a valuable oxidising agent, and, when exhausted, can readily be regenerated by allowing potassium permanganate to percolate through it; its commercial uses are for removing iron from, and oxidising the organic matter and bacteria in water. These results are easily brought about by allowing the contaminated water to flow through a layer of the permutit; in sterilising water by this means it may be necessary, in addition, first to add a little' permanganate to the water.