---------------------------------------------------------------------------- | Orthoclase. | Albite. | Oligoclase. | Labradorite. | ------------------|--------------|-------------|-------------|--------------| Silica | 65·72| 65·00| 67·99| 68·23| 62·70| 63·51| 54·66| 54·67| Alumina | 18·57| 18·64| 19·61| 18·30| 23·80| 23·09| 27·87| 27·89| Peroxide of iron | traces| 0·83| 0·70| 1·01| 0·62| -- | -- | 0·31| Oxide of manganese| traces| 0·13| -- | -- | -- | -- | -- | -- | Lime | 0·34| 1·23| 0·66| 1·26| 4·60| 2·44| 12·01| 10·60| Magnesia | 0·10| 1·03| -- | 0·51| 0·02| 0·77| -- | 0·18| Potash | 14·02| 9·12| -- | 2·53| 1·05| 2·19| -- | 0·49| Soda | 1·25| 3·49| 11·12| 7·99| 8·00| 9·37| 5·46| 5·05| ----------------------------------------------------------------------------| | 100·00| 99·47|100·08| 99·83|100·79|101·37|100·00| 99·19 | -----------------------------------------------------------------------------
It is obvious that soils produced by the disintegration of these minerals must differ materially in quality. Those yielded by orthoclase must generally abound in potash, while albite and labradorite, containing little or none of that element, must produce soils in which it is deficient. The quality of the soil they yield is not however entirely dependent on the nature of the particular felspar which yields it, but is also intimately connected with the extent to which the decomposition has advanced. It is observed that different felspars undergo decomposition with different degrees of rapidity but after a certain time they all begin to lose their peculiar lustre, acquire a dull and earthy appearance, and at length fall into a more or less white and soft powder. During this change water is absorbed, and, by the decomposing action of the air, the alkaline silicate is gradually rendered soluble, and at length entirely washed away, leaving a substance which, when mixed with water, becomes plastic, and has all the characters of common clay. The nature of this change will be best seen by the following analysis of the clay produced during this composition, which is employed in the manufacture of porcelain under the name of kaolin, or china clay--
Silica 46·80 Alumina 36·83 Peroxide of iron 3·11 Carbonate of lime 0·55 Potash 0·27 Water 12·44 ---- 100·00