520 ALUMINUM. 



But from the analysis of Mosander, the refractory clay of Stour- 

 bridge (a fire-clay) is a neutral silicate of alumina, A1 2 O 3 , 

 3Si O 3 . China-clay or kaolin, which is prepared from decay- 

 ing granite, being the result of the decomposition of the 

 felspar and mica of that mineral, is not uniform in its compo- 

 sition. 



A subsilicate of alumina exists, forming a very hard crystal- 

 lized mineral, disthene or cyanite, of which the formula is 2A1 2 

 3 , Si0 3 . 



Double silicates of alumina and potash are extensively diffused 

 in the mineral kingdom, forming a very considerable portion 

 of the solid crust of the globe. The most usual of these double 

 salts is the following. 



Felspar is composed of single equivalents of the neutral sili- 

 cates of potash and alumina. Its formula is therefore analo- 

 gous to that of anhydrous alum, silicon being substituted for 

 sulphur; KO. Si O 3 -f A1 2 O 3 , 3Si O 3 . It is one of the three 

 principal constituents of granite and gneiss. 



Amphigen or leucite occurs principally in the lava of Vesu- 

 vius in a crystallized state. The relation between the potash 

 and alumina is the same as in the preceding mineral, but it con- 

 tains one-third less silica. Hence the formula 3KO, 2Si O 3 + 

 3(A1 2 O 3 , 2Si O 3 ). A similar combination is obtained by pre- 

 cipitating a saturated solution of alumina in potash, by a solu- 

 tion of silicate of potash (Berzelius.) 



When a mixture of silica and alumina is fused with an ex- 

 cess of potash, and the fused mass washed with water, to with- 

 draw everything soluble, a powder remains in which the potash 

 and alumina are still in the ratio of single equivalents, but in 

 which the oxygen of the silica is equal to that of the bases. This 

 double salt has consequently the formula, 3KO, Si O 3 -f3Al 2 O 3 , 

 2Si O 3 . 



Albite or soda felspar much resembles felspar in properties. 

 Its composition is analogous, with the substitution of soda for 

 potash; NaO, Si O 3 +A1 2 O 3 , SSi O 3 . 



Analcime is the soda silicate proportional to amphigen. It is 

 crystallized like amphigen, but contains 8.27 P er cent or 6 atoms 

 of water. Its formula is 3Na O, 2SiO 3 + 3(Al 2 O 3 , 2Si O 3 ) + 

 6HO. 



A third compound may be prepared, corresponding with the 

 artificial potash compound above. It occurs also in hexagonal 



