1554 Journal of Applied Microscopy 



{d) Simple and alumosilicates, under action of heat, either unite to a com- 

 plicated substance or there is an interchange of metals of the RO group. 



{e) With complicated alumosilicates containing RO there are many known 

 reactions which produce aluminates of RO. 



The simple silicates are salts of known acids, but, while a few alumosilicates, 

 such as leucite KgAlgSi^Oj 2, could be considered double salts of known acids, 

 most are classed as salts of complicated hypothetical acids, and for some no 

 satisfactory acid has been found. 



Alumina may be regarded either as an anhydrous acid or as a weak base. 

 In the latter case the salts show many characters of so-called complex acids, and 

 as indicated by the following experimental data, it is much more probable the 

 alumosilicates are anhydrides, hydrates, and salts of complex alumosilicic acids : 



{a) Aluminates form under the same conditions as alumosilicates. 



(b) By splitting up of alumosilicates at high temperature aluminates are formed. 



(/) The action of water or carbonate solution often results in destruction of 

 alumosilicates, and formation of aluminates or alumina hydrates. 



The complex structure of the alumosilica nucleus is shown by the properties 

 of the compounds ; for instance : 



{a) Compounds of only AI2O3 and SiOg correspond in properties to acid 

 anhydrides; e. g., heated with carbonates they swell and evolve CO 2 rapidly, 

 and form an alumosilicate. Similar results are obtained by heating with sul- 

 phates, haloids, etc. 



(b) Kaolin and other clays act like acids, destroying haloid salts at compara- 

 tively low temperatures. 



In nature and the laboratory many substitution reactions occur in which the 

 alumosilica kernel is not destroyed. The general scheme is : 



Mx-j-MjAls^M^x+MAls, in which x is the acid anhydride, M and M^ 

 different metals, and Als the alumosilica kernel. 



Clay (alumosilicic acid) and minerals of sillimanite group (alumosilica anhy- 

 dride) form by destruction of alumosilicates under the same conditions as hydrates 

 and anhydrides, by the destruction of their salts. This is best seen by a com- 

 parison with silicates. 



(1) By heating opal we obtain silica ; by heating clay we obtain 



minerals of sillimanite group. 



(2) By destruction of simple silicates under the action of water and 



CO 2 in nature we obtain opal; by similar destruction of 

 alumosilicates we obtain clay. 



(3) At high temperatures in fusions rich in alumina, corundum or 



sillimanite separates, just as from fusions rich in silica, 

 tridymite or quartz separates. 



THE SIMPLE SILICATES. 



These are salts of known acids, and their derivatives, the two great groups, 



RO. .OR 



being the orthosilicatcs, with a structure formula /Si\ ^"*^ the 7neta- 



RO/ \0R 

 RO. 

 silicates, with a structure formula of \Si=0. 



RO/ 



