156 G. F. Becker — Silicic Acids. 



half of the lithium is replaced by sodium. In this case meta- 

 silicic acid actually breaks up into polysilicic and orthosilicic 

 acid. Were the process of decomposition to be reversed, or 

 were albite and eucryptite, together with some appropriate lith- 

 ium compound, to be mingled under the physical conditions 

 which prevailed when the spodumene formed, this pyroxene 

 would be regenerated. This case consequently appears to de- 

 monstrate that a metasilicate, one indeed of a peculiarly impor- 

 tant group of metasilicates, may be produced by the union of 

 orthosilicic and polysilicic components. 



It remains to be considered how disilicic acid is to be re- 

 garded. If the synthesis of molecules is practicable, the reso- 

 lution of molecules is also feasible. "Why then may disilicic 

 acid not be considered as a polysilicic acid from which ortho- 

 silicic acid has been isolated ? This process would be repre 

 sented by 



3H 4 Si s 8 -H 4 Si0 4 = 4H 2 Si 2 5 . 



Were such really the process by which disilicic acid is pro- 

 duced, one would expect to find some hints of it in the 

 occurrence of the disilicates. As a matter of fact, petalite 

 occurs at the original locality in Sweden with lepidolite, which 

 is an orthosilicate similar in composition to petalite. Tscher- 

 mak regards lepidolite as a mixture principally composed of 

 the silicate (Li, K) AlSiO\ If the formula of petalite is 

 (Li, ISTa) Al(Si"0 6 ) 2 , as it is usually written, then two mole- 

 cules of petalite might combine with the lepidolitic compound 

 and they would yield three of a poly silicate (Li, K, ISTa) 

 AlSi 3 8 , which *is simply an orthoclase containing lithium. 

 Such orthoclases are known to occur at various localities. It 

 is also well-known that among the decomposition products of 

 orthoclase, orthosilicates (muscovite, kaolin) are most abundant. 

 These facts evidently suggest that petalite may have resulted 

 from the separation of such a feldsjjar into two compounds. 



The resolution of orthoclase into orthosilicates and other 

 compounds is in progress in all parts of the world. Were 

 disilicic acid strongly electronegative and its compounds of a 

 stable character, one would expect therefore to find disilicate 

 minerals frequently associated with decomposing granite. This 

 expectation is not realized nor does it seem probable that the 

 absence of such compounds is due to their solubility, for micro- 

 scopical study shows that the separation of silica accompanies 

 the decomposition of the feldspar in most cases. 



Such facts seem to me to throw much doubt on the true 

 character- of the mineral petalite and these doubts are not 

 resolved by study of the data. Some of the analyses must be 

 considerably strained to give the formula usually accepted • 



