234 MINERALOGY 



Dimetasilicic acid is derived from two molecules of an ortho- 

 silicic acid by the elimination of three molecules of water, or 

 from two molecules of a metasilicic acid by the elimination of one 

 molecule of water. 



iH-0;\ X)-H H ~ 0i - 



i H-f O / \0-~H" H-0~/\ 0-H i ' 



orthosilicic acid 



/OfT TT _ (~) \ 



O = SK /Si = : H 2 Si 2 5 dimetasilicic acid. 



M);H H-Oi/ 



metasilicic acid 



Example, Petalite, LiAl(Si 2 O 5 )2. 



Trisilicic acid is an acid obtained from three molecules of the 

 orthoacid by the elimination of four molecules of water. 



Orthosilicic acid 



O = Si-0-H 



p r Hi o 



T _ H - C Nsi<( O '' - 1 ; H-O-Si-O-H; H 4 Si 3 O 8 = trisilicic acid. 



H _o_Si-0 



Example, Orthoclase, KAlSi 3 8 . 



In some minerals several silicic acids may be present in the mole- 

 cule, and a mineral may be a mixture of two or more of these acids 

 and yet on analysis yield the same percentage results of oxygen and 

 silicon. From inspection alone it is impossible to determine 

 whether the analysis represents a metasilicate or a trisilicate and 

 orthosilicate mixed; to this difficulty must also be added the 

 chances of basic, acid, and hydrated salts being present. For the 

 same analysis several structural formulae of the mineral may 

 be written, all of which will represent the results equally well ; 

 thus, andalusite, Al 2 SiO B , may be written as an orthosilicate, 

 O = Al Si0 4 = Al, or as a metasilicate (AlO) 2 Si0 3 , both of 

 which will yield the same percentage composition on analysis. 



The probable structural formula is derived from a study of the 

 decomposition products of any mineral, as well as by a thorough 

 study of its synthesis where that is possible. In the case of anda- 



