THE EARTH'S CRUST 47 



The numerous natural polysilicates (poly- means many) compos- 

 ing granite and most other rocks of the earth's crust are salts of 

 poly silicic acids, although the acids themselves are not known to 

 exist free from the basic elements or radicles. The following may 

 illustrate a few of the possible combinations, the last three being 

 known only in salts in which bases appear in place of the acid 

 hydrogen : 



Silicon dioxid, SiO 2 . . . . O = Si=O. 



Metasilicic acid, H 2 SiO 3 . . O = Si = (OH) 2 . 



Orthosilicic acid, H 4 SiO 4 . . (HO) 2 = Si = (OH) 2 . 



Disilicic acid, H 2 Si 2 O 5 . . . HO-Si=O 3 =Si-OH. 



Polysilicic acid, H 4 Si 3 O 8 . . (HO) 2 = Si=O 2 = Si = (OH) 2 . 



= Si " 2 = Si ^ 2 = Si =- Si = (OH) - 



Among the most common mineral compounds found in granite 

 is ordinary felspar, or orthoclase, or potassium aluminum poly- 

 silicate, KAlSi 3 O 8 , or (KAlSi 3 O 8 ) 2 , whose structural formula may 

 be represented thus: 



/o o o o 



K/ Si Si Si 





O O 



Si Si 



This is sufficient to illustrate what is meant by polysilicates. 

 Other silicates differ from the common felspar by the substitution 

 of other elements for potassium or aluminum or both, and also 

 by different proportions of the various constituents, as: 



Orthoclase (potassium felspar) .... KAlSi 3 O 8 . 



Albite (sodium felspar) ....... NaAlSi 3 O 8 . 



Anorthite (calcium felspar) ...... CaAl 2 Si 2 O 8 . 



Crysolite (magnesium iron silicate) . . . MgFeSiO 4 . 



In some cases hydroxyl groups are included, and when such com- 

 pounds are heated, two hydroxyl groups are broken, leaving one 

 oxygen atom in their place, thus yielding water and anhydrous 



