GUIDE TO THE MINERALOGIO COLLECTIONS 87 



It is formed in the same way as malachite and occurs asso- 

 ciated with it at the localities named under the latter mineral. 

 Azurite is an ore of copper. 



SILICATES 



The members of this division are mainly important as rock- 

 forming minerals. They are oxygen salts in which silicon is 

 present as the acid element and are classed according as they 

 are salts of disilicic acid (H 2 Si 2 5 ), polysilicic acid (H 4 Si,O s ), 

 metasilicic acid (H 2 SiO s ) or orthosilicic acid (H 4 SiOJ into 

 disilicates, polysilicates, metasilicates, orthosilicates. 



Subsilicates represent a group of basic silicates having a lower 

 oxygen ratio than the foregoing. 



Disi I lea tcs , poh/s i J i ca tes 

 Feldspar group 



For many reasons the feldspars are considered the most 

 important group of minerals in the large division of the silicates. 

 They form an essential constituent in a number of rocks such as 

 granite, syenite, gneiss, etc. which are of primary importance 

 as building materials and are largely quarried in all parts of the 

 world. As a group of minerals the feldspars present several 

 general characteristics which unite them in close relation to 

 each other. 



1 Crystallizing in the monoclinic and triclinic systems, the 

 feldspars agree closely in crystal habit, prism angle and methods 

 of twinning. 



2 They are characterized by two easy cleavages inclined to 

 one another at an angle which is close to 90°, the cleavage sur- 

 faces being smooth and of high polish. 



3 In hardness they vary between the comparatively close 

 limits of <; and 6.5. 



4 They range in color from clear and colorless through white, 

 pale shades of yellow, pink or green, to less common dark gray 

 tints. 



5 The feldspars are silicates of a luniiniiini and some other 



base, commonly potassium, sodium or calcium, less frequently 



barium. 



