137 



EOCK-EOPvMING MIIN^EEALS. 



The noil -metallic minerals that compose igneoua and other 

 rocks are arranged in seven cases (95-lOlj immediatelyfollowing 

 the foreign metallic series. These are for the most part silica or 

 ■silicates, and seldom" have any appreciable commercial value. 

 They comprise a large number of varieties, and are conveniently 

 arranged into the following groups : — 



I. Minerals composed entirely of silica (case 96 and part 

 of 95). e.g., quartz. 

 IT. Minerals consisting largely of silicates of alumina, with 

 potash, soda, or lime, e.g., felspar (cases 95 and 97). 

 III. Minerals consisting largely of silicates of iron and 



magnesia, e.g., mica (cases 97 and 93). 

 IV. Miscellaneous silicates and oxides, e.g., garnet (case 



100). 

 Y. Sulphates (case 99). 

 TI. Carbonates (case 101). 

 YII. Fluorides and phosphates. 



Very many of these minerals may also occur as gangue 

 material in lodes, and many of the most perfectly crystallised 

 specimens have been so formed. 



The great proportion of rock-forming minerals contain silica 

 and alumina ; so great, indeed, is this proportion, that it is 

 estimated that silicon constitutes one-half of the crust of the 

 earth, and aluminium one-fourth, leaving only the remaining 

 fourth for all the other elements. 



The rock-forming minerals are all placed together, so that 

 specimens from New South "Wales and other localities will all 

 be found in these cases. 



GROUP I. 



Minerals composed entirely of silica. Case 96, 



Cetstallised silica. Qcartz. 



Quartz is the most common of all minerals. It assumes 

 various forms and colours, is frequently beautifully crystallised, 



