Centre Valley Tin Ore 



Always in connection with these deposits of tin and 

 associated minerals there is much silicification of the 

 rocks ; that is to say, formation of secondary quartz. 



Let us now consider how these changes have been 

 brought about and how they are connected one with 

 another. In the first place it must be noted that the 

 rocks which form the walls of the veins are very much 

 altered, and the nature of the alteration often gives 

 valuable clues as to the mode of formation of the veins 

 themselves. 



The tin deposits are accompanied, as has been seen, 

 by the formation of minerals containing fluorine, and it 

 has been experimentally shown that at high temperatures 

 the elements tin and fluorine form a compound SnF 4 

 which is volatile. At lower temperatures this fluoride 

 of tin is chemically acted upon by water or steam, forming 

 oxide of tin (tin-stone) and hydrofluoric acid, thus : 



SnF 4 + 2H 2 O = SnO 2 + 4 HF. 



The hydrofluoric acid then acts upon the felspars and 

 other minerals of the surrounding rocks, producing fluor- 

 spar, topaz, and other fluorine-bearing minerals. 



The secondary quartz (SiO 2 ) which is such a common 

 feature of the rocks in the neighbourhood of tin veins 

 and also of the veins themselves, in all probability in 

 many instances is produced in a similar manner, as 

 silicon also forms a volatile fluoride which is decomposed 

 by water or steam, thus : 



SiF 4 + 2 H 2 O = SiO 2 + 4 HF. 



In cases where the element boron is also present the 

 minerals axinite and tourmaline are formed, and of these 



285 



