1108 A TREATISE ON METAMORPH1SM. 



It is therefore evident that, since iron sulphide is the most common 

 of the sulphides in nature and sodium carbonate an abundant carbonate, 

 the reaction given is likely to take place on a considerable scale, and there- 

 fore that sodic sulphide, which is available to dissolve the sulphides of 

 the other metals, is probably produced under natural conditions in large 

 quantities. 



Since experiment has shown the solubilit}- of the sulphides in sodium 

 sulphide and sodium carbonate, and observation has shown the frequent 

 presence of these compounds, especially sodium carbonate, in underground 

 water, there can be little doubt that the sulphides as such are transported 

 on an extensive scale in the ground waters. 



While the sulphides are somewhat readily soluble by various com- 

 pounds, it is not believed that the material of the sulphide ores is carried 

 to the openings in the rocks to be deposited by the ascending waters in the 

 form of sulphide only. It has been fully explained that the sulphides in 

 the belt of weathering are largely oxidized to sulphites and to sulphates, 

 mainly the latter, and taken into solution by the descending waters. In 

 this connection it is to be noted that the sulphates of iron, copper, zinc, 

 nickel, mercury, and silver are all readily soluble; and even the sulphate 

 of lead is dissolved to the extent of one part in 31,500 parts of water at 

 15° C., a which is probably entirely adequate for the purposes of under- 

 ground transportation. While the sulphur compounds of these metals are 

 very largely transported as sulphates, they may also to some extent be 

 transported as sulphites. Of these metals the sulphites of iron, copper, 

 zinc, and silver are rather readily soluble. 



PRECIPITATION OF SULPHIDES. 



We have now seen that the metals which may be deposited as sul- 

 phides in the ore bodies may be transported as sulphides in solution or as 

 sulphates. The question now arises as to the conditions which will result 

 in their precipitation. It will be necessary to consider separately the pre- 

 cipitation of compounds transported as sulphides and as sulphates. 



The more important metals which are known to be precipitated as 

 sulphides in sufficient quantities to constitute ore are those of iron, lead, 

 zinc, nickel, cobalt, copper, silver, mercury, arsenic, and antimony. As to 



«Comey, A. M., Dictionary of Chemical Solubilities: London, .1896. 



