Cadmium, and Mercury. 359 



geologists. We may call attention, in this connection, to the 

 fact that sphalerite can be formed from acid as well as alkaline 

 solutions. If the temperature is as high as 300°, only sphaler- 

 ite seems to be deposited from solutions containing 1 per cent 

 of free sulphuric acid, while only a few per cent of wurtzite is 

 formed when the concentration of acid is as high as 5 per cent. 

 However, when the temperature drops to 250° a solution con- 

 taining as much as 2*5 per cent acid deposits practically pure 

 wurtzite. In other words, from 350° down to 250°, which is 

 as low as we have been able to get crystalline products, the 

 lower the temperature the smaller is the percentage of acid 

 required to give pure wurtzite. Unfortunately we cannot say 

 what that percentage may be at ordinary temperature, because 

 we cannot imitate the slow rate of crystallization which pre- 

 sumably proceeds in nature. But certainly sphalerite may be 

 what geologists call a secondary mineral if the temperature and 

 acid concentration fall within certain limits. On the other 

 hand, to judge from the synthetic work wurtzite can never be 

 anything but a secondary mineral, * since we have obtained it 

 only from acid solutions. Our knowledge of natural wurtzite 

 is still rather limited. Mr. B. S. Butler of the U. S. Geolog- 

 ical Survey has acquainted us of an interesting occurrence of 

 this mineral, to which he has given careful study. In the 

 Hornsilver Mine near Frisco, Beaver Co., Utah, wurtzite seems 

 to be undoubtedly a product of secondary sulphide enrichment. 

 The original ore, still fresh in the lowel levels, contains galena, 

 pyrite and sphalerite, a little chalcopyrite, and possibly other 

 copper minerals. The upper part of the deposit has been 

 largely oxidized to sulphates, minor quantities of carbonates 

 and other secondary minerals forming also. Octahedral cavi- 

 ties in the rock bear testimony to the former presence of pyrite, 

 which, though plentiful lower down, has now disappeared from 

 the oxidized zone. Below this lies a zone of secondary sulphide 

 enrichment carrying secondary (reprecipitated) chalcocite, cov- 

 ellite, and wurtzite in large quantities, much of the last named 

 precipitated around original sphalerite cores. The chemistry 

 of these processes must have involved first the oxidization of 

 the sulphides of zinc, copper, and iron to sulphates, and since 

 the original ore contained pyrite, the oxidized solution must 

 have contained sulphuric acid. As this solution moved down- 

 ward, the sulphuric acid gradually decomposed the more sol- 

 uble sulphides of the unoxidized ore with the formation of 

 sulphates and hydrogen sulphide. At greater depths, after 

 the oxygen of the solution was entirely used up and the acid 

 reduced, the sulphides were reprecipitated. The difficultly sol- 

 uble sulphides of copper would readily precipitate on the more 

 * At least, it can form only from acid solutions. 



