358 Allen and Crenshaw — Sulphides of Zinc, 



tors which determine the product ; and if we omit two experi- 

 ments out o{ the twelve done at three different temperatures, we 

 find the following regularity: For a given temperature the higher 



the acid concentration the greater is the quantity of wurtzite 

 found in the product, and for a given acid concentration the higher 

 the temperature the greater is the quantity of sphalerite found 

 in the product. More work ought to be done on this point, 

 but the above conclusion gains support from the fact that the 

 same rule proved true in the case of the disulphides of iron.* 

 There the quantity of pyrite increased with the temperature 

 for a given acid concentration, and the quantity of marcasite 

 increased with the acid concentration for any given tempera- 

 ture. When it is remembered that pyrite and sphalerite are 

 stable forms, while marcasite and wurtzite are tonstable, the 

 analogy becomes striking. 



In one respect this synthetic work on sphalerite and wurtzite 

 may appear inadequate to the geologist, viz., the temperature 

 of formation. Field observation points to lower temperatures 

 in many instances than the lowest at which we have succeeded 

 in forming these minerals. Although it would be highly desir- 

 able to obtain positive results at lower tern peratu res, it may be 

 pointed out that there is nothing in our knowledge of the min- 

 erals to indicate anything more than continuous changes in the 

 necessary conditions as the temperature falls. "We must admit, 

 however, that the temperature interval is still too long to extra- 

 polate accurately, so that we might predict, for example, what 

 concentration of acid would be necessary at ordinary tempera- 

 ture to give rise to pure wurtzite. Since even quite dilute solu- 

 tions of soluble sulphides and zinc salts always give an immediate 

 amorphous precipitate at the ordinary temperature, and since 

 amorphous sulphides are seldom met with in nature, we are 

 constrained to believe that the natural sulphides which have 

 been deposited even as low down as 100°, must have been crys- 

 tallized from very dilute solutions indeed and therefore exceed- 

 ingly slowly. 



The genesis of the natural sulphides of zinc. 



The sphalerite of deep veins has in all probability been 

 formed from hot solutions, and the latter are generally alka- 

 line, as we know both from field observation of hot springs and 

 from our knowledge of the hydrolysis of alkali silicates, like 

 the feldspars, with hot water. In accord with this we have 

 found that alkaline solutions always give rise to sphalerite, 

 never to wurtzite. 



The conditions under which the sphalerite of the Mississippi 

 Yalley region was deposited have been much discussed by 



* Allen, Crenshaw and Johnston, loc. cit., p. 179. 



