Cornwall — Greenockite on Calcite from Joplin, Mo. 1 



Aet. II. — Occurrence of Greenockhe on Calcite from Joplin, 

 Missouri; by H. B. Cornwall. 



A YELLOW coating of greenockite on the sphalerite of the 

 Joplin zinc district is no uncommon occurrence, and having 

 received, through the kindness of Messrs. Geo. L. English & 

 Co., of New York, specimens of calcite from this locality 

 which showed a very distinct greenockite coating, the writer 

 made tests upon the material, to determine what proportion of 

 cadmium sulphide the coating might contain. 



The greenockite occurs as a bright yellow, dust-like coating, 

 which can be easily I'ubbed off with the finger. Beneath the 

 greenockite coating, and often extending beyond it, there is, 

 for the most part, a brown or yellowish brown, more adherent 

 deposit, in a thin layer which can be easily scaled off with the 

 knife-point. This seems to be sphalerite. 



The greenockite was removed by very gentle scraping with 

 the knife and subjected to the tests below. Although in 

 places it is very distinct, the coating is also very light, and 

 from an area nearly two centimeters square not more than 

 0*0014 grm. of the purest material could be obtained. Even 

 this portion was found to be mixed with zinc compounds from 

 the scale-like deposit below it. 



Gently roasted on platinum foil over a Bunsen flame, the 

 powder on cooling was brown in some parts, and nearly white 

 (distinctly yellow while hot) in others, indicating cadmium and 

 zinc oxides.* The roasted powder, mixed with an abundance 

 of charcoal dust and heated to moderate redness in a closed 

 glass tube, gave a brownish sublimate (cadmium oxide), and 

 on stronger heating a less volatile sublimate, yellow while hot, 

 white on cooling (zinc oxide). 



On roasting 0'0014 grm. in a small platinum dish, dissolving 

 the residue in sulphuric acid, evaporating and gently igniting, 

 a residue was obtained which weighed 0*002 grm. Theory 

 requires from pure cadmium sulphide 0*00202 grm. of cadmium 

 sulphate ; but as a mixture of sphalerite and calcite similarly 

 treated might give the same figures, this result does not 

 indicate with any certainty the proportion of cadmium in this 

 lot of material. 



A second lot, also weighing 0*0014 grm., was dissolved in 

 nitric acid, evaporated to dryness with sulphuric acid, and 

 heated to expel all free acid, the residue treated with a single 

 drop of hydrochloric acid, sp. gr. 1*20, and dissolved in four 



*When not too strongly heated at first, the yellow coating assumes a 

 transient dark brown or reddish-brown color, and becomes yellow again on 

 cooling. 



