220 Kraus and Cook — Iodyrite from Tonopah. 



potassium or sodium chloride. It was, therefore, decided to 

 use a cold concentrated solution of sodium chloride. Immersion 

 in such a solution for fifteen seconds produced excellent micro- 

 scopic etch figures of an hexagonal outline on the base and 

 located as shown in figure 10. Figures of the general form and 

 position, as illustrated in the diagram, were produced on the 

 prism faces after a treatment of about forty seconds. The posi- 

 tion and outline of these figures show conclusively that crystals 

 of iodyrite possess six vertical planes of symmetry and one polar 

 axis of hexagonal symmetry. These elements show that the 

 classification which has been followed, as indicated above, is 

 to be considered as the correct one. 



Chemical Analysis. 



For the chemical analyses somewhat over a gram of clear, 

 semi-transparent crystal fragments of a pale yellow color was 

 used. This material was from Tonopah, Nevada, and was 

 placed at our disposal by the Foote Mineral Company, to 

 whom we desire at this time to express our thanks for the cor- 

 dial manner in which assistance was rendered us during this 

 investigation. 



As to the analysis only the method of decomposition need 

 be given in detail. This consists of placing a small sample 

 of the finely powdered homogeneous material in an evaporat- 

 ing dish and covering the same with water acidified with one 

 or two drops of concentrated sulphuric acid. A small piece 

 of chemically pure zinc is then suspended in the solution so as 

 to just come into contact with the powdered mineral. Decom- 

 position is generally complete after about twenty- four 

 hours. The hydriodic acid which is liberated passes into 

 solution, while the silver collects in the dish in the finely divided 

 metallic state. In this way any loss of hydriodic acid by vola- 

 tilization was avoided. For the sake of convenience the zinc 

 was attached to a piece of platinum wire, so that after the decom- 

 position was complete, it could be readily removed and any 

 adhering silver washed off. In order to remove any traces of 

 silver which could not be washed off, the zinc was immersed for 

 an instant in nitric acid. After filtration the metallic silver was 

 dissolved in nitric acid and then precipitated and weighed as 

 the chloride. Silver nitrate was added to the filtrate and the 

 iodine precipitated and weighed as the iodide. 



Two analyses were made which show very close agreement 

 and indicate that the composition of iodyrite may be expressed 

 by the formula Agl. 



In 1854 J. Lawrence Smith* made two analyses of iodyrite . 



* Dana, System of Mineralogy, 4th edition, 1855, 506. 



