Kraus and Cook — Iodyrite from Tonopah. 219 



Etch Figures and Class of Symmetry 



Iodyrite is generally* placed in the class of symmetry which 

 is designated by Grothf as the dihexagonal pyramidal class. 

 This is based entirely upon the geometrical development of the 

 crystals, which, however, might allow of such interpretations 

 as to permit assigning the compound to at least three other 

 classes, since no faces of the most general form have as yet 

 been observed, Hence, it is necessary to rely upon etch figures 

 in order to determine with definiteness the symmetry of 

 iodyrite. 



Spencer \ indicates that repeated attempts were made to 

 obtain etch figures upon the crystals which he studied from 



Fig. 10. 



(p//o) 



(//oo) 



(/OTOJ 







(0//OJ 







{/ / oo) 



yo/o) 



Broken Hill, New South Wales. These attempts were unsuc- 

 cessful. Spencer, unfortunately, does not mention the solvents 

 which he employed. 



A solution of potassium iodide is considered by Roscoe and 

 Schorlemmer§ as a good solvent for silver iodide. A solution 

 of N/2 potassium iodide r was used by us for the production of 

 etch figures on carefully selected crystals of iodyrite from 

 Tonopah. Although the solution etches the crystals, we were 

 unable after repeated attempts to obtain figures possessing a 

 definite outline. 



According to Gmelin|| silver iodide is soluble in a solution 

 of mercuric nitrate and in a concentrated solution of either 



*See Dana and Naumann-Zirkel, loc. cit. ; also Bauer, Lehrtrach der Miner- 

 alogie, 2te Auflage, 1904, 439 ; Klockmann, Lehrbuch der Mineralogie, 4te 

 Auflage, 1907, 403 : Groth, Tabellarische Uebersicht der Mineralien, 4te 

 Auflage, 1898, 50. 



f Physikalische Krystallographie, 4te Auflage, 1904, 496. 



% Loc. cit. 



§ Treatise on Chemistry, 1907, ii, 468. 



f Handbook of Chemistry, 1852, vi, 157. 



