1912] Eakle: The Minerals of Tonopah, Nevada 11 



jarosite. The later crystallization of the iodyrite is evidenced by 

 its deposition on layers of the iron sulphate and on other second- 

 ary minerals lining the Assures. 



The crystals are mainly of a bright sulphur-yellow color, but 

 some incline to greenish yellow, and others are tarnished bronze- 

 brown. The brilliant crystals after exposure to light gradually 

 become cloudy and opaque. The best crystals for measurement 

 occur isolated in quartz cavities, generally perched on drusy 

 quartz. 



Iodyrite possesses considerable crystallographie importance 

 because it is one of the few known representatives of the 

 dihexagonal-pyramidal, or hemimorphoric, class of symmetry, 

 and many of the Tonopah crystals show this hemimorphie char- 

 acter quite prominently. With very few exceptions the crystals 

 are simple combinations of the unit prism (1010), base (0001), 

 and steep pyramid (2021). The prism is terminated on the 

 upper or positive end, according to the usual orientation, by 

 narrow faces of (2021) and a broad base, and on the lower or 

 negative end by (2021), often without a lower base, as a char- 

 acteristic habit. The few other forms are very rare and were 

 observed only once or twice in a lot of several hundred crystals 

 examined. Leaving out all doubtful forms, which were many 

 on account of imperfections, the forms determined were : 



c (0001) i (2021) u (4041) 



m (10T0) r (lOTl) £ (2021) 



a (1120) / (3031) c (0001) 



One crystal showed narrow faces of the second order prism 

 (1120). Several had the unit pyramid (10T1) as very narrow 

 faces. The steeper pyramids (3031) and (1041) were each 

 observed on two crystals. The only negative pyramidal termi- 

 nation was (2021) . 



The four forms (0001), (10T0), (2021), and (2021) make 

 up the crystals and the habit or type is governed by the size and 

 predominance of these forms. Few of the crystals are simple 

 because of the alternating growth of prism and pyramid. Suc- 

 cessive alternations of these two forms have produced hori- 

 zontally striated, furrowed, stepped, and tapered crystals 

 which make measurements very poor. A fact also observed was 



