Kraus and Cook — Iodyrite from Tonopah. 213 



a prismatic habit, a {1120}, being by far the predominating 

 form. The prism m{1010} occurs as very narrow faces trun- 

 cating the edges of ${1120}. This is the first time that the 

 simultaneous occurrence of the prisms of the two orders on 

 a single individual of either the artificial or natural compound 

 has been recorded. Furthermore, the prism, a {1120}, had not 

 previously been observed on natural crystals. The general 

 type of development, possessed by these crystals, is clearly 

 shown by figure 1. 



The averages of several of the more important readings 

 used for the identification of the forms given above are as 

 follows : 



Observed Calculated 



c : u = (0001) : (4041) 1h° 21' 75° 13' 



c : m = (0001) : (1010) 90° 4' 90° 00' 



m : a = (1010) : (1120) 30° 2' 30° 00' 



Through the courtesy of the American Museum of Natural 

 History of New York, we have been able to examine speci- 

 men 2609, which shows some excellently developed crystals. 

 Our examination of this interesting specimen was, however, 

 confined to the use of the hand lense, but it is evident that 

 the crystals are undoubtedly twinned according to the law 

 noted above. Not being able to carry out goniometric meas- 

 urements on any of the individuals on this specimen, we are 

 unable to state definitely what forms occur upon them. 



Iodyrite from Tonopah, Nevada. 



The material examined from Tonopah, Nevada, consisted of 

 several thousand isolated crystals and two matrix specimens 

 furnished by the Foote Mineral Company. From a study of 

 the matrix specimens, composed almost entirely of quartz, on 

 which the iodyrite occurs as isolated crystals or in crystalline 

 crusts, it is clearly evident that the mineral is of secondary 

 origin. 



The crystals vary considerably in color and size. Those 

 studied more closely varied from a bright lemon-yellow through 

 the darker shades of yellow to a yellow-green. They measured 

 from 1 to 5 mm in length. The crystals possess a greasy ada- 

 mantine luster and occur as simple crystals, parallel groupings, 

 twins, and rosette clusters. In all nine types of development 

 were observed, four of which appear to be characteristic of 

 simple crystals. 



Type 1. Crystals of this type possess a pronounced hemi- 

 morphic development. They show the following forms : 

 c{0001} 5 m{10l0}, ^2021}, ^{2021}, £{7073}, w{9092}, 



Am. Jour. Sci.— Fourth Series, Vol. XXVII, No. 159.— March, 1909. 

 15 



