124 E. V. Shannon — Occurrence of Ilvaite in the 



opposite side. The results obtained in measuring the 

 terminal faces were quite disappointing. Only one 

 value was obtained for the angle rr' (over c), 67°2 / , which 

 is 9' short of the angle given by Dana. The values 

 obtained for the angle oo'" form the most puzzling fea- 

 ture of the crystals. The angle given by Dana is 

 40 c 29'. The values read on the Idaho mineral range 

 from 36°34' to 42°45'. Two values, 36°34' and 39°14', 

 fall below 40° ; seven fall between 40°9' and 40°52' ; three 

 fall at 41°2', 41°21 / , and 41°24 / , respectively, and six 

 fall between 42°1' and 42°45'. On the whole there is 

 little difference in the relative quality of the faces upon 

 which these angles were obtained. Assuming that all 

 those values between 40°0' and 41°0 / represent this angle, 

 (the average for six of these values is 40°28' as com- 

 pared with 40°29' calculated), we have still the group of 

 six values between 42°0 / and 42°45', the average for the 

 group being 42°27'. This doubtless is another form in 

 the zone bob' having the value pyr. A pyr'" = 42°27' ± 5'. 

 It is possible that the other abnormal values obtained are 

 due to the tendency of the crystals to split in half through 

 the center of the crystal parallel to 010. Such rifts 

 might easily be concealed in the deep striations of the 

 101 face and might easily separate the crystal sufficiently 

 to change the angle between the faces of the pyramid. 



Composition. — Since a complete analysis was avail- 

 able, the mineral was not reanalyzed. The analysis of 

 Idaho ilvaite by Hillebrand is compared below with 

 analyses of ilvaite from a number of other localities. 



I II III IV V VI 



SiO., 



29.16 



28.09 



29.81 



29.30 



29.93 



29.3 



A1A 



.52 



.32 



.16 









Fe,0„ 



20.40 



20.80 



18.89 



20.30 



20.16 



19.6 ' 



Feb 



29.14 



29.93 



32.50 



33.50 



31.83 



35.2 



MnO 



5.51 



3.24 



2.22 



1.97 



3.02 



• a • • 



CaO 



13.02 



15.89 



13.82 



13.71 



13.71 



13.7 



MgO 



.15 



.18 



.30 











Na,0 



.08 Cr,0. 



, 0.13 









■ * * • 



h,o— 



.15 













11,0+ 



2.64 



1.62 



1.62 



1.90 



0.42 



2.2 



Total 



100.41% 



100.20 



99.32 



100.68 



99.07 



100.00 



Sp. Gr. 



4.059 





3.85 



4.05 



.... 



.... 



