246 



l*ord — StepJtanite Crystals from Ari&jpe, Mexico. 



which are shown on the stereoaraphic projection, figure 6, and 

 were as follows: h (010), c(001), i, (190), X (310), w(110), 

 tf(021), c?(114), A(112), /'(111), © (134), v (132), T(142). All 

 of these are well known and common forms and are included 

 in the lists given by Dana and Goldschmidt, with the exception 

 of i 1 (190). This prism is listed by Hint;;e and was observed 

 on crystals from Pribram, by Nejdl.* This form on the crys- 

 tals from Arizpe was observed five times and although it 

 occurred in each case as small faces four of the reflections 

 obtained from them were good. The average of the four 

 measurements 5/\/, = 10° 6'; calculated = 10° V. 



The habit of the simple crystals of this type is represented 

 in top view and clinographic projection in figures 2 and 3. The 

 prism zones are well developed and distinct, some of the faces, 

 either m or A,, being commonly large and prominent, giving to 

 the crystals their peculiar habit of being elongated in a diago- 

 nal direction. The pyramid faces are smaller, very irregularly 

 developed and frequently show repeated oscillation from one 

 to the other. In figure 3 such a characteristic oscillation is 

 shown where the base c caps the pyramids P and co and then 

 gives place again to the pyramid q. 



Among this type of crystals twinning according to the usual 



stephanite law, in which m becomes the twinning plane, 



was observed in several cases. The stereographic projection, 



figure 6, shows the observed forms in normal and one twinned 



*Zeitsclir. f. Kryst. u. Min., xxix, 408. 



