24S Ford — St&pJianite Crystals from Arizpe, Mexico. 



normal position, oscillating with each other, then three more 

 faces again in twin position, q', P' and m', and finally a few 

 others in normal position again. Figure 5 shows the top view 

 of another twin which showed two projecting arms twinned to 

 each other, each arm in turn being itself a twin. On the right 

 hand and upper side of the right hand arm the faces were 

 taken as in normal position, those on the lower and left hand 

 side being in twin position (' ) ; on the upper left hand arm the 

 faces are in twin position to ( ' ) and are marked with a double 

 prime ( " ), while on the lower side of this arm the position 

 reverts to that of the faces on the lower side of the right hand 

 arm ( ' ). The crystals of this second type averaged between 

 three and six millimeters for their greatest dimension. 



A chemical analysis was made of the mineral, the results of 

 which agree closely with the theoretical composition of steph- 

 anite. Tests were made for other elements likely to be 

 present but with only negative results. The method of analysis 

 was simple : the finely powdered mineral was digested for some 

 hours with aqua regia, the insoluble AgCl filtered off, dissolved 

 in ammonium hydroxide and the undissolved mineral, of which 

 there was always, at least, a small amount, collected on a weighed 

 filter and its weight deducted from the amount taken. The silver 

 was precipitated as AgCl by acidifying the ammoniacal solution 

 with nitric acid. After the nitric acid in the original solution 

 had been destroyed by repeated evaporation with hydrochloric 

 acid, the antimony was precipitated by H 2 S, collected on a 

 weighed filter tube, ignited in an atmosphere of carbon dioxide 

 and weighed as Sb 2 S 3 , following the method described by 

 .Bradley.* The sulphur was determined both by fusion with 

 sodium carbonate and potassium nitrate, and after treating the 

 mineral with strong nitric acid. The results of the analysis 

 are as follows : 











Theory for 





I. 



II. 



At. 



5Ag 2 S.Sb 2 S 3 



s 



16-23 



16-44 



16-33 



16-23 



Sb 



15-43 



15-18 



15-30 



15-25 





68-29 



68-42 



68-36 



68-47 



9999 100-00 



The Mineralogical Laboratory is indebted to Mr. Miller for 

 his kindness in sending these specimens to the School, and to 

 Prof. Huntoon for placing them at its disposal. 



Mineralogical Laboratory of the 



Sheffield Scientific School of Yale University, 

 New Haven, Conn., Feb. 1908. 



* This Journal, xxi, 453, 1906. 



