SOME CRYSTAL MEASUREMENTS OF CHILLAGITE. 217 



Form C. — In all the crystals two basal planes were 

 present, one of which showed a fairly good signal and the 

 other a poor one. The angles between the basal planes as 

 recorded vary from 4' to 21'. 



Forms y and I. — These are closely related as they 

 represent the 119 and 118 faces respectively. The former 

 has been found prominently developed on stolzite. 1 



Form f. — The faces belonging to the form f (115) are 

 developed on all the crystals measured, with the exception 

 of crystal 4. This crystal appeared on inspection to be 

 similar to the others measured. Very possibly this is due 

 to the form d (L14) being more prominently developed than 

 in the case of the other crystals. The readings for the 

 faces of the form f were so constant that this face was 

 taken as the standard. Eighteen faces of this form were 

 developed. The p's of four of them were exactly 23° 23'; 

 the mean reading of the 19 faces developed gave the value 

 as 23° 26'. 



Knowing that the crystals measured were related to 

 wulfenite and stolzite, the measurement of these minerals 

 as recorded in Goldschmidt were referred to, in order to 

 connect them with the crystal measurements if possible. 

 The standard value 23° 23' was nearest the value 26° 22' 

 which was the p for the 229 faces on wulfenite. Assuming 

 the indices of the standard face to be 115, the p for the 

 Oil face was brought into the closest agreement with that 

 for the Oil face of both wulfenite and stolzite. 



By means of this standard face the value of the "c" axis 

 was determined as : — 1. 5291 



That of wulfenite is 1. 5774| 2 



That of stolzite is 1. 56063 



1 E. Artini, Uber den Stolzit von Bena, Padru (Ozieri) Zeit. f. Kryst. 

 Vol. xliii., pp. 422-3, 1907. 



2 Goldschmidt, Krystallographische Winkeltabellen. 



