452 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



065 new? 



matter to obtain thin oriented plates of the mineral by crushing a 

 fragment on a slide when thin plates lying on the cleavage are. found 



to give a perfectly centered bi- 

 axial negative figure with 2E 

 very small. The index of re- 

 fraction and birefringence are 

 high. These properties defi- 

 nitely identify the mineral as 

 leadhillite. 



Leadhillitehas also been iden- 

 tified in minute tabular crystals 

 in a specimen from the Look- 

 out Mountain mine east of the 

 Forks of Pine Creek. The speci- 

 men is a residual lump of me- 

 dium grained galena contain- 

 ing a little chalcopyrite. It is 

 crusted with alteration prod- 

 ucts, which include cerusite, lim- 

 onite, chrysocolla, copper pitch, 

 linarite, caledonite, and the 

 leadhillite. The leadhillite 

 forms small tabular crystals of 

 hexagonal aspect, most of which 

 are now coated with or replaced by cerusite. A few were transpar- 

 ent and unaltered and one of these which was measured had the 

 development illustrated in the drawing (fig. 157) and gave the fol- 



Fig. 157 



— Leadhillite. Lookout Mountain mine, 

 Pine Creek, Shoshone County 



The measured crystal was examined under the microscope and 

 found to be biaxial negative with 2E very small, birefringence high. 

 The axial plane is parallel to the face adopted as a (100) and the 

 acute bisectrix perpendicular to (001) which makes the optical 

 orientation Y = a, Z = &, X = c. Dispersion r<v inclined marked. 



