186 



University of California Publications. 



[Geology 



The average of sixty-seven measurements gave p' = .7606, 

 and an average of ninety-five measurements gave q' = .5492. 

 The values of these elements are therefore : 



e' = .3974 ; ,1 = 68° 40' ; p'„ = .7606 ; q' = .5492. 



Since the mineral is monoclinic, the polar elements e, p , and 

 q„ are obtained by multiplying the values e', p' , and q' by the 

 sin fi. The polar elements become therefore : 



e = .3692 ; p (1 = .7069 ; q = .5104. 



Axial ratio. — The length of the axes c and a can be derived 

 from the equations c = q'„ and a = ~. 



The axial ratio therefore for neocolemanite becomes : 

 a : b : C = 0.7771 : 1 : 0.5492 ; p = 111° 40: 



For colemanite the axial ratio is 



& : b : c = 0.7768 : 1 : 0.5430 ; p = 110° 07: Eakle. 



a : b : c = 0.7755 : 1 : 0.5415 ; p = 110° 13: Goldschmidt. 



Optical orientation. — Neocolemanite is optically positive and 

 the plane of the optic axes lies normal to the clinopinacoid as 

 with colemanite, but in neocolemanite the axis of least elasticity 

 makes an angle of about 42° with the vertical axis in the acute 

 angle ^, whereas in colemanite this axis is inclined about 83° 

 from the vertical in the obtuse angle p. The position of the 

 acute bisectrix, determined with sodium light is as follows : 



Neocolemanite c A c = — 42° 30: 



c A c = +83° 44: Hiortdahl. 5 

 c A c = -4-82° 34: Bodewig and vom Rath. 



Colemanite 



Indices of refraction. — The indices of refraction were deter- 

 mined with the "Abbe Totalreflectometer, " using sodium light. 



5 Th. Hiortdahl. Colemanit, ein krystallisirtes Kalkborat aus Californien. 

 Zeitschr. fur Kryst., 10 (1884), 25. 



o C. Bodewig und G. vom Rath. Colemanit aus Californien. Ibid., 179. 



