234 



University of California. 



[Vol. 3. 



Interfacial Angles. — The calculated, and some of the meas- 

 ured, interfacial angles are as follows: 







Calculated. 



Measured 



(110) : 



: (HO) 



60° 30' 



60° 30' 



(010) 



: (120) 



40 36 



40 36 



(010) 



: (450) 



53 54 



53 52 



(001) 



: (HO) 



60 47 



60 40 



(001) 



: (011) 



19 34 



19 31 



(010) 



; (021) 



54 35 



53 40 



(001) 



: (201) 



68 06 



67 45 



(001) 



: (HI) 



41 57 





(001) 



: (T21) 



51 06 







PHYSICAL 



PROPERTIES. 





General Characters. — The crystals are transparent and have 

 a deep brick red color, vitreous luster and pale yellow streak. 

 They are very brittle, have a hardness of 1.5 to 2, and a specific 

 gravity of 2.075, determined with the Thoulet solution. Cleavage 

 is very perfect parallel to the clino-pinacoid \010\ and distinct 

 parallel to the unit prism ) 1 1 \ . 



Optical Characters . — The plane of the optic axis lies normal 

 to the clino-pinacoid. The mineral is optically positive, and the 

 acute bisectrix C makes an angle of about 12° with the vertical 

 axis in the acute angle /8. Sections cut almost normal to the 

 vertical axis show a good biaxial interference figure . The indices 

 of refraction were determined approximately on thin sections by 

 the Due de Chaulnes' method, and gave for sodium light 

 a = 1.544 ; f3 - 1.548 ; y = 1.572. 



The double refraction from this would be high = .028. The 

 optic angle calculated from the indices gives 



2F n , = 40° 54: 



A measurement of the optic angle of a section mounted in 

 Canada balsam, the latter having about the same index as P, gave : 



25,,,, = 40? 



The dispersion is p < v . 

 The crystals are strongly pleochroic; C, deep orange red; t>, 

 pale red; bright yellow. In thin sections the colors are much 

 paler, b and ft becoming almost colorless, while C becomes orange 

 yellow. 



