NOTES ON THE TOPAZ FKOM MINO. 



71 



and are sometimes coloured different tints of green and pink, which 

 disappear on long exposure to sun light. Pleochroism in large speci- 

 mens can be observed without an instrument. The following axial 

 colours vary in intensity, according to the pellucidity : 



a = sky-blue. 

 6 = pink. 

 c = yellow. 

 The absorption is a>*6^c 



Some of the larger specimens are coloured differently along their 

 horizontal axes. Nothing is to be specially mentioned, with respect 

 to the optical characters of this topaz, of which the following optical 

 constants have been measured. 



a ß r 2V 2E 



For yellow. 1 1.6134 1.6178 1.6233 62°52' 115°3' 



For red. 2 1.6113 1.6142 1.6162 62°24' 113°33' 



Enclosures are commoü ; they are cassiterite, tourmaline, chloritic 

 substance, as well as liquids containing bubbles. The specific gravity 

 is 3.556-3.566 ; mean 3.561. This is nearly the same as that of 

 crystals from the Urals and from Brazil. 



Crystal Faces. — The faces, which I have recognised are as 

 follows : 



Brachydomes. / = (021) 2 P do 



X = (043) i P do 

 Macrodomes d = (201) 2 P do 



Pyramids. it = (111) P 



i = (223) fP 

 r\ = (241) 4 P 2 



1. Using sodium carbonate. 



2. Using strontium chloride, 

 f These are faces now first observed in Japanese crystals 



