BY D. MAWSON. 



593 



The following measurements wei*e made with a Fuess universal 

 reflecting goniometer. 



Faces. 



Measured Angles. 



Calculated Angles. 



1. mh.ni 1 " 



110 A110 



56° 0' 





2. c A m 



001 A 110 



77° 54' 





3. dhb 



111 A010 



74° 15' 





4. dhm' 



111 AllO 



67° 55' 





5. d Ac 



111 A 001 



33° 8' 



33° 7' 



6. I)A d 



010A111 



Approx. 77° 



75° 51' 



7. b A id 



010 A 021 



60° 23' 



60° 25' 



8. bNu 



010AI31 



50° 35' 



50° 40' 



9. bhg 



010 A 251 



Approx. 45° 20' 



45° 55' 



10. b\ij 



010A2-10-1 



26° 45' 



26° 54' 



11. cNcj 



001 A2-5-1 





60° 54' 



Numbers 1, 2, 3, and 4 are measurements of the first order, 

 and were used for calculating the crystal indices; the figures 

 quoted are the mean angles for about 15 crystals measured. The 

 face D yielded in place of the signal only a faint blur of light, 

 the reading being indefinite, though the presence of the face was 

 certain. Faces g and y }delded only faint signals. 



Calculated crystal indices : — a :b:c= -54736 : 1 : -28404 



= 76° 16' = 001A100. 

 Twinning not observed. Cleavage -{ ilO }- perfect, ^ 010 \~ 

 less perfect. Brittle. Fracture, splintery. Hardness, quite 6. 

 Specific gravity determined by the pycnometer method on an 

 idiomorphic crystal after boiling in distilled water under a 

 suction pump, 3-35 at 26°C. Colour of powder, slate-blue. 

 Highly opaque until the slice approaches 0'06 mm. in thickness. 

 Colour by transmitted light, on sections 003 mm. thick, varies 

 from olive-green on (010) to sky-blue tints on (100), and yellowish- 

 grey on (001). Pleochroism on sections 0-03 mm. thick. — 

 A (near c) = yellowish-blue. 

 b = bluish-grey. 



C (near a) = faintly brownish-yellow. 

 Absorption strong, — a > b > £ 



