454 Genera: Notes. [May, 
; »— fo. Thin plates parallel to the cleavage are trans- 
lucent to transparent with very weak pleochroism if any. The 
positive bisectrix is probably perpendicular to the cleavage plane. 
The dispersion very large, v > p, so that for red light the mineral 
; is uniaxial. For green and blue light, however, the optical angle 
| is about 50°. In all its optical properties it resembles rutile very 
closely. In attempting to prove by means of etched figures! 
-that cryolite is without doubt a monoclinic mineral, Baumhauer 
has succeeded in showing that in the massive mineral two crys- 
tals are so united that (1) œ P and oP of the one are parallel re- 
spectively to œ P and oP of the other; or (2) o P of the first is 
Bie parallel to oP of the second, and vice versa, Crystals of struv- 
ite with a different habit from any heretofore described have been 
found at Homburg v. d. H., and investigated by Kalkowsky.’ 
The planes observed were Fe OF, 2P3) 22S, af Gree 
In physical properties the mineral from this locality also 
differs from that found elsewhere, a:b:c = .5685 : 1: .OII3. 
\ 
ae SIb., x, p. 366. 
n t 1b. XI, p.29. 
= $ Bulletin de la Société Minéralogique, July, 1885, p. 304. 
a 
