1894,] Mineralogy and Petrography. 423 
cules in the proportions 90.8 : 4.1 : 2.4, with the addition of 2.7% of the 
molecule Na Mn (SiO,),. The mineral is essentially a blue variety of 
diopside, differing from the anthochroite of Igelström” and from the 
blue pyroxene of Merrill and Packard.” 
Zonal Plagioclase.—Herz" has shown by a study of the position 
of axial planes in successive zones of zonal plagioclase, and by the 
values of the respective cleavage angles, that the zonal banding in this 
mineral is due to the concentric growth of envelopes of different com- 
position. The axial planes and the cleavage angles always correspond 
with the extinction angles in the corresponding band. It had been 
suggested by Grosser that the regular decrease in the extinction of the 
shells of a zonal plagioclase is due to difference in the orientation of 
the successive envelopes and not to a difference in their chemical com- 
position, Herz’s work proves conclusively that the decrease in the 
value of the extinction is not due to differences in orientation of the 
same chemical substance. 
Hercynite in Gabbro.—Small octahedra and large irregular 
masses of the green spinel hercynite occur in an altered gabbro at Le- 
Prese, in the Valtellina, Switzerland. According to Linck,” it is 
found as irregular granular masses within the rock, and as small octa- 
hedral crystals enclosed in its plagioclase and associated with corundum 
sillimanite and biotite. The spinel includes small quantities of biotite, 
small plates of ilmenite, resembling the plates in hypersthene, a little 
pyrite, etc. An analysis of tolerably pure material yielded : 
SiO, ALO, FeO, MgO FeO Total 
159 59.62 3.10 9.38 25.30 = 98.99 
which corresponds to (Fe Mg) Al,O, in which Fe : Mg = 3:2. 
Optical Constants of Topaz.—Four Japanese topaz crystals 
and one crystal of the same mineral from New South Wales are de- 
scribed by Hahn,” and some of the optical constants of the former 
have been determined. One of the crystals from Otamjama near 
15 AMERICAN NATURALIST, 1890, p. 74. 
*Ib., 1892. p. 848. 
Min. u. Petrog. Mitth. XIII, p. 341. Classe 
-o *Siteb. d. Kön-preuss. Akad. d. Wiss. zu Berlin. Phys.-Math.- = 
1898, p. 47. 
“Zeits. f. Kryst., XXI, p. 334. 
