1 16 The American Geologist- February, i899 
[Contributions to the Mineralogy of Minnesota. IV.] 
CHLORASTROLITE AND ZONOCHLORITE FROM 
ISLE ROYALE. 
By N. H. WiNCHELL, Minneapolis, Minn. 
Chlorastrolite . Although not known on the Minnesota 
coast is in the same rocks on Isle Royale. The literature of this 
American mineral shows an interesting history. Discovered 
by Dr. C. T. Jackson when he had charge of the survey of 
the United States mineral lands of the lake Superior land dis- 
trict, it was analyzed by J. D. Whitney in 1847,* after the 
charge of that survey passed to Foster and Whitney. It was 
analyzed by Hawes in 1875! who came to the conclusion from 
the chemical composition and its variation, that it is not a 
homogeneous mineral, and was believed by him to be an im- 
pure condition of prehnite. In 1888 Lacroix showed that its 
optical characters ally it with thomsonite, the optic plane be- 
ing transverse to the fibration. Dana, in the last edition of 
his mineralog}^ (1892), considers it of doubtful authenticity, 
placing it in an "appendix to zeolites," among synonyms and 
questionable species. 
It occurs as small pebbles on the beach, but is also in the 
trap rock. It has a finely radiated structure, and stellate 
markings of light and dark green on the smoothed surface. Its 
hardness and beauty have given it considerable value in the 
market as a gem. The individual specimens are rarely larger 
than a pea, but sometimes reach the size of half an inch in 
diameter. The fibres are fine, rigid and uniform, and are 
elongated with n^. They vary in brightness in convergent 
light in proportion as they expose to observation the obtuse 
or the acute optic angle. Specific gravity is 3.155. Hardness, 
5.5. Chlorastrolite has a higher index of refraction than thom- 
sonite, and its extinction varies to a maximum of about 20° 
from the fibration. It has a distinct pleochroism. being color- 
less and light-green. 
Hawes and Lecroix both mention impuraties in the speci- 
mens examined by them ; in the specimens belonging to the 
collection of the Minnesota survey, from which thin sections 
have been made, the mineral is quite pure, showing only a 
*J. Nat. Hist., Boston, V. 488, 1847. 
'Am. Jour. Sci., X. 25, 1875. 
