428 THE GEOLOGY OP MINNESOTA. 



[Chlorastrollte. 



polishes well. In a similar manner it fades out into a white structureless substance 

 whose hardness is less, but also sometimes into a pinkish zeolitic substance which 

 resembles mesolite. The idea is suggested by the examination of a large number of 

 such transitions, that the green structureless substance is a transition stage between 

 chlorastrolite and mesolite or thomsonite, the iron element prevailing on one side, 

 and not on the other. It seems likely that this green structureless mineral may be 

 that which has been analyzed and named zonochlorite, by A. E. Foote (American 

 Association for the Advancement of Science, 1873). Probably no definite mineral 

 composition or structure can be detected in this green substance, the extremes 

 only being identifiable, viz.: mesolite on one side and chlorastrolite on the other. 

 That the two minerals are closely allied in origin, structure and composition, 

 differing principally in the content of iron, is evident not only from the chem- 

 ical composition and optic characters, but also from their intimate association 

 often in the same amygdule. Such association sometimes illustrates a sudden 

 transition from one to the other, and sometimes a gradual one, with a considerable 

 amount of the amorphous green mineral. The re-examination of zonochlorite by 

 Hawes (American Journal of Science, x, 24, 1875) shows that it is not a homoge- 

 neous mineral. He describes it as having green earthy particles as impurities 

 disseminated in a white mineral. It is evident also that the material examined by 

 Hawes, under the name chlorastrolite, was not a fair sample of that mineral. 



As has been remarked under No. 566, this mineral has a well-marked individ- 

 uality, structurally, while its chemical composition is quite different from that of 

 thomsonite, viz.: 



Thomsonite (Table mountain, Colorado, Hillebrand). Chlorastrolite (Isle Royale, Whitney).* 



SiO 2 40.52 SiO 2 - 36.99 



A1 2 O, 29.22 A1 2 O 3 25.49 



Fe 2 O 3 0.79 Fe 2 O 3 - - 6.48 



CaO 12.43 CaO 19.90 



Na 2 O 4.31 Na 2 O 3.70 



H 2 O 12.79 K 2 O 0.40 



H 2 O - 7.22 



100.06 100.18 



Mic. The marked superficial characteristics are expressed on the interior by a 

 finely radiated fracture, the fibres of which start from points and run unequally to 

 the right or left, so that the spherules are eccentric and the fibres of one spherule 

 abut transversely or obliquely on those of another. 



Extinction takes place parallel and perpendicular to the fibres. 



The fibres are parallel with the mean axis of elasticity ( m ) and hence sometimes 

 where they have their elongated sections, they show a positive and sometimes a 

 negative sign, the axial plane being transverse to the fibration. 



* Geology of (he Lake Superior Lund District, part ii, p. 97, 1851. Journal Boston Society of Natural History, vol. v, 

 p. 488, 1847. 



