424 THE GEOLOGY OP MINNESOTA. 



[Chlorastrolite. 



neutral tint, separated by intervals of light, or light yellow. These changes shift 

 from one to the other, not abruptly, but by the gradual oncoming of streaks or spots 

 of the other tint. The transitions between the yellow and the neutral tints are 

 always through a narrow interval of white which blends on one side with the neutral 

 and on the other with the yellow. 



If the convergent light lens be used it is at once apparent by the nature of the 

 interference figure, that the fibres that undergo these changes of color are all situated 

 uniformly in the same position with respect to the section, from the center to the 

 circumference of the spherulith. They may all give the bisectrix n e . Or, in another 

 spherulith or in another part of the same spherulith, while these same variations are 

 exhibited, the interference figure seen is that indicative of ,, from one end of the 

 fibres to the other. It is inferred therefore that these variations of color are not due 

 to a shifting of the fibres with respect to the position of their axes, but to an over- 

 lapping succession of fibres cut obliquely, producing a section that might be compared 

 to the layering of shingles on a roof. 



It is also apparent that the fibres are parallel to n m , and hence that the axial 

 plane is perpendicular to the elongation, as in thomsonite. 



The specific gravity of this mineral is found to be 3.155. 



Lacroix has investigated chlorastrolite from an optical point of view* and 

 reached the conclusion that it should be associated with thomsonite. Hawes con- 

 sidered it a variety of prehnite, but the optical properties distinctly show that it 

 belongs with the group of thomsonite, in so far as its optical plane is transverse to 

 to the elongation. According to Dana,f who follows Hawes, it is not a homogeneous 

 mineral, and he places it in an appendix to the zeolites, with doubtful species and 

 synonyms. The specimens examined by us, however, are pure and optically uniform 

 in all their characters. There are seen smaller spheruliths which are green and which 

 between the nicols appear uniformly blue, embraced scatteringly in the fibres of the 

 chlorastrolite, but, as they are probably a radiated chlorite (delessite?) their presence 

 does not affect the purity of the surrounding fibres. Lacroix also mentions inclusions 

 of quartz and oxide of iron, and an amorphous substance occupying the centre of 

 the spheruliths. These, however, make no part of the mineral. They would, how- 

 ever, be very likely to be embraced in any chemical analysis which might be made; 

 and this is the only plausible explanation which we can make of the analyses which 

 vary, and of the view of Hawes that the mineral is not homogeneous. Such chemical 

 analyses have no validity as a guide to the nature of the mineral. The mineral has 

 a strong fibrous character, structurally, and a constancy of optical orientation which 

 give it an individuality of its own, and until chemical analysis of some of the pure 



* Bulletin de la Sociiti de Mtnfraloijie de France, x. p. 147. 

 t System of Mineralogy, p. 610 (sixth edition). 1892. 



