WARREN AND PALACHE. — QUINCT PEGMATITES. 157 



Crocldolite and the Slender Blue or Black Amphibole Crystals of the 

 Pocket. — As seen under the microscope the crocidolite appears as a 

 tangled felt of the most exceedingly minute fibers, pale blue to almost 

 colorless, a lies with the elongation, the extinction is apparently 

 small, and the index of refraction for the ray vibrating perpen- 

 dicular to their length seems to be the same, as near as can be told, 

 as the yellow ray of the riebeckite. If the crocidolite be immersed 

 in water the fibers straighten out in a most remarkable manner. The 

 crocidolite is shot through and through with the slender black am- 

 phibole crystals and tiny quartz crystals. It is therefore impossible 

 to get pure material for analysis. Some carefully picked material, 

 freed from quartz, so far as possible, by hand, was analyzed for ferrous 

 iron and was found to contain 17.8 FeO. This figure must be much 

 under the true value, since considerable quartz weighed out with the 

 crocidolite must have been dissolved during the decomposition with 

 hydro-fluoric acid, and the content of hydroscopic water is undoubt- 

 edly large. Making allowances for these, the value for the ferrous iron 

 is probably close to that found in the riebeckite. 



The blue or black amphibole crystals occurring in the crocidolite, in 

 pockets, and on exposed surfaces of the pegmatite fi'agments have the 

 form of elongated, relatively flat prisms, deeply striated parallel to 

 the elongation (parallel c') but not so far as observed, terminated with 

 definite planes. 



The smallest of the crystals will measure in thickness but little 

 more than the larger crocidolite fibers. The width may in general be 

 said to run from 0.04 to 0.003 mm., although the larger crystals may 

 sometimes be observed as wide as 1.5 mm. The thickness will vary from 

 about one fourth to one tenth of the width. The majority of the crystals 

 as they lie on the flat side are opaque except at the very edges. The 

 pleochrism and absorption are the same as for the riebeckite and the 

 index of refraction for the yellowish colored ray appears to be also the 

 same. 



Both the crocidolite and the minute prisms are believed to be near 

 to or identical with the riebeckite in chemical composition, 



Aeg trite. — The aegirite of the central pocket is also prismatic in 

 development, sometimes, and especially in the smaller crystals, showing 

 distinct and measurable terminations. There is, however, even in the 

 best crystals much facetting and curvature of part of the terminal 

 planes, especially in those highly inclined to the vertical axis. The 

 faces of the prism zone are generally plane and measurements of suf- 

 ficient accuracy were obtained to make it clear that these crystals may 

 be referred satisfiactorily to the axial elements of aegirite as described 



