83 



face. In their centre there is often a microscopical inclusion 

 of some foreign substance, e. g. a feldspar grain or the like. 



The colour of the mineral is a light sky-blue or greyish 

 blue. The streak is light, nearly ash-grey faintly inclining to 

 blue. Translucency hardly observable even in thinnest splinters. 

 Such small portions in which the fibres are arranged in parallel 

 position, show a marked silky lustre, but in general the fracture 

 is earthy. 



In microscopical sections the mineral is translucent 

 with dark Prussian-blue colour. The fibrous texture is of ex- 

 treme fineness, so that the single fibres are hardly observable. 

 What can be seen are bundles or sheaves of parallel fibres, 

 bent and interlaced, forming felt-like aggregations. There is 

 no possibility of ascertaining, by what crystallographic elements 

 the single fibres are bounded. The double refraction is extremely 

 weak, and hardly perceptible. They seem to extinguish nearly 

 parallel to their longitudinal direction. The pleochroism, on the 

 other hand, is fairly distinct, the light -absorption being con- 

 siderably stronger along the fibres than at right angles to them. 

 The axial colour in the former direction is dark sky-blue, in 

 the latter bluish grey. 



It has already been remarked that crocidolite occurs on 

 Narsarsuk as secondary inclusions in, or alteration products of, 

 other minerals. These other minerals are arfvedsonite and 

 aegirine. In the collection of minerals of the Stockholm Uni- 

 versity there is a large crystal having the form of aegirine, but 

 its substance is wholly altered into compact and hard crocidolite. 

 This specimen formerly belonged to the Liitzen collection. In 

 the former collection there are also some cleavage fragments, 

 the chief cleavages of which are those of arfvedsonite, and 

 one end of which consists of crocidolite. The unaltered 

 part of such a cleavage fragment does not, however, consist 

 exclusively of arfvedsonite. Certain enclosed portions of it 

 show cleavage in other directions than those of arfvedsonite. 



