116 



In the following I shall suppose that the first of the men- 

 tioned possibilities is the one really existing. 



The crystals of type 2 have a very fine, and in most cases 

 indistinct, microscopical structure. Where this structure is 

 seen tolerably distinctly, it appears almost always as systems 

 of striæ that are always parallel to the hexagonal m(l010}; 

 each single part of the crystal has generally only one direction 

 of the striæ, and is composed of two individuals whose optic 

 axial planes form an angle [of 60° with the direction of the 

 striae; if the optic axial plane is parallel to the monoclinic 

 (OlO), the twinning plane is parallel to {310}. The structure 

 is shown diagrammatically in fig. 7, where each single line does 



■'^'.r 



:^^ 



'■^^ 



-f^ 



;?^ 



Fig. 7. Diagrammatic figure of the twin-formation in catapleiite, 

 type 2. The figure shows a part of the crystal where two indi- 

 viduals alternate with each other; here the lines show the bound- 

 aries between the individuals. In the upper part of the figure the 

 individuals become so narrow, that they cannot be distinguished 

 from each other under the microscope; consequently the two axial 

 planes become one, which is designated by the dotted axial figure. 

 The hexagon, as in fig. 6, represents the contour of the crystal. 



not, as in the other figures, represent a very thin twin-indi- 

 vidual, but the boundary between two such individuals. If the 

 thickness of the single striae decreases to dimensions that 

 cannot be distinguished under the microscope, the whole field 

 gets the appearance of a single individual with the axial plane 

 orientated as shown in the dotted figure, that is to say, per- 

 pendicular on the hexagonal ш{1010}. By this the circum- 

 stance is explained that large parts of the crystals show no 

 visible lamellae, but are almost homogeneous with undulating 



