43J 



{a :h\c -:= 0,5871 : 1 : 0,5585). hill it is evidently unknown, to wliicli 

 svmnietn-class of the tliree possible ones it helon^us. Its [)scn(loliexagonal 

 habitus is obtained in two ways: a. by twin formation parallel 

 to |JJO|, consisting' in an intergrowing of three individnals in such a 

 manner, that the faces of JHOj will function as the apparent prism- 

 faces of the psendoliexagonal combination ; h. by a twiiniing parallel 

 to |130|, in which three individnals form either a threefold twin 

 by contact, or a threefold one by intergroioth, the faces of |1 lOj 

 being turned outward (fig. '\a). In crystals of the sti-ucture, described 

 sub a, a plate cut per()endicularly to |001|, will appear to be divided 

 into six sectors, of which ev^ery one is optically biaxial, the planes 

 of the optical axes being situated in three successive sectors under 60° 

 one to the other, while they are of course equally directed in every two 

 diametrically opposite sectors. In crystals of the type h there will 

 be either three sectors, in which the axial [danes are orientated 

 along the larger diagonal of the kite-shaped sectors (fig. J /'J; or 

 there appear six sectors, in which the axial planes are orientated 

 perpendicularly lo a diameter of the rhomboidal boundary of the whole 

 complex (fig. U).^)\ in this last mentioned case the axial planes in 

 two diametrically situated sectors will appear, as in the case sub a, 

 orientated in the same direction. The considered possibilities are 

 elucidated by some schematical drawings in fig. 1. 



(0Ï0) 



Fig. 1. 



In most cases the boundaries of (he sectors are not distinct; the 

 ditferent individuals on the contrary, will penetrate each other 

 partially. The cleavage occurs parallel (o |010|, but it is not very 

 distinctly pronounced. The optical axial plane is parallel to jlOOj ; 

 the c-axis is first bisectrix, and the dispersion is only weak: (,)<]?'• 

 Cordierite is one of the most striking instances of polychroitic minerals 

 {(lichroite) ; the here nsed crystals also showed this phenomenon in 

 a very marked degree. 



§ 3. In our experiments we could use cordierites of the following 

 three places: a colourless cordierite of Madagascar; a pale blue 



