97 



Occasionally a re-entrant angle is observable, formed by (101 II 

 on the lower half and (101 f I on the upper. The optically bi- 

 axial character of the mineral proves that its structure cannot 

 be hexagonal, and the circumstance that the acute bisectrix is 

 normal to the base refers the mineral to the rhombic system. 

 If the mineral is referred to this system, we calculate from the 

 angles 



(no) : (iTOI = 120° and (ill) : lOOl) = 59° 9' 



the following axial ratios: 



a : b : с =^ 1.7320-5 : 1 : 1,3411. 



The forms present then become 



с = {OOI}, 



= {lOl} and {112}, 

 p = {201} and {ill}, 

 m == {100} and {110}. 



By transmitted light these catapleiite crystals are like those 

 of type I, i. e. they are faintly yellowish brown to nearly colour- 

 less. Their most marked characteristic is a splendid metallic 

 iridescence exhibited by their faces. Probably owing to some 

 thin surface film the faces exhibit very vivid rainbow^ colours, 

 especially red and green. The basal plane generally displays 

 a bright copper-red hue, but yellow, when the tarnish is thinner. 

 The pyramidal faces, on the other hand, exhibit green to blue 

 tints. These small crystals would probably not have been 

 observed, had not their brilliant colour reflections attracted my 

 attention. 



In parallel polarized light the crystal tables prove to be 

 composed of extremely thin lamellae crossing one another at 

 angles of 60° or 120°. These lamellae however, seldom 

 show total extinction in any direction, because lamellae in dif- 

 ferent directions almost always lie over and across each other. 

 XXIV. 7 



