19a 



To understand the law, according' to which the apparently 

 hexagonal crystal is built of rhombic individuals, it must 

 be remembered, that the angle between (llO) and (01 0), the 

 two faces, which most frequently appear, is 58° 13' or near 60°. 

 The examination of the hexagonal transverse sections showed, 

 that the optic axial plane in the far predominant number of 

 cases makes a right angle with the side-lines and that these 

 accordingly belong to the brachypinacoids. At the corners of 

 the hexagon a little of the prismatic face generally shows itself, 

 after which the side-line is shghtly bent (60° — 68° 13'= Г 47'), 

 because the next individual's brachypinacoid then begins. 

 Schematically the «hexagonal» prism then consists of 6 single 

 rhombic individuals, whose optic axial planes all meet in the 

 crystal's middle-^line. The prismatic face {lio} is the plan of 

 contact between the separate individuals. 



On the edge between {llOJ and (oio) appears sometimes 

 the prismatic face {l30}; it is this one, which on the "hexa- 

 gonal» prism has the appearance of the prism of second 

 series. 



This w^ay of formation could in single cases be seen on 

 the outside of the «hexagonal" prism itself. It showed 

 itself especially distinctly, when the brachypinacoid and the 

 appending brachydome on the one individual protruded a little 

 in front of the next individual's prism and pyramidal face. It 

 was very seldom however, that both these sets of faces were 

 so distinct, that a measurement could be made. When such 

 a crystal was examined in transverse section , it seemed on 

 directly observing it, us if the individuals had joined together 

 in a parallel position, as the angle most frequently appearing is 

 very near 60°, however on determining the position of the 

 optic axial planes, it was found, that these crystals also are 

 constructed in conformity with the above mentioned law. 



Physical jproperties. The mineral is brown and opaque. 

 Luster from greasy to vitreous. The double refraction is slight. 

 XXIV. 1 3 



