Jrom Altenberg, near Aix-la-Chapelte. 



109 



Crystallographic Structure *. The perfect 

 crystal of this mineral is shewn in the an- 

 nexed figure. The angles taken by the re- 

 flecting goniometer were, 



a upon a 

 b upon b 



= 78 36' 

 = 81 34 



If we suppose the faces c, c, c, c enlarged 

 till all the others disappear, the crystal will 

 assume the form of a scalene four-sided pyramid (as shewn in 

 the annexed figure) whose edges have the following values : 



SB - 107 2' 

 y 139 41 

 z 86 49 



Hence the ratio of 

 the axes will be 



a : b : c = 1 : s/4.443 : ^1.493. 



As the form resulting from the enlargement of the faces 

 c, c, c, c is the fundamental one according to MOHS'S System, 

 and designated by P, we obtain, by taking the angle of the ho- 

 rizontal prism at the apex of the pyramid, a Pr 101 24', 

 and b (Pr + oo) 3 = 81 34', the angle of the vertical prism 

 corresponding to the obtuse angle of the base of P. The mi- 

 neral has two cleavages perpendicular to each other ; the one pa- 

 rallel to d, which is easily obtained, and distinguished by its 

 pearly lustre ; and the other parallel to e, which is less distinct. 



* I have been indebted for this crystallographic description of the mineral to Mr 

 HAIDINGEU. 



