( 194 ) 



reflexes. The crystals are also IVequoiitly bordered bv curved planes 

 and by vicinal forms in the prism-zone. These circumstances render 

 an accurate investigation very difficult; occasionally, however, I 

 obtained better formed crystals, which gave very sharp reflexes and 

 served for the following accurate measurements. 



The}' are -inonoclino-prismatic with the axial relation : 



a : 6: c= 0,5217:1 : 0,7803 

 ^=i85°12'. 



Forms observed are: »?= |110| and 7> = |120|, broad and lustrous; 

 a = |100} and » = |130|, very narrow ; a is genei'ally hazy; /;=:|()10i, a 

 little broader, but is generally absent ; c=|001|, large and very lustrous; 

 r=|ÏÖl|, well developed and lustrous; t = \104:\, narrower and 

 is often absent; ('=r|ri2|, generally small and dull, occasionally a 

 little broader and bettor reflecting: s := \TS2\, large and lustrous, but 

 generally there are oidy two parallel planes present. 



Combinations of nil the forms rarely occur. Generally such wilh 

 m,p,c,r; i», j), h, c, r, t, s and o: m,[),<(,h and r, etc. The more 

 typical crystals are shown in figures 1-3. 



I ! 



i : I 



i : 1 



M ! : 



jf , ; - if 



!' i i 



i '\ \ 



I i ; 



•' I ■' 



I ' ' 



c 



Fig. 2. 



Fig. 3. 



In the properly formed crystals, the angular values are very 

 constant; the reflexes are as sharp as possible. The substance has 

 a decided tendency to twin-formation; in this, one form |102| is a 



