158 



MINERALOGY 



the angle omi = c A i and omu = c A u and omc = c A o ; where these 

 lines, mi, mu, me, intersect the axis c will be their intercepts on c 

 when it is unity on b. As u is the unit pyramid, ou will be unity 

 on c, and by measurement and comparison to the unit on b = .47 + . 

 The axial ratios as determined are, a:b:c = .52 + : 1 : .47 + ; as cal- 

 culated they are, a : b : c = .5285 : 1 : .4769. 



In comparison, the intercept of the pyramid o on the axis c, 

 oc, is just twice ou, and the intercept of i, oi, is 2/3 of ou. The 

 parameters and indices of the three pyramids may now be written 

 as follows : 



u = a:b:c, (111). 



= a:b:2C, (221). 



1 = a:b:2/3C, (223). 



There are usually two brachydomes present, and these may be 

 measured next by remounting the crystal with an edge of this zone 

 perpendicular to the holder and adjusting an edge in the goniom- 

 eter as before. Starting with the first reading from one of 

 the faces further from the base the results obtained are as 

 follows : 



FACE READINGS ANGLES 



= 179 20' c A y = 6219' 



= 160 42' c*f =43 41' 



= 117 1' 

 = 73 23' 

 = 55 41' 



cy = 62' 20 



c * f = 43 38' 

 c A y'=6221' 

 c A f =43 39' 



The parameters and indices of these two 

 forms are determined as follows : as they are 

 in the brachypinacoidal zone they will be 

 parallel to the a axis. In Fig. 308, make 

 ob equal to unity on the macroaxis and 

 draw the vertical axis at o, then draw 

 bf making the angle obf = 43 39' ; 

 if oc is unity on c, then of is twice 

 oc or 2 c and the parameters and 

 indices of the form f are oo a : b : 

 26, (021). 



In the same way draw by, 

 making the angle oby = 62 

 FIG. 308. 20', then oy is 4 c and the 



