120 



MINERALOGY 



Crystalline Characters 

 In the orthorhombic system, where the unit on each axis is a 



o / 



different one. there are two axial ratios, and - , b being the unit 



D b 



of comparison, or as they are generally written, a : b : c = .8152 + 



1 : 1.31359" 1 ", the axial values of barite. 

 On calculating the axial ratios it 

 will be necessary to measure the angle 

 of the unit prism, or the angle be- 

 tween the unit prism and either the 

 macro- or brachypinacoid, when & in 

 terms of b may be calculated. To 

 determine c, the angle of the unit 

 dome or the angle between the dome 

 and a pinacoid must be measured. 



Example. In the mineral stauro- 

 lite, the angle 100 A 110 = 25 20'; as 

 this is the angle between the poles, 

 the actual angle of the right-angled 

 triangle is 25 20', with the side & 



FIG. 237. 



opposite, therefore, tan 25 20' = = = .4734+, as b = 1. 



D 1 



In calculating the value of c, the angle 101 A 101 = 110 32', 

 which being the angle between the poles, the actual angle between 



the faces, Fig. 237, cao = 1/2 dod' = 55 16' ; - = tan 55 16', c = 



ft 



a X tan 55 16' ; a = .4734. 



Log tan 55 16' =10.159083 - 10 

 Log .4734 = 1.675228 



Log c = 9.834311 - 10 



c = .6829 

 Axial ratio of staurolite, & : b : c = .8152 : 1 : .6829. 



THE MONOCLINIC SYSTEM 



The monoclinic system includes all those crystals referable to 

 three axes, two of which, c the vertical, and b the orthoaxis, are at 

 right angles ; the third, or clinoaxis, is at right angles to b and in- 

 clined to c, and is designated by a. Here the three diametral planes 



