24 



unit, or fundamental form, to which all other faces are referred. 

 The intercepts of the unit form on all interchangeable axes are 



a 



equal, their ratio - = 1 ; the intercepts on axes that are not inter- 



a, 



a 

 changeable are always an indeterminate quantity, r = 0.81520 + ; 



= 1. 31359 + , when b is taken as unity and express the ratio of 

 b 



the units on the axes. The axial ratios of barite are written a : b : c = 

 0.81520:1:1.31359. For chemically pure substances the axial 

 ratios are constant and are characteristic of the substance, just as 

 much as any of its chemical properties. The axial ratios and the 

 value of the interaxial angles in the monoclinic and triclinic systems, 

 which are also constant for pure substances, are termed the 

 crystalline characters or elements. The crystalline characters in 

 the isometric system are determined by all the axes being inter- 

 changeable; they are the same for all substances that crystal- 



lize in the system ; 



c c 



In the tetragonal system, - = v = 



SL 1 



0.1644154 + 

 c 



axial ratio of rutile. In the hexagonal system 

 - = T = 9.734603 + , apatite. In the orthorhombic system there are 



3. I 



two axial ratios, and ^, a : 5 : c = 0.81520+ : 1 : 1. 31359 + , axial 



ratios of barite. In the mono- 

 clinic system the two axial ratios 

 and the value of the angle p 

 are the crystalline characters ; 

 a : 6 : c = 0.658510 + : 1 : 0.55538 + , 

 P = 63 56' 46", orthoclase. In 

 the triclinic system there are three 

 angles in addition to the axial 

 ratios : 

 * : b : c = 0.49211 + : 1 : 0.47970+ ; 



o = 82 54' 13" ; 



p = 91 51' 53"; 



Y = 131 32' 19", axinite. 

 Holohedral, holosymmetric, or 

 normal, are terms denoting a type of crystals in each system, in 

 which the symmetry requires all the faces possible to be repre- 

 sented by one set of parameters to be present to complete the 



FIG. 33. Holohedral Form a : c : 3 a, 

 (331). 



