HEXAGONAL SYSTI.M 



sr> 



unity p, is a function of the other intercept n. The value of p <-x- 



il in terms of n is : will increase as n decreases, 



n i n i 



until n is unity, when 



n i 



becomes infinity, or if the value of n 



increases, 



decreases, and when n = 2, 



= 2 



the value 



n i n i 



of n may vary between i as its minimum limit and 2 as its maxi- 

 mum limit ; m, the coefficient of c, is independent of n and may 

 vary between and oo . There will also be four terms in the in- 

 dices of any plane, thus hkil, where i represents the smallest inter- 

 cept. Of the three indices hki, two will always be of the same 

 sign and the third of the opposite sign, and the algebraic sum of 

 these three indices is always zero. Their relative values are 

 i>h>k and h + k = i. In writing the indices 1 always stands 

 last and represents the c axis not interchangeable with the lateral 

 axes. Twelve of the thirty-two types are included in the hexag- 

 onal system, all of which possess at least one axis of trigonal 

 symmetry. 



CLASS, HEXAGONAL HOLOHEDRAL (HOLOSYMMETRIC) 

 TYPE 20, DIHEXAGONAL EQUATORIAL 



Symmetry. Crystals of this type possess one dihexagonal axis, 

 the c axis, 6 didigonal axes, all lying in the equatorial plane and 

 inclined at an angle of 30 to each other, three of which are the 



FIG. 152. Type 20, Dihexagonal 

 Equatorial. 



