COMPOUND CRYSTALS. 



43 



is evident to the eye, either of six crystals meeting in a point, 

 or of three crystals crossing one another. Besides, there 

 arc nuinorous minute crystals regularly arranged along the 

 fays. Figure 73 represents a cross (cruciform) crystal of 

 staurotide, which is similarly compound, but made up of 

 fewer crystals. " Figure 74, is a compound crystal of gypsum, 

 and figure 75, one of spinel. These will be understood fiom 

 the following figures. 



Figure 76 is a simple crystal of gyp- 

 sum ; if it be bisected along a 6, and 

 the right half be inverted and applied 

 to the other, it will form figure 74, 

 which is therefore a twin crystal, in 

 which one half has a reverse position 

 from the other. Figure 77, is a simple 

 octahedron ; if it be bisected through 

 the dotted line, and the upper half, after being revolved half 

 way around, be. then united to the lower, it produces figure 

 75. Both of these therefore are similar twins, in which one 

 of the two component parts is reversed in position.* Com- 

 pound crystals are generally distinguished by their reentering 

 angles. 



Besides the above, there are also geniculated crystals, as 

 in the annexed figure. The bending has here 

 taken place at equal distances from the center 

 of the crystal ; and it must therefore have 

 6een subsequent in time to the commence- 

 ment of the crystal. The prism began from 

 a simple molecule : but after attaining a certain 

 length, an abrupt change of direction took place, 

 of geniculation is constant in the same mineral species ; for 

 the same reason that the angles of secondary planes are 

 fixed ; and it is such that a cross section directly through the 

 geniculation is parallel to the position of a common secon- 

 dary plane. In the figure given, the plane of geniculation is 

 parallel to one of the terminal edges. 



Mention illustrations. Explain their structure in the case of gypsun 

 and spinel. What is said of geniculated crystals'? 



* Such crystals have proceeded from a compound nucleus in which one 

 ;f the two particles was reversed. Compound crystals of the kind 

 above described, thus differ from simple crystals in having been formed 

 lorn a nucleus of two or more united molecules, instead of from a simple 

 Kiclc un. 



