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 

 are numerous minute crystals regularly arranged atong the 

 rays. Figure 73 represents a cross (cruciform^crystal of 

 staurotide, which is similarly compound, bu^/made up of 

 ewer 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 b, and 

 me right half be inverted and applied 

 to the other, it will form figure 74, 

 which is therefore a twin crystal, 

 which one half has a reverse positi 

 from the other. Figure 77, is a smipie 

 octahedron ; if it be bisected though 



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

 way around, be then unite# to the lower, it produces figure 

 75. Both of these therefore are similar twins, in whichjme 

 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 genioulated 

 in the annexed figure. The bending has Kere 

 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 directum took place. The angle 

 of geniculation is constant in the/ same mineral species ; for ' 

 the same reason that the anafes of secondary planes are 

 fixed ; and it is such that a cr/ss section directly through the 

 geniculation is parallel to tbe position of a common secon- 

 dary plane. In the figure ^iven, the plane of geniculation is 

 parallel to one of the terrmnal edges. 



Mention illustrations. Explain their structure in the case of gypsum 

 and spinel. What is said pf geniculated crystals? 



j _ 



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

 'jf the two particles was reversed. Compound crystals of the kind 

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

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

 lucleus. / 



