181 



substances J ) in general. Although it is obvious, that the symmetry is 

 only rhombic, it is remarkable, how closely the image obtained 

 approximates to that of a true hexagonal crystal (fig. 146). 



5. If repeated twinning occurs in several directions at the 

 same time, compound penetration-twins of a very complicated 

 structure may be formed, and the approximation to higher 

 symmetrical individuals obtained in this way may go remarkably far. 

 As an instance of the way in which the approximation to the higher 

 symmetrical form may take place, we here mention the mineral 

 philUpsite, a zeolithic silicate of the approximate composition: 



This mineral is undoubtedly monoclinic; but its angle (110): (110) 

 is 6042', while (001): (101) = 90!'. It has, therefore, an approximate 

 rhombic, as well as a pseudo-hexagonal symmetry. The result of 



Fig. 141. PhilUpsite. 



the repeated twinning with (001) and (Oil) as twinning-planes, is 

 almost a square prism formed by four individuals, the faces of which 

 are finally striated. Now, three compound individuals of this kind 

 may interpenetrate, with (110) as twinning-plane, to form a single, 

 yet more compound individual, as shown in fig. 141. 



If the re-entrant angles be now gradually removed by the develop- 

 ment of the faces indicated, this complex of twelve crystals 

 passes finally into the form of an apparent rhombicdodecahedron, as 

 it occurs in the cubic system. Each rhombic face of this form may 

 then be subdivided into four fields by striations diverging from 

 the centre, and parallel to the principal edges of the pseudo- 

 dodecahedron thus obtained. 



In general such remarkable mimetic forms are often met with in 

 the group of the zeolithic silicates, a fact which may be coordinated 



!) F. M. Jaeger and H. Haga, Proceed. Kon. Akad. van Wet. Amsterdam, 

 18, 1357, (1915). 



