GUIDE TO THE MINERALOGIC COLLECTIONS 



15 



Trisoctahedron. The trisoctahedron (fig. 33) is composed of 24 

 faces, each of which cuts two axes at equal distances and the 

 third at a distance which is relatively ^ 

 greater. The faces are isosceles triangles y/ / VV 



and are disposed in groups of eight about y /y\ p / p )0\ 

 the axes of tetragonal symmetry and in v~-A//_L__— ^ 

 groups of three about the axes of trigonal \/\^ /y 

 symmetry. The trisoctahedron is occa- \A // 



sionally found in combination with other 

 forms as in galena. 



Trapezohedron. The trapezohedron (fig. 34) is composed of 24 

 faces each of which cuts two axes at equal distances and the 



third at a distance which is relatively less. 

 Garnet, leucite, analcite and other minerals 

 crystallize in trapezohedrons. 



Of the above named crystal forms the cube, 

 dodecahedron and octahedron alone present an 

 unvarying constancy of form, the cube and 



Fig. 34 octahedron being identical with the familiar 



geometric forms. In the hexoctahedron, tetrahexahedron, trisoc- 

 tahedron and trapezohedron the variations in the relative values 

 of the axial intercepts give rise to a number of variations under 



Fig- 35 Fig. 36 Fig. 37 



each form, each subject to the law of simple mathematical ratio. 

 The series of tetrahexahedrons shown in fig. 35-37 serves to 

 illustrate this point. Fig. 35 and. 37 are forms occurring in 

 copper and fig. 36 is frequently observed on crystals of fluorite. 



