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NEW YORK STATE MUSEUM 



sect, as shown in fig. 28. The shaded planes of fig. 28 intersect 

 in axes of tetragonal symmetry, the white planes intersect in 

 axes of trigonal symmetry. 



Hexoctahedron. The hexoctahedron (fig. 29) 

 is composed of 48 faces, each cutting the 

 three axes at relatively different distances. 

 The faces, which are scalene triangles, are 

 grouped around the trigonal axes in groups 

 of six. 



Fi *- 29 Cube. The cube (fig. 27, model 2) is com- 



posed of six square faces each of which is parallel to two axes. 

 This crystal form is represented by a number of minerals, the 

 most common being galena, fluorite, halite, etc. 



Dodecahedron. The dodecahedron (fig. 30, 

 model 3) is composed of 12 rhombic faces, each 

 of which cuts two axes at the same relative 

 distance and is parallel to the third. This crys- 

 tal form is quite common in garnet, and is 

 found to a less degree in magnetite and other 

 minerals. 



The tetrahexahedron (fig. 31) is composed 

 of 24 faces each of which is parallel to one 

 axis and cuts the other two at relatively 

 unequal distances, The faces, which are 

 isosceles triangles, are grouped in fours 

 about the axes of tetragonal symmetry 

 and the long edges are parallel to the 

 edges of a cube or 

 hexahedron. This 

 crystal form, in combination, is well illus- 

 trated by copper, fluorite and other miner- 

 als. 



Octahedron. The octahedron (fig. 32, 

 model 1) is composed of eight equilateral 

 triangular faces which cut the three axes 

 equally. Good examples of this form may be found in crystals 

 of magnetite and spinel. 



Fig. 30 



Tetrahexahedron. 



Fig. 31 



Fig. 32 



