ADAMS : FUNDAMENTAL POLYHEDRON OF DIAMOND 



241 



and the dodecahedral faces will be relatively larger on the large 

 figure and small, or even wanting, on the smaller. At the limit, 

 one becomes a tetrahedron of infinitesimal size, while the other 

 reduces to the dodecahedron of the face-centered lattice. 



Fig. 1. Orthographic projection. Fig. 2. Clinographic projection. Fig. 

 3. Development of surface; may be used as net for constructing model. 



In chalcopyrite,^ where alternate planes of iron and copper 

 atoms replace the zinc of sphalerite, the axis perpendicular to 

 these planes is unique and the crystal is therefore tetragonal. 

 Replacing alternate iron planes by tin, as in stannite, leaves the 

 system still tetragonal. 



' BuRDicK, C. L., and Ellis, J. H., Journ. Amer. Chem. Soc. 39: 2518. 1917. 



