302 MINERALOGY 



less yellow when cold, from the presence of cadmium; this -coat 

 moistened with cobalt solution and again heated in the O. F. be- 

 comes green ; the soda fusion yields a sulphur reaction on silver. 

 Soluble in hot HC1, evolving sulphuretted hydrogen. 



General description. Crystals are usually combinations of the 

 plus and minus tetrahedrons with the cube or the dodecahedron, 

 less often with the trigonal trisoctahedron (311). They are often 



FIG. 399. Sphalerite and Calcite. Joplin, Missouri. 



rounded on the tetrahedral edges and striated parallel to the inter- 

 section of the two tetrahedrons ; on the cube face these striations 

 are diagonal, showing its hemihedral symmetry. 



Twins after the spinel law are common and may be developed 

 polysynthetically. Granular, compact, fibrous, or foliated varie- 

 ties are common. Pure sphalerite is white and occurs at Frank- 

 lin, New Jersey, and at Nordmarken, Sweden. It is usually colored 

 with iron or manganese, even though these metals are present only 

 in very small quantities. Cadmium sulphide, being isomorphous 

 with zinc sulphide, is usually present, in some localities as high as 

 5 per cent. The very dark specimens contain small quantities of 

 indium, gallium, or thallium ; gallium was first discovered by Lecoq 

 de Boisbaudran in 1875 in a specimen of blende from Pierrefitte, 

 Pyrenees. Indium was discovered in 1863 by Reich and Richter 



