T8 Dana and Wells — Selenium and Tellurium minerals. 



polarized light they give an extinction parallel to the vertical 

 axis ; and in the 45 degree position the thickest crystals ex- 

 hibit an interference color which is red of the second order. 

 In convergent polarized light an obtuse bisectrix emerges at 

 right angles to a, 100, the plane of the optic axes being paral- 

 lel to the base. The optical orientation is d=c, b=a and 6=h. 

 Since a is the acute bisectrix the double refraction is negative. 

 These optical properties not only agree with orthorhombic 

 symmetry, but also with the determinations made on the 

 Yellowstone Park fayalite. 



When treated with hydrochloric acid the crystals are decom- 

 posed, and gelantinize. They contain ferrous iron, and after 

 the separation of the iron from solution with ammonia, they 

 yield no micro chemical reaction for magnesia. Their chemi- 

 cal composition, therefore, is undoubtedly the same, as that of 

 the fayalite from Obsidian Cliff, namel} ,r : orthosilicate of iron. 



The occurrence of fayalite in the hollow spherulites and 

 lithophysse in the obsidian of the Lipari islands, while not so 

 abundant as in that of the Yellowstone Park, is identical. It is 

 associated in the same manner with tridymite and alkali feld- 

 spars, and its development is unquestionably due to the same 

 causes in the two regions. 



Art. X. — On some Selenium and Tellurium minerals from 

 Honduras ; by Edward S. Dana and Horace L. Wells. 



Through the kindness of Mr. Henry S. Durden, of the 

 State Mining Bureau in San Francisco, we have received a 

 number of specimens of minerals containing selenium and 

 tellurium, two of which have proved to be of unusual interest. 

 The presence of selenium in some of them had been already 

 determined by Mr. Charles G. Schneider before they were 

 sent to us, but further than that the examination had not 

 gone. The locality from which they were obtained, as Mr. 

 Durden informs us, is the El Plomo mine, Ojojoma District, 

 Department of Tegucigalpa, Honduras. 



The mineral, to which our attention was first directed, 

 proved upon blowpipe examination to contain selenium and 

 tellurium, while the metals proper were absent. It presents 

 itself in massive forms only, with indistinctly columnar 

 structure and shows a perfect cleavage parallel to a prism of 

 60°. The color is blackish gray, the streak black. It is dis- 

 seminated through a gangue consisting chiefly of quartz with 

 some barite. An analysis (Wells) of the mineral proved it 

 to contain selenium and tellurium only. The separation of 



