ART. 24 BEKJAMINITE^ A NEW MINERAL SHANNON 5 



is medium gray and the luster is metallic with a greasy appearance. 

 At first glance the mineral suggets massive tetrahedrite. It has a 

 moderately good cleavage in one direction, somewhat interrupted 

 by the later cracks. The masses are coarse equigranular and the 

 mineral shows no tendency to platy, fibrous or prismatic form. 

 Cleavage surfaces indicate the crystals to reach a maximum size of 

 15 mm. in the masses. Upon exposure the cleavage surface remains 

 bright or becomes slightly yellow while in another direction the grains 

 tarnish coppery red and in a third direction they become dull lead 

 gray. Granular aggregates which have been exposed for some time 

 look like a mixture of three minerals and the nondescript appearance 

 is heightened by the quartz filled cracks, minute chalcopyrite grains 

 and scattered pyrite and molybdenite. The streak is dull lead gray 

 and only assumes a barely perceptible reddish gray tinge with long 

 rubbing. The mineral scratches calcite but with difficulty and its 

 hardness is thus about 3.3 to 3.5. 



PYROGNOSTICS, ETC. 



Benjaminite is soluble in hot concentrated nitric or hydrochloric 

 acid and the solution gives the usual qualitative reactions for silver, 

 copper, bismuth, and lead. Alone on charcoal it yields sulphurous 

 fumes but is not reduced. With potassium iodide-sulphur mixture 

 the usual conspicuous bismuth coating is obtained. In the closed 

 tube it gives only a ring of sulphur and in the open tube only sulphur 

 dioxide without the formation of any sublimate. 



MICROSCOPIC PROPERTIES. 



In polished sections under the metallographic microscope the ben- 

 jaminite is medium gray, the color being about that of the average 

 tetrahedrite. Examined with polarizing reflecting apparatus the 

 mineral is found to be uniformly and very decidedly anisotropic but 

 it exhibits no color pleochroism. When the surface is treated with 

 the standard microchemical reagents hydrochloric acid, ferric chloride, 

 mercuric chloride, and potassium hydroxide give negative results. 

 With reagent nitric acid the surface effervesces and blackens and the 

 fumes tarnish brown. These properties, according to Davy and 

 Farnham's scheme, would identify the mineral as aikinite, thus 

 seemingly substantiating the qualitative identification. 



ANALYSES AND COMPOSITION. 



The specimens were at first labeled aikinite and the mineral not 

 only gives the qualitative reactions of aikinite but greatly resembles 

 the aikinite from Beresov, Siberia in the Museum collections. The 

 analysis was at first undertaken as of aikinite from a new locality 

 and was temporarily discontinued owing to the inhomogeneous 



