596 PROCEEDINGS OF THE NATIONAL MUSEUM. voi,. r.s. 



BOULANGERITB FROM PERU. 



All exceptionally fine specimen of typical " mineral wool," recently 

 received by the United States National Museum, has been examined 

 and found to be boulangerite. The specimen came to the depart- 

 ment of geology indirectly through the Division of Mineral Tech- 

 nology, and little is known regarding its history other than that it 

 probably came from the Panama-Pacific exposition. It was with a 

 lot of ores labeled Peru. A small label on one end of the specimen 

 bears the words " Atlante No. 10," and a fragment of a label found 

 with the specimen, so far as it could be deciphered, read : " S. S. 



Fluke veta nueva distanci 7 ds de Huanca Seca, 



Se llama San Antonio." 



Occurrence. — The boulangerite coats the upper surface of the 

 specimen with a soft mat of interlaced fibers, covering an area 12 by 

 16 centimeters. The specimen, which is 12 by 16 by 9 centimeters 

 in maximum dimensions, evidently represents the crust from one 

 w^all of a partly filled fissure. The specimen illustrates very beauti- 

 fully the paragenesis and association of the minerals of the vein. 

 The first deposit on the walls of the fissure was resinous dark-brown 

 sphalerite, which, mixed with a very little pyrite and galena, forms 

 a layer averaging 3 millimeters in thickness. Next came bluish 

 white to white greasy quartz, which forms a massive layer up to 

 5 centimeters in thickness, upon the surface of which was deposited 

 a crust or comb of long needlelike transparent quartz crystals, aver- 

 aging 1.5 centimeters in length, with a thickness of from 1 to 2 

 millimeters. Upon the comb of quartz crystals was next formed a 

 thin layer of arsenopyrite in crystals up to 3 millimeters in diam- 

 eter, followed by pearly buff manganiferous siderite, which makes 

 a crust composed of large and somewhat curved rhombohedral crys- 

 tals 2 centimeters or more on an edge. The boulangerite forms a 

 wool-like mass of matted fibers, 1 to 2 centimeters thick, covering 

 the siderite. It was the last mineral in the vein to be deposited. 



Physical properties. — The boulangerite is in the form of exceed- 

 ingly thin dark steel gray fibers, matted together in a loosely coher- 

 ent felt. The individual fibers are very flexible. Owing to the mi- 

 nuteness of the crystals and their manner of aggregation, the specific 

 gravity and hardness of the mineral could not be determined. The 

 streak is distinctly brownish gray to brown. 



Chemical composition. — A sample of the wool-like material picked 

 from the felted mass was analyzed, yielding the composition given in 

 the following table: 



