408 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



SAMARSKITE (529) 



Variable complex columbate and tan- Orthorhombic. 



talate of various bases approximat- 

 ing the formula 3(Fe,Ca,U0 2 )0. 

 (Ce,Yt) 2 3 .3(Cb, Ta) 2 5 . 



BOISE COUNTY 



A sample of a heavy concentrate from a sand from Idaho City 

 labeled "P654, olivine," was found to be strongly radioactive. Care- 

 ful microscopic examination showed this material to be composed in 

 large part of a coal-black glassy mineral with a brown streak and 

 conchoidal fracture. The mineral occurs in rounded grains and in 

 dull pitted square prismatic crystals which are either broken at the 

 ends or are terminated by a chisel-shaped dome. All of the grains 

 and crystals are very much corroded and are dull and 

 brownish in color on the outside. One of the smooth- 

 est of the crystals was measured by light reflected from 

 the faces and gave approximate measurements of 90° 

 between the pinacoids and 86° between the faces of 

 the dome, which compares well with the angle e(101) : 

 e'(101) =87° for samarskite. The radioactivity of the 

 mineral, its crystal form, and its physical properties 

 suggest that it is samarskite. The identity is by no 

 means definitely established, however, and it is to be 

 understood that this and several other of the rare 

 earth minerals of these sands are but tentatively as- 

 signed to the species under which they are described. 

 The hardness of the samarskite is 5-6. The streak 

 is dark brown. When powdered and examined under 

 the microscope the mineral is found to have a dark- 

 brown color and to be transparent only on very thin edges. It is 

 isotropic throughout as are most such rare earth minerals. The 

 form and appearance of the crystals are as shown in Figure 126, which 

 also shows the tendency of two or more crystals to occur in parallel 

 position. The samarskite makes up about 60 per cent of this 

 material, which apparently is the heaviest fraction of a concentrate 

 from a sand obtained from a dredge operating at Idaho City. In 

 addition to the 60 per cent of samarskite, this concentrate contains 

 about 10 per cent of columbite in sharp crystals, the remaining 30 per 

 cent consisting of various other unidentified rare earth minerals, 

 zircon, monazite, garnet, and much metallic lead, the latter evidently 

 being artificial and fragments of solder, shot, or something of the 

 sort whose source is not known. Several other samples labeled 

 "P654, chromite,' , "P654, garnet," etc., are apparently other frac- 

 tional concentrates from the same original lot of sand. The one 

 labeled garnet consists of about 50 per cent by volume of brownish- 



FlG. 126.— SAMAR- 

 skite from 

 black sand, 

 Idaho City, 

 Boise County. 

 Drawn with 

 6(010) in front 



