AUT. 3. BLACK SANDS FROM IDAHO SHANNON. 19 



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 smoothest of the crystals was meas- 

 ured by reflected light from the faces and gave approximate meas- 

 urements of 90° between the pinacoids and 86° between the faces of 

 the dome, which compares well with the angle e(lOl) A e'(TOl) 

 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 referred 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 on very thin edges. It is isotropic 

 throughout, as are most such rare-earth minerals. 

 The form and appearance of the crystals are as / x\\ 



shown in figure 15, which also shows the tendency f r \^ 

 of two or more crystals to occur in parallel posi- 

 tion. 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 addi- 

 tion to the 60 per cent of samarskite, this concen- 

 trate contains about 10 per cent of columbite in fig. i5._cbystal 

 sharp crystals, the remaining 30 per cent consist- °^ samarskite 



r ■ ,-, -J °-c J ,, . (?) FROM IDAHO 



ing of various other unidentified rare-earth min- citt. 

 erals, zircon, monazite, garnet, and much metal- 

 lic lead, the latter evidently being fragments of solder, bab- 

 bitt, or of lead bullets. Quartz is frequently attached to the 

 samarskite, and in a few instances what appears to be monazite is 

 intergrown with it. Several other samples which were 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 pale to 

 deep brownish-red almandite in sharp trapezohedral crystals, the 

 remaining 50 per cent being largely samarskite and columbite. The 

 columbite is relatively more abundant than in the first sample exam- 

 ined. The samarskite is entirely like that already described, showing 

 rounded pitted grains and rough crystals. Some of these have grains 

 of quartz and crystals of muscovite attached to them, while others 

 seem to show either two minerals or two generations of samarskite, 

 some of the grains, where broken, showing an inner crystal sur- 

 rouned by an outer shell of a similar substance. The sample 



