99 



PE0CEEDIXG5 OF THE XATIOXAL, MUSEUIM, 



VOL. 66 



POLTCRASE. 



A sample of " oversize " coarse sand from Centerville contains 

 abundant grains and rough crystals of a dark-brownish or greenish- 

 black mineral not very different in appearance from the samarskite. 

 The crystals, which reach 1 cm. in diameter, are orthorhombic in 

 aspect and vary from square prismatic to thin tabular. They are all 

 coated with an exterior crust of a pale-yellow alteration product. 

 Within this shell the crystals and grains consist of a brownish-black 

 glassy material having a conchoidal fracture and a brown streak. 

 Under the microscope thin fragments are transparent, isotropic, and 

 brown in color. The mineral is intensely radioactive. Mr. Frank 

 L. Hess had previously recognized this or a similar mineral in placer 

 gravels from Centerville. and recently has turned his sample over to 

 the present writer for chemical investigation, 

 which it is hoped may yet be undertaken. The 

 properties and appearance of the mineral are 

 identical in most respects with the polycrase 

 from Marietta County, X. C. and for the present 

 it will be referred to that mineral. This mineral, 

 recognizable by its light-colored coating, occurs 

 sparingly also in the samarskite and columbite 

 bearing concentrates from Idaho City. A crystal 

 from this lot gave measurements on the pinacoids 

 and on two pyramid faces indicating roughly the 

 form s(lll) of polycrase. The remaining faces 

 were coated. The form and appearance of this 

 crystal, which was tabular, are shown in the drawing (fig. 22) . There 

 was, as shown in the figure, a smaller crystal in parallel position pro- 

 jecting from the face of the larger individual. 



OTHER RARE-EARTH MINERALS. 



In addition to the rare earth minerals described above under the 

 headings " samarskite," " columbite,*' and " polycrase," it is probable 

 that a nmnber of others occur in lesser amount in the sands. In 

 the columbite-samarskite material certain glassy to resinous grains 

 without crystal form gave light-brown internal reflections and had 

 a light-brown streak, while other grains gave green internal reflec- 

 tions and fragments were brownish green under the microscope. 

 These may include euxenite and yttrotantalite or possibly ferguson- 

 ite. One small crystal which was noted under the microscope ap- 

 peared to have the tetragonal form and pyramidal hemihedrism of 

 fergusonite. In the polycrase-bearing sand from Centerville, in ad- 

 dition to the abundant polycrase, there are occasional grayish iron- 

 black tetragonal crystals having dull faces, which may be one of 



Fig. 22. — Cetstal or 



POLICEASE (?) FEOM 

 IDAHO CITT. 



