AET. 3. BLACK SAXDS FEOM IDAHO SHAXXOX. 23 



the minerals related to tapiolite. Other grains appeared to be much 

 worn isometric crystals of a brownish-gray color, resembling micro- 

 lite or pj^rochlore. When all the recognizable rare-earth minerals 

 had been picked out of a sample of this sand, the residue was found 

 to still be radioactive and to contain heavy opaque gray-black grains 

 with submetallic luster. 



PYRITE. 



Unaltered pyrite was seen only as one or two grains in one sand, 

 but jDseudomorphs preserving the crystal form of pyrite perfectly 

 are present in greater or less number in almost every sample of sand 

 examined. These are often deceptively lustrous and black in color, 

 with polished faces, but occasional particles in each sand are 

 brownish in color or have an ocherous external coating. Most of the 

 pseudomorphs consist of limonite, as is shown by their brown streak, 

 but a few are attracted by a magnet and consist wholly or in part of 

 magnetite. The forms exhibited by these pyrite pseudomorphs are 

 cubes, or cubes with the corners truncated by octahedral planes in the 

 majority of the sands, but those in monazite sand from Centerville 

 are octahedral and are frequently much elongated and distorted. 

 The most complex forms occur in the titanite and allanite bearing 

 sand from Bear Creek. In this sand the altered pyrite crystals, 

 which are abundant, show combinations of the cube, octahedron, and 

 pentagonal dodecahedron, with possibly other forms. They would be 

 exceedingly hard to identify by form alone were it not for the 

 presence on all of them of the higlily characteristic striations and 

 grooves produced by oscillation between the cube and the pyrito- 

 hedron. Altered pyrite crystals are sparingly present in all of the 

 fine sands from Snake River and are very abundant in some of the 

 coarser sands of the Boise Basin region. 



ALLANITE. 



Allanite was positively identified only in a sand from Bear Creek, 

 in Camas County, although crystals of the same form and habit 

 were noted in small number in several other sands, especially those 

 from Minidoka and other Snake River localities. This mineral is 

 difficult to distinguish by its form from certain prismatic black 

 crystals of hornblende which occur occasionally. The Camas County 

 sand consisted largely of magnetite, which, when extracted with a 

 magnet, left a light-colored residue consisting mainly of irregular 

 fragments of quartz. Scattered through this residue were crystals 

 of titanite, zircon, garnet, etc.. together with small black prisms 

 with wedge-shaped terminations, which resembled augite crystals. 

 One of these crystals upon being measured on the goniometer gave 



