AKT. a. 



BLACK SANDS FEOM IDAHO SHANNON. 



21 



labeled " chromite " contains a little samarskite, but is for the most 

 part composed of ilmenite. 



COLUMBITE. 



The samarskite concentrate from Idaho City described above con- 

 tains important amounts of a mineral in black crystals which proved, 

 upon measurement, to have the angles of columbite. x^side from the 

 difference in form, which is not always manifest, this mineral greatly 

 resembles ilmenite, which occurs commonly in the sands. The colum- 

 bite makes up about 10 per cent of the high samarskite sand and is 

 more abundant than samarskite in the garnet-bearing sand. The 

 crystals vary considerably in habit, ranging from tabular parallel to 

 the pinacoid h{010) to square prismatic. The common forms and 

 habits are illustrated in the drawings (figs. 16 to 19, inclusive). The 

 color is black, and the luster is more vitreous than metallic. The 

 prismatic planes are usually very brilliant, but the terminal faces 

 are frequently more or less dull and pitted. This is especially true 

 of the unit pyramid w(lll), the faces of which are most frequently 

 dull and often show rounded depressions. Under the microscope 

 the powdered mineral is translucent on thin edges, wath a brown 

 color. Frequently several similar crj^stals are grown together in 

 parallel position and many crystals are attached to small masses of 

 quartz and muscovite. In the coarse polycrase-bearing sand from 

 Centerville crj^stals up to 1 cm. in length occur sparingly which 

 have the form and appearance of columbite. These are invariably 

 dull with a grayish-black color and more metallic luster. They also 

 are more opaque than those described above. Judging from appear- 

 ance alone it seems probable that these crystals from Centerville are 

 more nearly pure iron columbate, while the brilliant black crystals 

 from Idaho City are probably higher in their content of tantalic acid, 

 and possibly they contain some manganese. The forms and angles 

 measured on crystals from Idaho City are given in the following 



table : 



Forms and angles of coluvibite from Idaho City. 



