18 



PROCEEDINGS OF THE NATIONAL MUSEUM. 



VOL. 60 



in a crystal and another crj^stal was penetrated by a tabular crystal 

 of ilmenite. 



TITANITE. 



Titanite is not rare as a constituent of the heavy sands, although 

 it is nowhere very abundant and its distribution is not so universal 

 as might be expected. It occurs in the nonmagnetic portion of the 

 sand from Bear Creek, Camas County, along with zircon, allanite, 

 and gold, as small irregular grains and flat crystals which vary from 

 yellow through various shades of green in color. Except for a cer- 

 tain greasy appearance and luster the irregular grains are hard to 

 distinguish from irregular grains of augite and olivine which are 

 common in other sands. The majority of the titanites, however, 

 show some crystal faces, and the form is quite characteristic, being 

 unmistakably different from the forms assumed by olivine and 

 augite. The titanite crystals show the familiar " envelope " com- 

 bination of the base c(OOl), the clinopinacoid a(lOO), and the unit 

 pyramid n{lll), the appearance of the crystals being as shown in 

 the drawing, figure 26. Usually the thin edges and corners of 

 the crystals are more or less worn and broken, and where this is not 

 the case the interfacial angles often have a rounded or fused appear- 

 ance. The basal pinacoid is usually irregular or dull and pitted. 

 The angles measured by which the several forms were identified are 

 as follows : 



Interfacial angles of titanite J rom Bear Creek, Camas County. 



Flat crystals of this same form are abundant also in a sand from 

 Cow Creek, in the Pierce City district, which contains much ilmenite 

 with rose-pink pyrope in much-rounded crystals, and some monazite. 

 It also occurs in small amount in most of the monazite-bearing sands 

 of the Boise Basin region. In these latter sands the titanite has a 

 pale-brown color not very different from the color of the monazite. 

 It may be distinguished from the monazite by differences in luster 

 and crystal form. 



SAMARSKITE. 



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

 ("P654, olivine") was found to be distinctly radioactive. Careful 

 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 



