310 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Measurements oj j or sterile, Figure 77 



CHRYSOLITE (376) 



OLIVINE, PERIDOT 



Magnesia-iron silicate, (2Mg,Fe)O.Si0 2 . Orthorhombic. 



Chrysolite or olivine has been reported from several localities as a 

 constituent of basaltic igneous rocks in which it occurs as micro- 

 scopic grains. It has also been described as a constituent of heavy- 

 concentrates from sands worked for placer gold from several localities 

 along Snake River of which the following description is typical. 33 



MINIDOKA COUNTY 



While olivine occurs sparingly in all of the sands from Snake 

 River localities as clear pale-yellow angular grains, in several samples 

 from Minidoka it is present as clear pale lemon-yellow crystals with 

 highly lustrous faces. These resemble small topazes or crystals of 

 chrysoberyl and their identity was not suspected until they were 

 measured and found to have the angles of olivine. The dominant 

 forms present are the prism m(110) and the dome K(021) with the 

 prism s(120) and the brachypinacoid 6(010) less prominent. The 

 macrodome d(l0l) and the pyramid /(121) occur rarely as very small 

 faces as shown in Figure 78. The combination of forms is the same 

 on all of the crystals measured, but they vary in development, ranging 

 from short prismatic parallel to the vertical axis (fig. 78) to moderately 

 long prismatic by elongation on the a axis (fig. 79). Similar clear 

 yellow olivine grains from other Snake River localities do not show 

 measurable faces. Occasionally the brilliant yellow crystals of 

 olivine have an outer coating of pale brown clay, and many of them 

 contain included grains of magnetite. The forms present were 

 identified by the following measurements : 



83 Earl V. Shannon. Mineralogy of some black sands from Idaho, etc., Proc. U. S. Nat. Museum, vol. 

 60, art. 3, p. 27, 1921. 



