ART. 8. 



BLACK SANDS FROM IDAHO SHANNON. 



27 



OLIVINE. 



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 w,(110) and the dome -ST (021) with the 

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

 machrodome d{101) and the pyramid /(121) occur rarely as very 

 small faces, as shown in figure 24. 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. 24) 

 to moderately long prismatic by elongation on the a axis (fig. 25). 

 Similar clear-yellow olivines from other samples of Snake River 

 sands do not show measurable faces. Occasionally the brilliant 

 yellow crystals of olivine have an outer coating of pale brown clay. 

 Many of the crystals contain abundant small included grains of 

 black iron ore. The forms present were identified by the following 

 angles : 



Angles of olivine from Minidoka. 



QUARTZ. 



Quartz in the form of angular dirty white or brownish sand occurs 

 abundantly in the sands which have not been too far concentrated, 

 this being the most abundant of the lighter constiuents eliminated 

 by panning. This mineral also occurs frequently in concentrates of 

 the heavy materials as small brilliant transparent colorless crystals 

 formed by the combination of the plus and minus rhombohedrons 

 without prismatic planes, as shown in figure 20. Many of these 

 crystals resemble octahedral crystals of diamond where the hex- 

 agonal form is not easily distinguishable. Just why they should 

 persist in the heavy concentrate rather than be eliminated by pan- 

 ning is not clear. 



