MINERALOGY OF SOME BLACK SANDS FROM IDAHO, 

 AVITH A DESCRIPTION OF THE METHODS USED 

 FOR THEIR STUDY. 



By Earl V. Shannon, 

 Assistant Curator, Department of Geology, United States National Museum. 



INTRODUCTION. 



The term " black sand " is used commonly in gold placer mining 

 districts to indicate the heavy concentrate from the placer gravels 

 which accumulates with the gold in sluice boxes and on concentrating 

 tables. The name, which is in very general use, comes from the 

 fact that the predominant constituent is usually black in color. 

 This black constituent, although commonly magnetite is sometimes 

 largely ilmenite or in regions of serpentinous rocks it may consist 

 in considerable part of chromite. AATiere the dominant constituent 

 is not black in color local miners usually designate their heavy 

 residues by some more appropriately descriptive name. Thus in 

 southeastern Alaska much of the gold is recovered from " ruby 

 sand " consisting predominantly of garnet ; in the Florence and War- 

 ren districts in Idaho the heavy sand consisting very largely of 

 colorless zircon is called " white sand " and sand rich in monazite 

 in the placers of the Boise Basin is locally designated " yellow sand." 

 In the present treatment these several varieties are referred to col- 

 lectivel}^ as black or heavy sands. 



These sands consist ordinarily of the heavier and rarer constitu- 

 ents concentrated from a great volume of disintegrated rock and 

 may contain a great variety of unusual minerals. Many of these, 

 in addition to being of high specific gravity, are quite hard, and as 

 a consequence the heavy sands concentrated from stream gravels are 

 in many places aggregates of glittering faceted crystals of minerals 

 of various colors. Even relatively soft minerals occurring in the 

 sands at times show few signs of abrasion, having escaped wear 

 doubtless because of their small size and the rapidity of erosion. 



No. 2398.— Proceedings U. S. National Museum, Vol. 60. Art. 3. 



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