4 PROCEEDINGS OF THE NATIONAL. MUSEUM. vol. 80 



present study it was found impossible to determine whether many 

 tetragonal crystals were zircon, rutile, xenotime, or thorite by crys- 

 tallographic measurements- The angles of the commonly occurring 

 pyramids on these minerals are more closely similar than the meas- 

 urements usually obtained from adjacent faces of the same crystal. 

 This may be shown by comparing the angle from the pole to the 

 unit pyramid on each of these minerals as given below : 



Theoretical angles of similar tetragonal minerals. 



Zircon c ( 001 ) a p (111 ) =42 ° 09'. 

 Thorite c ( 001) a p ( 111 ) =42 ° 10'. 

 Xenotime c(OOl) a«(111)=42'' 12'. 

 Rutile o(001)A«(lll)=42"' 19'. 



The spectroscope is of very doubtful value in distinguishing be- 

 tween these troublesome tetragonal minerals because of its predom- 

 inantly qualitative character. A crystal of either of the four min- 

 erals mentioned above will ordinarily yield spectroscopic reactions 

 for the essential constituents of each of the others. The microspec- 

 troscope is open to the same serious objection. 



The following table is put forth in the hope that it will be of 

 service in aiding in the identification of some of the commoner con- 

 stituents of black sands, especially when used in conjunction with 

 the descriptions and figures of the following pages. The determina- 

 tive table is almost entirely confined to the minerals observed during 

 the examination of some 50 sands from Idaho loc;alities, and there 

 are very numerous others which might reasonably be expected to 

 occur in heavy residues of this sort. When the sands of any region 

 are found to have any important content of any unusual looking or 

 unidentificable minerals they should be submitted to a competent 

 mineralogist for study. 



Table of visible properties of minerals occurring i7i heavy sands. 



Extracted by a common magnet: 

 Color black: 



Form isometric (octahedrons, etc.) ; magnetite. 

 Form trigonal (hexagonal and triangular, tabular) ; ilmenite. 

 Form broken or irregular ; magnetite, ilmenite. 

 Color white to gray, opaque, metallic ; platinum metals. 

 Not extracted by a common magnet: 

 Opaque or nearly so : 

 Color black : 



Form trigonal ; ilmenite. 



Form isometric; chromite (streak brown), limonite (streak 



brown), hematite (streak red). 

 Form tetragonal, prismatic: 



Crystals dull; tapiolite (?). 

 Crystals brilliant, striated ; rutile. 



