4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 65 



garnet from gem gravels in the collection, in distinguishing green- 

 ockite from other minerals occurring as yellow coatings, and in the 

 identification of a number of other minerals. The microspectroscope 

 may also be applied to the measurement of the thickness of iridescent 

 films and the discovery of the origin of various color phenomena, 

 but this phase of the subject has been fully discussed by Keeley in 

 the paper cited and need not be further considered here. 



THE RARE EARTH MINERALS 



The strong absorption bands shown by salts of certain of the rare 

 earth metals have long been recognized as a good means for their 

 detection in solutions, and several writers have pointed out that 

 minerals containing them also show the bands, and have called 

 attention to the value of this property for identification of these 

 minerals. In the preparation of this paper all available minerals 

 known to contain appreciable amounts of the rare earths have been 

 examined. Most of the light colored ones, as listed in the tables 

 below, were found to exhibit two or more of these bands, all except 

 the violet calcite yielding much more intense effects when viewed at 

 an angle to the path of the light than when observed in the direction 

 of the transmitted ray. Not only is the presence of these absorption 

 bands useful as a means of distinguishing rare earth minerals from 

 all others, but it may even serve to differentiate certain of the indi- 

 vidual species ; the positions and intensities of the bands vary from 

 one to another in a fairly characteristic way, although identification 

 on this basis alone is not always certain, since slight variations may 

 occur between different grains of the same mineral. 



The presence of the rare earth metals in calcite from Joplin, 

 Missouri, was discovered by W. P. Headden 1 by analytical pro- 

 cedure, and has recently been reaffirmed by Pisani, 2 the amounts 

 present being mostly less than 0.05 per cent. Headden found that 

 the violet calcite from this locality gives " didymium " absorption 

 bands. With the microspectroscope this material shows, by trans- 

 mitted light, two distinct bands, matching approximately those of 

 neodymium in the " didymium " glass comparison spectrum, and 

 being probably due to that element, the salts of which have a violet 

 tint. The most deeply colored specimens show these bands when as 

 thin as 3 millimeters, although the paler tinted varieties show them 

 only in greater thicknesses, while the colorless and yellow portions 



Amer. Journ. Sci., ser. 4, vol. 21, 1906, p. 301. 

 Compt. rend., vol. 158, 1914, p. 1121. 



