COMPOSITION OF ALLANITE 481 



allanite is made up of two and probably three distinct minerals. Dr. 

 Larson's results, which, with his permission, are published here for the 

 first time, follow: 



ROANOKE COUNTY, VIRGINIA^ 



A microscopic stud}^ of the black, vitreous allanite, from what appeared 

 to be the freshest part of the specimen, showed that it is made up of two 

 different minerals — a pale olive green, sensibly isotropic mineral, and a 

 pale green, birefracting mineral. The birefracting mineral is derived 

 from the sensibly isotropic mineral and replaces it along streaks and 

 irregularly. (A thin-section would show this better.) 



The isotrophic mineral has an index of refraction of about 1.69 7 it .003. 

 The birefracting mineral is faintly pleochroic with x = pale green and 

 2; = yellowish. The mean index of refraction is about 1.71 and the bire- 

 fringence is about 0.01. 



AMHERST COUNTY, VIRGINIA <^^ 



All of the material examined is birefracting. It is probably optically 

 negative ( — ), with a large axial angle, but no good interference figure 

 was had, probably owing to some abnormal dispersion. jS = 1.755, varies 

 0.01. Birefringence = 0.01 about ; x = pale yellowish, z = deep reddish 

 brown. 



ALBANY, WYOMING <^2 



The blackish brown, vitreous material is made up of two or probably 

 three distinct minerals. Thin sections show the relations clearly. A 

 nearly colorless, isotropic mineral is replaced in irregular areas by a pale 

 greenish, faintly birefracting mineral, and these two are in turn altered 

 along the borders of the grains and along cracks and streaks to a reddish 

 brown, rather strongly birefracting mineral. This latter makes up over 

 half of the sections of the large piece and all but a small core of the 

 small grains — up to several millimeters across. The alterations suggest 

 the alteration of olivine to iddingsite, and in both the oxidation of iron 

 is probably an important change. 



(a) The colorless to pale greenish, isotropic mineral has a rather con- 

 stant index of refraction of 1.685±: 0.005. 



(h) The pale green, birefracting mineral has a somewhat higher index 

 of refraction and a weak birefringence. It is pleochroic with x = nearly 

 colorless and z = pale green, 



»0T. L. Watson. 

 «i F. L. Hess. 

 «2F. L. Hess. 



