486 T. L. WATSON WEATHERING OF ALLANITE 



I II III IV 



DizOg®* none 1.21 0.65 none 



LaaOs none trace 3.27 none 



Fe^Oa 37.14 47.31 34.48 12.24 



FeO .* none trace none none 



BeO 0.94 none none 1.95 



MgO none trace 0.29 none 



OaO none 2 . 45 none none 



NaaO none none 0. 43 none 



K2O none none 0. 20 none 



H2O 29.55 29.24 17.16 21.37 



99.64 99.86 99.72 99.49 



I. Inner red layer of weathered allanite crust from Amherst County, Vir- 

 ginia. J. R. Santos, analyst. 

 II. Weathered allanite crust from Roanoke County, Virginia. S. D. Gooch, 



analyst. 

 III. Weathered allanite crust from Nelson County, Virginia. E. P. Valentine, 



analyst. 

 IV. Outer white layer of weathered allanite crust from Amherst County, Vir- 

 ginia. J. R. Santos, analyst. 



These analyses represent approximately a similar stage of advanced 

 allanite weathering. They clearly show the composition of the allanite 

 deca3^ed product to be made up in each case of essentially the same con- 

 stituents, but in widely varying percentage amounts. Without regard to 

 their quantitative order, these are silica, alumina, ceria, ferric oxide, and 

 water. Some didymia and lanthana are indicated in two of the analyses, 

 and in one (III) lanthana exceeds ceria by 2I/2 times in amount — a re- 

 versal in the usual order of solubility of the salts of the two metals. 

 Likewise the heterogeneous character of the weathered product has been 

 shown from microscopic study composed of isotropic and of weakly bire- 

 fracting types of variable physical properties and probably composition. 

 Probably the bulk of the product is made up of the isotropic type, the 

 general character of which, from the nature of many colloidal materials, 

 argues for a single mineral in colloidal or metacolloidal form of variable 

 composition rather than composed of a mixture of several minerals. 



Some qualitative tests carried out by Prof. A. W. Giles at the request 

 of the writer, on the reddish brown decayed product of allanite from four 

 different localities, agree in most cases in showing the lack of homogeneity 

 of the material. The tabulated results follow below. Similar tests made 

 by Valentine on the decayed product from Nelson County, Virginia, gave 

 the following results: Decomposed by HCl; heated on charcoal, the 



68 Di is the symbol for old didymium, which is now known to be a mixture of neody- 

 mium, praseodymium, samarium, etcetera. 



