DISCUSSION OF RESIILTS 497 



stage of weathering the weathered product is composed of a few constit- 

 uents, chiefly silica, alumina, ferric oxide, ceria,^^ and water, all the other 

 constituents of the fresh mineral having heen nearly or completely re- 

 moved in soluble form by leaching. When compared with analyses of the 

 corresponding fresh mineral from which the weathered product has been 

 derived, each constituent on a ferric oxide constant basis has suffered loss 

 from leaching, the more soluble ones having entirely disappeared and the 

 more refractory ones having l)een removed in unusually large amount. 



In common with the weathering of rocks and minerals in general, the 

 change in allanite has been accompanied by a large increase in water 

 (hydration), there being present in the decomposed product nearly 30 

 per cent of water. Allanite which appears fresh, but on analysis contains 

 several per cent of water, is regarded as an altered form of the mineral, 

 which often contains some COo, and slight oxidation is sometimes indi- 

 cated. 



The most patent reaction involved in the alteration, and one which ac- 

 companied hydration, was oxidation, whereby ferrous oxide was changed 

 to ferric oxide. For the cases studied ferric oxide was retained in largest 

 amount, and has been assumed as the constant factor for calculating the 

 percentage amounts lost and saved of the other constituents. It seems 

 clear in one case at least that this constituent has suffered some loss from 

 leaching ; but the process of oxidation must have taken place, as a rule, in 

 a sufficient supply of free oxygen to convert the iron into the insoluble 

 ferric form. 



One of the most important as w^ell as interesting facts developed in this 

 study is the large but imequal percentage amounts of the rare earths lost 

 by leaching. With only one exception (Nelson County, Virginia), cerium 

 has proved to be the most refractory of the rare earths, and although lost 

 in large amounts it has been partly retained in every case, and is an im- 

 portant constituent of the weathered product. In several cases the analyses 

 indicate that other rare earths present in the fresh mineral have suffered 

 complete loss. Spectroscopic examination by Santos of the outer white 

 layer of the weathered allanite crust from Amherst County, Virginia, 

 failed to show the absorption bands of didymium, so complete was its 

 removal. 



The soluble form in which the cerium earths have been leached from 

 allanite on weathering is important, l)ut is probably less conclusive than 

 for other constituents, since their chemistrv is not so well known. It is 



^^ The decayed product of allanite from Nelson County, Viri?inia, contains lauthana lu 

 amount about 2i/^ times greater than ceria. 



