498 T. L. WATSON WEATPIERING OF ALLANITE 



known that the neutral and basic carbonates of the cerinm metals are 

 insoluble in water, while the double carbonates are sparingly soluble and 

 are decomposed by water. Cerons and eerie carbonates are reported to 

 be insoluble in cold water and carbon dioxide, while lanthanum carbonate 

 is insoluble in cold water, but is slightly soluble in carbonated water. All 

 carbonates of the cerium metals are soluble in dilute acids. 



These are results obtained in the laboratory, where the time factor is 

 very brief and the concentrations greater than in nature, and are not 

 comparable with the natural conditions in the belt of weathering where 

 the mineral allanite in common with all others may be exposed for an in- 

 deiinite period of time to a constant supply of weak solutions. Again, the 

 rare earth elements, considered as trivalent at present, are known to bear 

 a general chemical resemblance to those of the alkaline earths, which 

 under conditions of weathering are always removed as soluble carbonates. 



From our knowledge therefore of the chemistry of the cerium group of 

 metals, it seems reasonable to regard their loss on weathering as due to 

 the formation of soluble carbonates removed in solution. That such car- 

 bonates are formed from the alteration of allanite has been stated by 

 Clarke,^* who says: "Allanite is often much altered, yielding carbonates 

 of the cerium group, together with earthy products of uncertain char- 

 acter." 



Although tests were made for the presence of carbonates on portions 

 of the weathered crust of the mineral from different localities with posi- 

 tive results in only two cases, carbonation is regarded as one of the im- 

 portant reactions in the alteration, and the cerium earths are believed to 

 have been removed in the form of soluble carbonates, as were certainly 

 the lime and magnesia. Of the rare earths present in allanite, cerium is 

 the least soluble and is partly retained in every case, entering as an im- 

 portant constituent into the composition of the weathered product. 



Lanthanite, a hydrous lanthanum carbonate, is reported from a few 

 localities and is the only known native carbonate of the cerium group of 

 metals. It has been reported in scales and scaly incrustations on allanite 

 from Essex County, New York,^^ and Baringer Hill, Llano County, 

 Texas,^^ and is undoubtedly a secondary mineral derived by alteration 

 from allanite. 



The variable but unusually large percentage losses of the remaining 

 essential constituents, silica and alumina, have their analogy in rock 



8* F. W. Clarke : The data of geochemistry. Bull. 616, U. S. Geol. Survey, 1916, p. 

 408. 



8^ E. S. Dana : A System of Mineralogy, 1900, p. 302. 



88 F. L. Hess : Bull, 340, U. S. Geol. Survey, 1908, p. 292. 



