484 T. L. WATSON WEATHERING OF ALLANITE 



partly encrusted, and others still exhibit only small patches here and there 

 over the surface of the mineral. 



In no case observed has the thickness of crust coating the larger masses 

 of allanite exceeded a quarter of an inch and is usually less, while some 

 of the grains and smaller masses may be entirely so altered. With only 

 one exception (Anderson County, South Carolina), the weathered crust 

 is sharply defined from the fresh appearing mineral, without gradation 

 from one into the other. A typical specimen, however, from Anderson 

 County, South Carolina, showed that weathering had developed in part 

 along cleavage directions, and it was clearly observed to extend from the 

 completely weathered crust into the fresh allanite along the cleavages. 



The crust is usually friable and pulverulent, approaching at times an 

 earthy character, but is often granular in appearance, exhibiting more or 

 less of a waxy luster which becomes pronounced under the microscope. 

 Its general appearance is that of a highly oxidized material which resem- 

 bles more or less closely some forms of hydrated iron oxide. The analyses 

 on page 485 confirm the high contents of ferric oxide and water in the 

 composition of the weathered product. 



As noted above, the prevailing color is reddish brown, but lighter shades 

 of brownish yellow, grayish buff to nearly white, are observed. Masses 

 of the mineral from several localities, especially emphasized in specimens 

 from Amherst County, Virginia, sometimes exhibit a rudely zoned or 

 layered crust, an inner reddish brown layer, and an outer lighter colored, 

 very thin layer which may be almost white in some specimens from the 

 Virginia locality. Separate analyses (I and IV of table on page 495) 

 have been made of these two portions of the crust on specimens collected 

 from Amherst County, Virginia. 



Although the heterogeneous character of the fresh mineral from the 

 different localities is indicated microscopically, there is no evidence for 

 regarding the two unlike layers of the weathered crust as having been 

 derived from a zonal arrangement of chemically different molecules in 

 the fresh mineral. Indeed, the rudely layered structure of the weathered 

 product is the result of alteration, and probably has its analogy in the 

 lateritization of rocks as recently developed by Professor Lacroix,^^ who 

 distinguishes in the laterites of French Guinea an inner zone of leaching 

 (depart) and an outer zone of concretion. 



MICROSCOPIC CHARACTERS 



Microscopic study of the weathered allanite crust from all the localities 

 enumerated above (page 485), except Baringer Hill, Texas, and Fauquier 



«^ A. Lacroix : Nouvelles Archives du Mus6e, ser. V, torn, v, pp. 255-356. 



