COMPOSITION OT' ALLANITE 483 



claimed to be common for certain minerals like homilite, gadolinite, 

 allanite, etcetera, in the "Brevik" region of Norway. 



Gadolinite, which is normally double refracting and crystalline, is, 

 according to Dana,^^ more commonly completely isotropic both in the 

 massive form and in crystals, and both forms may be observed in the 

 same specimen in thin section. Dana attributes this change in optical 

 structure to alteration from molecular rearrangement, for Petersson^® 

 has shown that both varieties have the same composition. The amorphous 

 form of the mineral was made anisotropic and crystalline on heating, 

 accompanied by other physical properties. Both forms are rendered 

 brown by alteration — oxidation and hydration. 



It is evident from the microscopic results detailed above that the com- 

 position of ordinary black vitreous allanite^*^* is open to doubt and should 

 be investigated. Such study will involve a separation of the birefracting 

 type from the isotropic form, and complete accurate analyses made of 

 each and their optical and other properties determined. Microscopical 

 and chemical studies of the two types will be continued with the hope of 

 definitely determining whether the birefracting form is derived from the 

 isotropic one by alteration or by inversion. 



For the purposes of the present problem, however, investigation strongly 

 points to sufficient variation in the composition of the so-called fresh 

 allanite to at least influence and possibly to account for. the heterogeneous 

 character of the altered allanite product representing the same stage of 

 weathering. 



Weatheked Product (Ceust) of Allanite 



megascopic characters 



Studies made of allanite from Ellenville, New York; Chester County, 

 Pennsylvania; Amherst, Fauquier, Nelson, and Eoanoke counties, A^ir- 

 ginia ; Iredell, Madison, and Mitchell counties, North Carolina ; Anderson 

 County, South Carolina, and Baringer Hill, Llano County, Texas, show 

 the formation of a reddish brown crust about the original mineral, which 

 is clearly an alteration product derived from weathering. Many of the 

 specimens are entirely encrusted 1)y the alteration product, others are only 



«=E. S. Dana: A System of Mineralogy, 1900, pp. 506, 507, 510, 511, 512. 



«6 G. Petersson : For. Forh., vol. 12, 1890, pp. 275-347 ; quoted by E. S. Dana. 



^a In his studies of radio-active minerals found in Pennsylvania, Wherry has pub- 

 lished a radiograph of allanite from East Bradford which is suggestive and may bear on 

 the possible difference in composition of allanite. In the radiograph the outline of the 

 allanite specimen is observed, but there are shown two small but pronounced white areas 

 of very much greater intensity than other parts of the radiograph. Wherry designates 

 the intensity of the mineral as "faintly active ; barely discernible print in three weeks." 

 (Edgar T. Wherry : .Tour. Franklin Institute, vol. clxv, 1908, pi. iv, facing p. 66.) 



