480 T. L. WATSON WEATHERING OF ALLANITE 



Texas. — In the famous giant pegmatite forming Baringer Hill/^ Llano 

 County, Texas, allanite is reported to occur in masses weighing up to 300 

 pounds, imbedded in purple fluorspar, and is associated among other min- 

 erals with a variety of rare earth ones in unusually large masses and 

 quantities. In addition to allanite are found other minerals containing 

 yttria and its companions, such as gadolinite, yttrialite, thorogummite, 

 nivenite, fergusonite, tengerite, cyrtolite, rowlandite, mackintoshite, and 

 yttrocrasite. Several of these are peculiar to this locality. Gadolinite, 

 the chief source of the yttria earths, occurs in crystals and masses of 

 irregular shape up to 200 pounds in weight. 



Hess reports the excavations thus far made to be comparatively shallow 

 and the minerals found to be more or less weathered. Allanite exhibits 

 weathering around the edges and along cracks to the usual reddish brown 

 product, which is well shown by a specimen of the fresh dense black and 

 vitreous mineral from this locality in the collections of the United States 

 ISTational Museum. 



Composition of Allanite 

 general observations 



Examination of the 85 analyses of allanite listed by Hintze^^ from all 

 parts of the world shows the widest possible variation both in composition 

 and in specific gravity. (See analyses of allanite from the Atlantic 

 States, tabulated on pages 474 and 479.) Some of the analyses show con- 

 siderable water, which is probably due to alteration. Most of the analyses 

 were made many years ago, some are less complete than others, and none 

 can be counted as modern. Again, the degree of purity in each case of 

 the sample analyzed is unknown. To these facts must be accorded some 

 weight in accounting for the variation in composition, but to what extent 

 it is not possible to state. 



It is believed that the chief cause for variation in the composition of 

 allanite is to be attributed to the heterogeneous nature of the mineral. 

 Kecent microscopic study by Dr. E. S. Larsen, United States Geological 

 Survey, of five specimens of black, vitreous, and supposedly fresh allanite 

 from as many different localities demonstrates its heterogeneous charac- 

 ter and leads to the conclusion that for the localities examined at least 



58 Frank L. Hess: U. S. Geol. Survey, Bull. 340, 1908, pp. 286-294. 



W. E. Hidden and J, B. Mackintosh : Am. Jour. Sci., vol. 38, 1889, p. 474. 



W. E. Hidden : Am. Jour. Sci., vol. 42, 1891, p. 430 ; ibid., vol. 19, 1905, p. 425. 



W. E. Hidden and W. F. Hillebrand : Am. .Tour. Sci., vol. 46, 1893, pp. 98, 208. 



W. F. Hillebrand: Am. Jour. Sci., vol. 13, 1902, p. 145. 



W. E. Hidden and C. H. Warren : Am. Jour. Sci., vol. 22, 1906, p. 515. 

 ^8 C. Hintze : Handbuch der Mineralogie, 1897, pp. 272-276, 



