482 T. L. WATSON WEATHERING OF ALLANITE 



(c) The reddish brown mineral has the following optical properties, 

 which vary a little: Optically negative (— ), 2Y = rather large, 8 > Y 

 rather strong. 



a = 1.727 ± 0.005, X = pale yellowish. 



/? = 1.739 dz 0.005, Y = reddish brown. 



7 = 1.749 dz 0.005, Z = reddish brown. 



Evidently the "allanite" of these specimens is made np of at least two 

 and probably three distinct minerals. The one is pale colored, sensibly 

 isotropic, and has an index of refraction of about 1.68 to 1.70; another, 

 possibly related to the former, has a somewhat higher index of refraction, 

 a weak birefringence, and is pleochroic in green and yellow. The third 

 is clearly derived from the others and has the following optical properties : 

 Optically negative ( — ), 2Y==: rather large, 8>Y rather strong. 

 ^ = 1.71 to 1.76. Birefringence, 0.01 to 0.02. Pleochroic in red brown 

 with absorption Y and Z > X. 



LLANO COUNTY. TEXAS ^ 



In mass, black and vitreons; in thin section rather deep olive green. 

 Isotropic and nearly homogeneous, n^^ 1.725 zb 0.005. 



GARTO, ARENDAL, NORWAY ^^ 



Pale olive green in section and sensibly isotropic; ?i = 1.670 ziz 0.005, 

 varies somewhat. Much of this specimen is a brownish, vitreous altera- 

 tion product. It is red brown in section and different grains vary greatly 

 in optical properties. Part is isotropic, part is birefraeting. The index 

 of refraction varies greatly, but much is near 1.60±. 



Larsen concludes from his microscopic study of the above five black 

 and vitreous allanites that the so-called "allanite'' is made up of a mix- 

 ture of at least two minerals, one of which is birefraeting, the other iso- 

 tropic. The two types are distinct and their proportion in the same 

 specimen is subject to great variation. Apparently the red, birefraeting 

 type is secondary, derived from the isotropic form, possibly without much, 

 if any, change in chemical composition. Such relation between the two 

 types might be the result either of alteration or of inversion, probably 

 alteration. 



The change from an anisotropic to an isotropic form, which may in- 

 volve simply molecular rearrangement (paramorphism) or more or less 

 chemical change, is regarded by some mineralogists as a form of alteration 

 and is reported to have been observed in many minerals. Such change is 



«su. S. National Museum 84416. 

 «*U. S. National Museum 49022. 



