110 Iddings and Cross — Occurrence of Allanite 



following additional particulars as to its characteristics and dis- 

 tribution may be of interest. 



Crystals which are large enough to be seen without the aid 

 of a lens are easily recognized in the hand specimen by their 

 brilliant black color, uneven fracture and oily luster. They 

 vary greatly in form and size in the different rocks examined, 

 in some cases being long, slender prisms, reaching a maximum 

 length of 1 cm., though usually appearing in short, stout 

 prisms, or in quite irregular grains of microscopic dimensions. 

 In thin sections from a great number of localities the color is 

 uniformly a rich chestnut brown with the strong pleochroism 

 already mentioned, but in a few instances a zonal variation has 

 been noticed, the color growing lighter from the center of the 

 crystal outward. Cleavage is in most cases wholly absent. 



A confusion with hornblende or biotite is only possible 

 when these minerals are of nearly the same shade of brown r 

 and are so cut by the section as to show no distinct cleavage. 

 But the lack of pleochroism and the nearly uniaxial character 

 of a basal section of biotite — the only one not exhibiting 

 cleavage — sufficiently distinguishes it from the strongly pleo- 

 chroic, biaxial allanite. In the exceedingly rare cases where 

 hornblende possesses the peculiar chestnut-brown color, and 

 shows neither characteristic cleavage nor outline, one can only 

 distinguish the allanite by its higher index of refraction, which 

 produces more brilliant interference colors between crossed nicols. 



From its mode of occurrence and association allanite must 

 be added to the group of primary, accessory rock constituents, 

 similar to zircon, sphene, and apatite, though much rarer than 

 any of these. In one instance it was noticed enclosing zircon, 

 in others, sphene, apatite, and magnetite, but it has also been 

 found in such connection with these minerals as to indicate a 

 contemporaneous growth. Its nature as a primary constituent 

 in eruptive rocks is further attested by its occasional inclusion 

 in biotite, feldspar, and quartz. 



In some regions it appears to be quite uniformly distributed 

 through certain types of rock. Thus, in the porphyrites and 

 allied porphyries of the Ten Mile district, Colorado, prismatic 

 crystals of allanite may be seen in numbers upon nearly every 

 hand specimen — one exhibiting as many as forty on its surface 

 — and few thin sections of these rocks were examined without 

 one or more being discovered. On the other hand, in the 

 vicinity of Eureka, Nevada, it seems to be very irregularly 

 disseminated through a considerable range of rock types, two 

 or three microscopical individuals being recognized in quite a 

 number of thin sections — and in one instance as many as 

 eleven — but in the majority of sections it is absent altogether. 

 In most of the rocks studied the mineral occurs very sparingly. 



