A DECIMAL GROUPING OF THE PLAGIOCLASES 1 



F. C. CALKINS 



United States Geological Survey, Washington, D.C. 



The accompanying diagram, which represents about half of the 

 methods according to which the soda-lime feldspars have been 

 grouped, reveals a surprising diversity of usage. Consistency, 

 which undeniably would be of some advantage in the long run, 

 is not likely to obtain until it is demonstrated that some particular 

 scheme is better than all others. That this has not been done is 

 perhaps because it has not been attempted; for, once the matter is 

 given any critical attention, the most convenient and logical 

 adaptation of current nomenclature seems rather easy to find. 



The ideal plan should, first, accord with the modern doctrine 

 that the plagioclases form a continuous series; schemes that 

 imply a limited number of compounds (Nos. 1-3) must, therefore, 

 be rejected. 



Secondly, the intervals that separate the species in certain plans 

 (Nos. 5, 8, 9) are needless, even when they imply no discontinuity. 

 They then result in eleven-fold instead of sixfold division, com- 

 pound names, like "andesine-labradorite," being applied to the 

 intervals between the main species; but six terms, with qualifying 

 adjectives, will suffice for as close discrimination as is worth while, 

 and the awkward compound names may be reserved for feldspars 

 that lie virtually at the junction of two species. 



Finally, the division should be regular. The question arises here 

 whether the names "albite" and "anorthite" shall be applied only 

 to the pure soda and pure lime feldspars respectively, as in Zirkel's 

 quite regular plan (No. 7), or whether they shall denote a certain 

 range of composition. As absolutely pure end terms are mere 

 abstractions, the answer to this question amounts to a choice 

 between a fourfold and a sixfold division. It is only following 

 universal practice to prefer the sixfold one. Now, there are but 



1 Published by permission of the Director of the United States Geological Survey. 



157 



