154 NEW YORK STATE MUSEUM 



Section 5 



THE OCCURRENCE OF PEGMATITE IN NEW YORK 



GENERAL FEATURES, FIELD RELATIONS AND USES OF 



PEGMATITES 



The coarsely textured modifications of granite that are called 

 pegmatites have a special interest that seems best to recognize here 

 by their separate description. This interest is connected not only 

 with their scientific features in regard to origin, methods of occur- 

 rence and mineral contents, but also with their industrial uses which 

 cover certain fields quite apart from those belonging to ordinary 

 granites. Pegmatites are sources of feldspar, quartz and other min- 

 erals of commercial importance. 



The most striking physical character of pegmatites — their 

 coarseness of texture — is a relative one, but important in determin- 

 ing their utility. Almost every variation may be found in the field 

 between the coarser granites which are available for constructional 

 or ornamental stone to the coarsest *' giant " granites or pegmatites 

 in which the individual minerals attain dimensions of several feet 

 and weights of a ton or more. It is evident that other things being 

 equal, the larger the size of the icrystals, the more readily can their 

 separation be carried out, and ease of separation is an important 

 factor in the success of quarry operations for the production of 

 feldspar and quartz. 



Pegmatite is commonly associated with granite in its field occur- 

 rence. It is rare enough to find any large granite exposure without 

 more or less of pegmatite either as included bodies or as distinct 

 but apparently related intrusions in the surrounding country. The 

 relation is so constant as to lead to the view already expressed 

 earlier in this report that pegmatite is really bitt a modified form 

 of granite, the textural differences being ascribable to variations in 

 the process of crystallization. The presence in pegmatites of min- 

 erals containing fluorine, chlorine, boron, water, and other ingre- 

 dients that are regarded as powerful solvents or " mineralizers," is 

 significant. It appears very probable from this and other consider- 

 ations that the rock represents the residue of a granite magma that 

 was still held liquid after the main body had reached its consoli- 

 dation temperature. This residue would tend to gather in the lower 

 part of the magma as a result of the forcing out of the solvents from 

 the cooling and crystallizing zone above. With the solvent vapors, 



