DESILICATED GRANITIC PEGMATITES^. 

 BY SAMUEL G. GORDON. 



The granitic pegmatites are among the most interesting mineral 

 assemblages known to the mineralogist, containing as they frequent- 

 ly do so many rare minerals. The present paper describes an ab- 

 normal group of granitic pegmatites, composed either of plagioclase, 

 usually albite, alone (albitite) ; of plagioclase and corundum (plu- 

 masite); or largely of corundum; all of which occur exclusively in 

 peridotites or their altered equivalents. The conclusion has been 

 reached that they owe their peculiarities to the reactions which oc- 

 curred between the original pegmatitic solutions and the peridotite 

 or serpentine into which these solutions were intruded. Such an 

 origin has been demonstrated for the plumasite of South Africa by 

 Du Toit^; a study of the albitites of Pennsylvania and Mary- 

 land indicates a similar genetic history ; and the corundum deposits 

 of Pelham and Chester, Massachusetts, of western North CaroHna, 

 and of Georgia present analogies which show them to belong in this 

 group. 



Previous Work. 



Albitite. 



California : The name albitite was given by Turner^ in 1896 to 

 an albite rock occuring as dikes in serpentine in the Grizzly Hill area, 

 about Meadow Valley, and near Big Bar Hill, Plumas County, 

 California. The albitite dikes were composed of large grains of 

 albite with marked cleavage, and showing occasionally a twin 

 lamella. 



Previous to this, Palache^ had described a soda amphibole (cros- 



1 The writer is indebted to Dr. Edgar T. Wherry and Dr. N. L. Bowen for a 

 critical examination of this paper. The Pennsylvania and Maryland albitite 

 were studied during the summer of 1920. An account of the minerals of the 

 district will appear shortly in The American Mineralogist, and a description of 

 the chromite deposits in a later paper in the Proceedings of this Academy. 



2 Trans. Geol. Soc. S. Africa, 21: 53-73, 1919 (see abstract below). 



5 Am. Geol. 17: 375-388, 1896; Fourteenth Ann. Rep. U. S. Geol. Surv., 1892- 

 93, (1894) 477; U. S. Geol. Surv. Folio 43, 1898. 



^Charles Palache, Univ. of Calif. Publ., Dept. Geol., Bull. 1: 181-192, 

 1894. 



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