W. M. Thornton, Jr. — A Feldspar Aggregate. 219 



composed essentially of feldspar and bine quartz and in places 

 much hornblende. The accessory minerals ilmenite and pyrite 

 and apatite are also present. The feldspar is by far the domi- 

 nant portion of the rock ; and, since it presents some unusual 

 features, it was thought by the author that a study of its com- 

 position would prove of some interest. Of course the natural 

 procedure would be to isolate mechanically the feldspathic por- 

 tion and to analyze the most homogeneous material obtainable. 

 But three analyses of the pegmatite were required for geolog- 

 ical purposes; and since time was lacking in which to make a 

 fourth of the feldspar alone, it was decided to employ the 

 analysis of the extreme acidic phase for calculating the compo- 

 sition of the feldspar. 



The color of the feldspar is light bluish gray. Under a 

 magnifier of twenty diameters it appears decidedly transparent 

 and glassy. The texture is one of very close crystalline 

 aggregation. Specific gravity = 2 - 68. From all outward 

 appearances one would suppose it to be a definite species ; but 

 the analysis and portioning of the molecules to form the respec- 

 tive feldspars in the accompanying table shows it to be a 

 mixture of orthoclase and plagioclase, and that the plagioclase 

 is made up of albite and anorthite in the ratio of 10 to 7. 

 This is also confirmed by microscopic examination of thin 

 sections.* 



Pegmatite (feldspathic fades) near Rose's Mill, Nelson Co., Virginia. 



Per Mo. Rel. no. 



cent wt. mos. Rel. no. feldspar mos. 



Si0 2 . 59-92 -r- 60 = 0-9987 2KAlSi 3 8 = 0-03117 



A1 2 3 -- 24-23 -=-102 = 0-2375 2NaAlSi s 8 =0-081 



Fe 2 3 0-29 CaAlsSisOa = -1137 



FeO 0-24 [Ca 3 P 2 O e = -0006] 



MgO 0-23 



CaO 6-47 



Na 2 5-03 



K 2 293 



56 = 0-1155 .-. NaAlSi 3 6 : CaAl 2 Si 2 8 

 62 = 0-081 2 : 14 = 10 : 7 



94 = 0-03117 



H a O(atllO°C.) 0-08 



H 2 (above 110° C.) 0-28 



COa trace 



Ti0 2 0-22 



P 2 5 0-09 +142 = 0-0006 



S trace 



MnO trace 



10001 



Pegmatite: = ,\ orthoclase — 17'337 per cent, plagioclase (Ab ]0 An,) 

 74-057 per cent. 



Composition of feldspar, aggregate : orthoclase 18 -96 per cent ; plagioclase 

 (Ab 10 An,) 81-04 per cent = 100-00. 



*Bull. 430-D. U. S. Geological Survey, p. 57. "The Virginia Rutile 

 Deposits," by T. L. Watson and S. Taber. 



