120 RE-EXAMINATION OF AMERICAN MINERALS. 



It will be seen at a glance that the only difference between 

 this mineral and orthoclase is the large amount of soda, and in 

 analysis 1 and 2 a small amount of lime, this last, most of which 

 is doubtless an impurity, alters somewhat the oxygen rates. 



No. 1 gives , 1 : 3.10: 11 



No. 2 •• 1:3:10.60 



No. 3 •• 1:2.90:11.08 



No. 4 •• 1:2.74:10.83 



This slight difference in the ratio (produced by the presence 

 of a considerable amount of soda) is not uncommon in ortho- 

 clase. In that from Hohenhagen, Schnedermann found 4.15 

 potash and 7.53 soda; the flesh-red feldspar from Bathurst, 

 Canada, gave Hunt 6.36 potash, 5.37 soda; and G-melin found 

 in the feldspar from Laurvig 6.55 potash and 6.14 soda, and in 

 that from Fredicksvarn 7.03 potash and 7.08 soda. These 

 numbers affect to some slight extent the oxygen ratio, but the 

 correspondence of the minerals in physical characters denotes 

 their identity with orthoclase. Moreover, the identity of loxo- 

 clase with orthoclase is made obvious when we take the ratio 

 between the silica and alumina, which in the purer varieties 

 (analyses 3 and 4) is as 4 : 1, and analysis 4 gives the ratio 

 12:3.04:1.11, or it Si + £ Si 3 . 



The specimens examined were received from Professor Sil- 

 liman, jr., and Mr. Samuel W. Johnson. 



13. D anbury Feldspars: 1. Oligoclase; 2. Orthoclase. 



1. Oligoclase. — The feldspar in which the danburite occurs 

 has so strong a resemblance to the oligoclase from Sweden that 

 we have been led to analyze it; the results of our examination 

 prove its identity with that species. The analyses gave 



1 2 



Silica 64.03 63.50 



Alumina 22.37 22.75 



Peroxide of iron trace trace 



Lime 2.91 ' 3.28 



Magnesia trace trace 



Soda 10.06 9.37 



Potash 60 .50 



Ignition 30 .21 



100.27 99.61 



These give the oxygen ratio 1:3:9 and the formula R Si-f-^tl Si 2 , which 

 are the ratio and formula for oligoclase. • 



