RE-EXAMINATION OF AMERICAN MINERALS. 123 



green in mass, and pale yellowish -green by transmitted light. 

 Hardness 2-2.5 ; specific gravity 2.80. The laminae are en- 

 tirely devoid of elasticity. It has been called pyrophyllite by 

 some mineral collectors, but upon what grounds we are igno- 

 rant, as it does not possess the remarkable property of exfo- 

 liating and swelling up by heat, so peculiar to pyrophyllite. 

 Analysis shows its composition to be identical with biotite. 



1 2 



Silica 39.62 39.49 



Alumina 17.35 17.06 



Peroxide of iron 5.40 5.21 



Magnesia 23.85 23.65 



Potash 8.95 



Soda 1.01 



Water 1.41 



Fluorine 1.20 



Chlorine . .27 



99.06 



Analysis 1 gives oxygen ratio £ 11.22 : & 9.73: Si 20.58, or 1: 1:2; 

 and the same formula as for the mineral last mentioned, 

 S 3 Si-f B Si- 



The specimens examined were received from Mr. Silas E. 

 Horton, of Craigville, New York. 



17. Margarodite. 



This mineral occurs at Lane's Mine, Monroe, Conn. It has 

 been analyzed by W. H. Brewer,* but owing to some impurities 

 in his specimens he obtained an excess of silica. 



Specimens very carefully selected, to avoid the fluor spar 

 and other minerals with which it is associated, gave 



1 2 



Silica 46.50 45.70 



Alumina 33.91 33.76 



Peroxide of iron 2.69 3.11 



Magnesia 90 1.15 



Potash 7.32 7.49 



Soda 2.70 2.85 



Water 4.63 4.90 



Fluorine 82 .82 



Chlorine 31 .31 



99.78 100.09 



* Dana's Mineralogy, third edition, p. 359. 



