50 PROCEEDINGS OF THE ACADEMY OF [1889. 



ON TWO MINERALS FROM DELAWARE COUNTY, PA. 

 BY F. A. GENTH. 

 Gahnite. 



In the summer of 1887, Mr. John H. Smedley observed in his 

 feldspar quarry, in Delaware County, some dark green crystals, 

 partly imbedded in muscovite and associated with quartz, albite and 

 garnet. 



I am indebted to Col. Joseph Willcox and Dr. John M. Cardeza 

 for a few of these which furnished the material for an analysis prov- 

 ing them to be Gahnite. 



The crystals, from 2 to 15 mm in size, are mostly very imperfect; 

 only the smallest show smooth faces of the octahedron in combina- 

 tion with the dodecahedron ; generally their faces are rough and 

 frequently show impressions from the associated minerals. The 

 larger crystals are much distorted, the dodecahedral planes some- 

 times deeply striated, producing, with the octahedral, rounded, rag- 

 ged faces upon which, occasionally, minute octahedral planes are 

 visible. In some of the crystals an indistinct octahedral cleavage and 

 frequent interlaminations with scales of muscovite may be observed. 

 A few have been partly altered into muscovite and are coated with 

 a thin fibrous crust of this mineral, in a similar manner as the gah- 

 nite from the Deake Mine, Mitchell County, N. C, described by me 

 in Am. Phil. Soc. Proc, August 18, 1882. 



Their color is blackish-green, in thin splinters translucent with 

 bright dark-green color. 



The crystals are so much mixed with muscovite that only with 

 great difficulty and the aid of a strong lens, a small quantity of pure 

 material could be obtained for analysis. 



Spec. Grav. = 4.587. 



The analysis gave : 



AP O 3 = 57.22 

 Zn O = 38.14 

 Cu O = 0.06 

 Mn O = 0.70 

 Fe O = 3.55 

 Mg O = 0.26 



99.93 



