COMMUNICATIONS FROM THE HARVARD MINERALOGICAL 



MUSEUM. 



OCCURRENCE OF NATIVE COPPER AT FRANKLIN 

 FURNACE, NEW JERSEY. 



By J. E. Wolff. 



Presented April 13, 1898. 



In 1897 a large specimen of ore containing native copper from the 

 new (Parker) shaft at North Mine Hill, Franklin Furnace, was ac- 

 quired for the Harvard Mineralogical Museum. 



The specimen, some seven inches square, is broken flat parallel to the 

 banding of the ore body, and is composed of a central band of apple- 

 green willemite interbanded with coarse white calcite and smaller masses 

 of franklinite and zincite in the usual association. The cleavaaje sur- 

 faces of the calcite masses are curved by pressure, and show the develop- 

 ment of the gliding planes parallel to — ^ R. In the central zone of the 

 specimen these evidences of pressure become intensified, and small 

 slickensided surfaces are developed in the willemite and calcite, while 

 the former mineral is irregularly cracked in addition to the develop- 

 ment of the imperfect cleavage. In this zone native copper occurs, 

 forming a band half an inch wide running parallel to the banding of the 

 ore. The copper has no distinct crystalline form, but occurs as thin 

 films or thicker sheets filling the cracks in the willemite and occasion- 

 ally in the calcite. Except for traces of oxidation on its surface, the 

 copper is pure and associated with no other mineral. 



Of two other specimens loaned by Mr. J. A. Van Mater, superin- 

 tendent at North Mine Hill, one has the same association described 

 above, and shows a veinlet of copper penetrating calcite along a gliding 

 plane, in addition to the penetration along fissures in the willemite. In 

 the other the copper occurs in .willemite which is associated with yellow 

 garnet, black biotite, and calcite. The masses of copper are thicker 

 here, and a projecting point laid bare by the removal of the matrix 

 has some resemblance to a spear-head twin of copper, but is probably 

 a matrix mould. 



