Bornite

Bornite is uncommon Franklin, though some specimens of chalcopyrite seem to contain minute grains of darker-colored bornite. The following analysis of a specimen from Franklin proves the presence of bornite mixed with sphalerite.

**Analysis of mixture of bornite and sphalerite**
(L. H. Bauer (243), analyst)
	Percent	Molecular ratio	Ratio deducted (sphalerite)		Molecular ratio of remainder (bornite)
Cu	38.16	0.600			0.600 = 5 x 0.120
Zn	22.17	0.338	0.338*
Fe	10.41	0.186	0.066*	0.419	0.120 = 1 x 0.120
Mn	0.80	0.015	0.015*
SiO₂	0.60	0.010	0.010	0.010
S	28.38	0.885	0.409	0.409	0.476 = 4 x 0.119
	100.52

[* Figures reflected in the value 0.419 shown at right]

In the specimen analyzed sphalerite was visible as abundant dark-brown grains and was assumed to be the source of the zinc and manganese and part of the iron found. Computation based on such assumption shows that about 40 percent of the material was sphalerite. Deducting and the equivalent of the SiO₂ found, the molecular ratio of the remainder gives exactly the bornite formula: Cu₅FeS₄. Many specimens of this bornite, in part almost pure, were found in the Stanton collection.


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