X-ray photoelectron spectroscopic investigation of the tarnishing of bornite

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Abstract
Changes in the surface composition of natural bornite after prolonged exposure to the atmosphere have been studied by X-ray photoelectron spectroscopy. Freshly cleaved surfaces oxidize rapidly, producing a hydrated iron oxide and a copper sulfide; as tarnishing proceeds, the iron oxide segregates to the surface forming an over-layer of uneven thickness on the copper sulfide. The tarnish is usually close to its limiting thickness after about two days and its thickness and composition are such that a purple colour is produced. Although surfaces often maintain this appearance for at least a year, blue patches may develop after between one and two days of exposure. Blue surfaces have also been observed when the mineral is cleaved along an existing interface. Copper(II) has been found to be present at all blue surfaces investigated. Silver, which exists as a minor element in the mineral, becomes concentrated in the tarnish layer where it is present in the copper sulfide component.

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