Chalcopyrite leaching followed by precipitation of other minerals

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Leaching of the mineral chalcopyrite (CuFeS2)which then is precipitated as a mixture of chrysocolla (light blue copper silicate) and malachite (green copper-carbonate-sulfate). The rust-brown in the picture is a precipitation of iron (oxide/hydroxide), i.e. another product of the chalcopyrite leaching reaction. The picture shows the south part of the open pit in Aitik (about -230 m, on the west side of the open pit). Without knowing for sure Roger Nordin believes it would be possible to show that at least some microbes are increasing the rate of oxidation. By courtesy of Roger Nordin
Leaching of the mineral chalcopyrite (CuFeS2)which then is precipitated as a mixture of chrysocolla (light blue copper silicate) and malachite (green copper-carbonate-sulfate). The rust-brown in the picture is a precipitation of iron (oxide/hydroxide), i.e. another product of the chalcopyrite leaching reaction. The picture shows the south part of the open pit in Aitik (about -230 m, on the west side of the open pit). Without knowing for sure Roger Nordin believes it would be possible to show that at least some microbes are increasing the rate of oxidation. By courtesy of Roger Nordin
Chrysokolla and perhaps some malachite. By courtesy of Torbjörn Kjellsson
Chrysokolla and perhaps some malachite. By courtesy of Torbjörn Kjellsson

Prerequisites for leaching (mobilisation) and precipitation (immobilization) of the mineral chalcopyrite (CuFeS2) are that the chalcopyrite is in close contact with pyrite which, in conjunction with rain/snow/fog may be oxidated (i.e. in air) and form diluted sulfuric acid which will leach copper from chalcopyrite and then precipitate it as chrysocolla or malachite.

Precipitation takes place as pH regains neutrality, that is when the pyrite has been diluted with enough rain/snow/fog on its way down the mountain (on condition that no more pyrite is oxidated on the way down). Precipitation may also be influenced by for example carbonate-streaks along the steep rock face which the solution passes. If it passes a calcite-streak the sulfuric acid will be neutralised and malachite can be formed. Air also contains carbon in the form of carbon dioxide (today about 400 ppm, and increasing because of the massive carbon combustion and the traffic), so ordinary oxidation with O2 + CO2, and also CO, may cause thin surfaces of malachite (and/or azurite - deep azur-blue carbonate mineral).

Pictures showing leaching, with or without microbes, look exactly the same in macro scale.

Reaction of leaching and precipitation

Chalcopyrite + Pyrite + O2 + H2O => chrysocolla / Malachite + Fe-oxid/hydroxid

Geology picture 2002, Sweden. By courtesy of Torbjörn Kjellsson
Geology picture 2002, Sweden. By courtesy of Torbjörn Kjellsson
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