Hope this is not too technical!

Glad you understood it Carnkie. Analyses were by inductively coupled plasma optical emission spectroscopy and ion chromatography, backed up by fourier transform infra-red spectrophotometry, X-ray powder diffraction and classical gravimetric methods. I also studied the water vapour sorption isotherms to gather data on the internal porosity of the material and found it to be a mixed micro/meso porous material. By careful calculation of the stoichiometries I was able to prove that some of the copper is unaccounted for by the analysed anions and so must be bound to the silica surface which is consistent with literature data on the coagulation of silica sols by the Cu(II) cation. I'll not go into detail though.

Azurite is blue, and requires too high a concentration of carbon dioxide to form in old mines.

Azurite only forms at much higher CO2 concentrations (CO2 dissolves in water and generates HCO3). When in contact with normal air water cannot dissolve enough CO2 to get the HCO3 concentration high enough for azurite. You need very high concentrations, which you will not get in a ventilated mine but may get within the natural, confiined cracks and pockets in the orebody, especially if in a carbonate-rich environment (eg. limestone).
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I’m curious as to what you consider a ventilated mine Stephen. Many Cornish mines were extremely poorly ventilated. This from the PP 1864 ‘On the conditions of all miners’.
There can be no doubt that the air of the mines, especially in the deeper workings, and at considerable distance from the shafts, becomes extremely impure from the very imperfect ventilation, the deteriorating influence of the respiration of the men at work, and the combustion of the endless candles which they burn. The cause seems indeed to be the most important in determining the unhealthy conditions of the miners. The ventilation generally becomes more difficult and defective with the increasing depth of the mines, and the state of the miners is therefore found most unsatisfactory in the districts where the mines have attained the greatest depth. The ventilation is, however, most seriously interfered with by the distance of the levels from a shaft, and several of the men complained much of the character of the air they had breathed in workings which were very near the surface,
Ergo; in many areas of the mines ventilation was virtually non-existent.

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The past is a foreign country: they do things differently there.

Hi Adam. It's all down to hydrogen carbonate ion (HCO3 one minus charge - can't do sub and superscripts here) activity, pH and stability fields. Malachite is easily formed under commonly encountered conditions, so you find it almost anywhere there's copper and not too much acid. Azurite only forms at much higher CO2 concentrations (CO2 dissolves in water and generates HCO3). When in contact with normal air water cannot dissolve enough CO2 to get the HCO3 concentration high enough for azurite. You need very high concentrations, which you will not get in a ventilated mine but may get within the natural, confiined cracks and pockets in the orebody, especially if in a carbonate-rich environment (eg. limestone).
The most interesting environments for post-mining copper minerals are pyrite-poor deposits in siliceous rocks. In limestone there's too much carbonate and only malachite forms. If too much pyrite (like at Avoca and Anglesey) it is too acidic so you only get transient sulphates like chalcanthite and maybe antlerite, plus metallic copper replacing the iron rails (great examples of this in 850 level, Avoca). In quartz-chalcopyrite veins, with little or no pyrite (eg. Coniston, and many Irish and Cornish mines) the balance is just right for a range of minerals. Malachite, langite, brochantite and the soft squishy blue & green silica gels (which are not true minerals) are the usual ones. Rarely devilline, and if near the sea (source of chloride) atacamite and connellite.
Langite is a greenish blue, and may be mistaken for azurite, but azurite is a deeper, purer blue and fizzes in acid. Langite does not fizz in acid. As langite is quite a common crystalline encrustation on copper mine walls I wonder if that was what you found?