FMF - Friends of Minerals Forum, discussion and message board

[Roger Warin]

Thank you Daniel for this informative photo.
I presume the observation was made in polarized light.
What would cross-polarized vision look like?
Can I say that the area on the left is an azurite crystal already partially transformed into malachite?
You are very lucky to have TS of different rocks.
I will soon give my macroscopic photos which take into account the external morphology.

[Daniel Garcia]

Thanks Roger

Here is the same view under normal light.
Azurite is still blue. It seems to be one single crystal, that inherits its shape from the pore geometry. Some other pores in the same section are filled with azurite only, and some others with boitroidal malachite. This one is the only one where the replacement is "ongoing"

[Roger Warin]

Hi Daniel and all lovers of pseudomorphs,
Still about my malachite pseudomorph after azurite.
A general macrophoto shows the appearance of this specimen.

This old name corresponds well to the destructive sequence of azurite in malachite. The time scale is long under normal conditions.
This phenomenon corresponds to a natural alteration of the initial azurite (unit cell vol.= 301.97 A³) into malachite (unit cell vol.= 364.35 A³).
It was taught to me in the 1950s (and certainly before) but without a description of the chemical process.
To highlight this process, I have chosen a strange little sample of Tsumeb, although it takes faith to observe it!
Azurite-malachite#1-02-A_RIn this view (FOV=8 mm), I have the impression of seeing the initial shape of the azurite crystal continue downwards, with however a gradual increase in volume.

Even in the region of the dotted lines, one notes (it seems to me) a certain differentiation in the granulation of the grains of malachite, over an elongation of approximately 2 mm at the most.
I remain convinced that this pseudomorphosis is the result of an intimate transformation of the initial mineral, without the presence in the solid state of liquid as such, but perhaps of coordinated water molecules. This is not a solution. The transition takes place, inside the crystal, even if isolated water molecules can act as ligands for the copper cations. However, the azurite crystal has been found in hydrothermal situations.
In a museum, partially azurite remains stable, in the Tsumeb mine not, depending on the circumstances.
I don't know if you can compare this process to the progressive rusting of a nail.
Then in a second time, this association served as the seed for a second generation of fibrous malachite under usual conditions.
This habitus of secondary malachite could perhaps specify the physicochemical conditions of the transition. What is odd is that this "dry rot" did not attack the rest of the azurite crystal.

[Daniel Garcia]

Thanks Roger, for this nice "macro" case.
It seems to me that in the dotted area of your last picture, the termination of the malachite cristals simply mimic the striated outer shape of the former azurite.

[Cesar M. Salvan]

[Roger Warin]

Really thank you for this demonstration.

[Cesar M. Salvan]

Following this little game. I changed slightly the calculations: I introduced a different, more actualized set of thermodynamic data, and also introduced sulfur species and copper mobile complexes.

To show it to you more practical and interesting, I prepared a sequence of plots, from reducing conditions to oxidant conditions. You can see the mineral sequence generated, which is very familiar to you all.

As you can see, the formation of carbonates is important and depends on the CO₂ concentration. To keep azurite stable, a pure CO₂ atmosphere is necessary. So, unless you store your azurite specimens in pure CO₂, expect the slow transformation into malachite, which will fade the blue of your favorite azurites, giving it green tinges. This transformation is quite slow, but noticeable in a lifetime. Sunlight and heat increase the rate of the phase transition.

These schemes explain (very simplified) the most common copper mineral associations and sequences you find in your specimens. For example, it explains why brochantite is a common sulfate species, and antlerite is way less common. Also, to form antlerite a higher copper concentration is necessary. Overall, malachite occupies a central position and the most stable in the majority of conditions. This explains why it is very common.

"Cesar"-someone could ask-"in your plots, tenorite, and not malachite, is the thermodynamically favored species in normal atmosphere, whose log fCO₂ is around -6. Why malachite does not turn black so easily as azurite turns green?". This could be explained by the additional stability of the malachite structure by Jahn-Teller effect, which is very important in copper coordination. To blacken the malachite more heat is necessary, to expel water and CO₂, and break coordination octahedra.

[Daniel Garcia]

Hi everybody,

Thanks, Cesar, for this expanded picture of the stability relationships in the H₂O O₂ CO₂ SO₄ CuO system. I have two questions and a suggestion.
(1) Which is the thermodynamic database you used for these diagrams?
(2) Did somebody study the geometry of the replacement front between azurite and malachite in old pigments? Does it look like a corona as in common corrosion cases?
(3) and since the conversion of azurite to malachite at constant Cu involves the addition of water while removing some CO₂, why not simply keeping our nice azurites in dried air to prevent greening...

[RayStraw]

This is very interesting.

Please advise the locality.

Ray