FMF - Friends of Minerals Forum, discussion and message board

[RayStraw]

I am still working on my paper which attempts to explain how malachite replaces azurite to form a pseudomorph where the azurite shape and faces are present but only malachite remains.

1. What I need is some partial pseudomorphs with broken azurite terminations. Pictures would be fine or any damaged specimen donations appreciated.

2. I would also like a complete pseudomorph to malachite from Ajo Arizona. These often are pseudo cubic. What I need is one where the pseudo is broken say at the edge of the specimen. Pictures are fine.

Thanks for any help.

Ray Straw
rsplinters(@)gmail.copm

[Kevin Schofield]

[Linus]

Ray, here is a favorite of mine from Milpillas...

[Roger Warin]

Hello,

I wrote an article on pseudomorphosis in my club magazine AGABminibul.
I envision many examples including obviously malachite after azurite.
You should know that this is a new chemical equilibrium which is being established, replacing the azurite material by malachite, without destroying the morphology of the crystal. Obviously, there is no longer any crystal lattice in this pseudo-crystal.
This reaction depends on the partial pressure of CO2. This evolution towards a new chemical equilibrium occurs especially in the arid zones.
But there is a problem, the text of my article is in French, but the photos are in Esperanto !
Google translation can help easily, I think.
If you are interested, I send you the pdf.
I remain at your disposal.
Roger.

[Roger Warin]

Here is a totally metamorphosed azurite crystal and another partially.
Roger.

[Jordi Fabre]

[RayStraw]

Roger:

The partial pseudomorph would be ideal for my purposes which will involve cutting sections of the crystal.

Please advise specimen dimensions.

Thank you,

Ray

rsplinters(@)gmail dot com

[Roger Warin]

Hi Ray,

Another fine example of pseudomorphism is green malachite after blue azurite, especially since often the crystals found have only been partially degraded. In this case, it is no longer an oxidation-reduction alteration which intervenes but another process completely unexpected for the amateur.

Malachite Cu2 (CO3) (OH)2 is an insoluble hydroxylated copper carbonate. It is close to the azurite Cu3 (CO3)2 (OH)2. It is the partial pressure of CO2 that conditions the appearance of one or the other of these minerals. In geological conditions, azurite is also not stable if the partial pressure of CO2 decreases. Pseudomorphoses of malachite are then formed because the malachite occupies a smaller volume, thus retaining the shape of the azurite crystal broken down atom by atom. As the immediate comparison of unit cells (different Z) is not possible, here is the explanation for the strong in theme.

Pseudomorph can be total or partial as frequently in Tsumeb (Namibia). This last example shows a chipped crystal on which we can clearly see the conchoidal fracture. In addition, the brackets emphasize the similarity of the streaks in the crystal before and after pseudomorph. The crystal morphology has really not changed.

The decrease in molecular volume allows the subsequent slow reaction without any breaking of the father-crystal (azurite) since the daughter (malachite) is smaller. This chemical reaction only depends on the partial pressure of CO2. A tropical climate seems favorable (temperature) to the formation of azurite. Cold geographic areas may slow the phenomenon of pseudomorphism.

The microcavities of pseudomorphs result from the absence of a lattice and the decrease in the volume of matter. Malachite is microcrystalline.
It is for this reason that in Nature, the reaction is irreversible. Azurite is never regenerated.
Here are the comparisons of these two mineralogical species:

(see the chemical reactions)

With translations:
Maille élémentaire = unit cell.

The alteration involves the replacement of part of the carbon dioxide (CO2) of the azurite molecule by water (H2O), in the ratio of 1 : 1.
We can write the azurite degradation reaction like this:

The dependence of the partial pressure on CO2 is obvious, a low partial pressure favoring the reaction when the CO2 is eliminated. It is conditioned by pH and temperature.

Roger.

[basti]

Here is a historic azurite-malachite specimen from Moravske Zemske Muzeum in Brno, Czech Republic.

I saw some partial pseudomorphs from Kerrouchen, Morocco - these could be available and not too expensive, some were even below $10 price tag.

[Peter Lemkin]

If it is due to the partial pressure of CO2 [and I don't question that], how does one explain the partial and patterned alteration. I have some azurite xx that partly altered to malachite in the most apparently random and even un-connected patterns and portions. I assume it must have to do with crystal defects, impurities or some such.......

[Roger Warin]

Hi Peter,
I misspoke. It is not the gaseous component of the atmosphere that controls this equilibrium, but rather the concentrations of CO2 gas dissolved in hydrothermal waters, under geological conditions.
Of course I made the same observations as you.
I suppose that hydrothermal waters find defects in the structure (the lattice) of the azurite crystal and several different and concomitant alterations can then occur. So I have the same conclusion as you.

[RayStraw]

Thanks everyone for your interpretations of azurite to malachite pseudomorphs.

All the information will be distilled and included in my paper.


Best,
Ray Straw

[RayStraw]

Roger:

This Tsumeb pseudomorph appears to be about 3cm maximum. Is that correct?

Is it still in your collection?

Best,
Ray Straw

[bob kerr]

Ray - this is probably beyond the scope of your study but there was an azurite specimen displayed at the TGMS Show that appears to be azurite after azurite and the second generation of azurite only was a thin coating on the outside of the primary azurite (???). or - was this one form of azurite morphing to another form "from the inside out"???

photo is attached and also, here's a posting I placed on Mindat where the general consensus is that this is a damaged or wall contacted specimen (which I doubt) but the two significantly different colors was not explained. here's the mindat link:
https://www.mindat.org/mesg-505011.html

if this is beyond what you want to study then - "never mind".
bob

[RayStraw]

This specimen from Roger Warin is just what I need for my azurite to malachite pseudomorph research (Malachite-psm-aurite_R.jpg):
Azurite crystal with chipped termination and malachite pseudomorph away from the termination but visible underneath the azurite. Size is approximately 3 cm.
Since I will be cutting up the crystal through the thickness in the length direction, a damaged crystal is desirable.

Roger's specimen is not available.


Does anyone have any like this that will donate the specimen for my research?

Ray Straw
rsplinters(@)gmail dot com