FMF - Friends of Minerals Forum, discussion and message board

[RayStraw]

I have a 11cm high broken azurite which shows internal growth of botryoidal malachite.

The rough chemistry requires 2 portions of solid azurite and one portion of liquid water to yield three portions of solid malachite and one portion of carbon dioxide (gas?). (The reaction yields 9% porosity as malachite is more dense than azurite.)

Is there an intermediate reaction that dissolves the azurite to permit in the formation of malachite? I assume the reaction is at the sub microscopic level.

Any comments would be appreciated including the pH of the dissolution.

[Peter Lemkin]

There are many papers that speak to all or part of your question. Here is one.
https://kipdf.com/stability-relations-of-malachite-and-azurite_5aee93577f8b9ac34f8b45b7.html
(link normalized by FMF)

I have quite a few azurite/malachite pseudomorphs and it is interesting how some have converted totally, others partially from one side toward the other and yet others that have patches of changes that seem to have no logic. I can see if they were bathed in a fluid in equilibrium unless there were areas of azurite with different properties of some kind that caused preferential attack and transformation. All interesting and complex. I have a book on pseudomorphs I'm sure went into this in detail, but it is packed away now, and I can't access it.

[Roger Warin]

Excellent debate.

[Daniel Garcia]

Hi all

You may find some answers and a general overview about the chemical aspects of mineral replacement reactions in the paper by A. Putnis.
Mineralogical Magazine, October 2002, Vol. 66(5), pp. 689–708.
Mineral replacement reactions: from macroscopic observations to microscopic mechanisms

The question you ask has many facets. In most cases, replacement proceeds by coupled dissolution and precipitation. How close to perfect pseudomorphism the result will be depends on conditions, of course. Putnis' paper gives a kind of rule of thumb for that point.
Concerning the appropriate pH for the conversion of azurite into malachite, there is very probably a range, not a single value, of P, T and fluid chemistries that may allow the reaction to proceed.

[Roger Warin]

I believe in abandoning the concepts of basic chemistry.
It is a chemical transformation in the solid state.
The crystalline edifice of azurite remains, but there are exchanges of atoms in the solid state by diffusion processes, which are obviously slow.
The proof (for me) is that the crystalline edifice of azurite remains and malachite settles on this lattice without any coincidence. Solid malachite micro-aggregates occupy sites inconsistent with the whole. Formally, there is no lattice of malachite, but that of the disappeared lattice of azurite remains.

Based on these considerations, the laws of chance guide the diffusion of atoms.
It would perhaps be appropriate to say whether coherent and homogeneous microdomains of malachite exist.
What is your opinion ?

[Jordi Fabre]

[Peter Lemkin]

[quote=]
Thank you for the reply.
I have the Vink paper but my chemistry training did not include orebodies. I will try to review the paper again.
Do you remember the author and/or title of the book you mentioned?

[/quote]
Yes, the Vink paper is pretty 'dense' with chemical formulas, but it seems to be regarded as the 'bible' on azurite <==> malachite chemistry. The book I was talking about is: Photo Atlas of Mineral Pseudomorphism 1st Ed. - J. Theo Kloprogge, Rob Lavinsky, Stretch Young. With the part on the chemistry and physics by Kloprogge. Sadly, the printing of the photos was not well done for the price. The photos were [I think] well done, but the printing process was not. The various colors were not put in register, and it gave a blurry effect to the images of otherwise fantastic specimens. Perhaps they corrected it in subsequent printings or I was the recepient of one of a few bad copies [although I complained to Elsevier, they sent me another free copy, but it was the same!]
It is also available as an e-book. https(:)//www(.)sciencedirect(.)com/science/article/pii/B9780128036747120018
The separate text part of the book is technical and quite comprehensive! If one advances through the chapters on this website, you can see the sub-topics that are covered! If one collects and likes pseudomorphs [as I do!], this book is a must-have!

Elsevier; 1st (18 August 2017)
English
290 pages
ISBN-10 978-0128036747
ISBN-13 978-0128036747
2.36 pounds [~ 1Kg]
8.5 x 0.8 x 10.9 'inches' [almost A4]

[RayStraw]

[Daniel Garcia]

"Superb reference"

Indeed, one of my top ten geoscience papers.
A copy is available upon request by MP.

Pseudomorphism, and more generally mineral replacement is a fascinating topic.
I don't know if it is of interest for this forum, but I can share some experimental examples of mineral replacement (under controlled conditions of course).

[Roger Warin]

Hello Daniel
I am obviously interested in additional information.
Can you give your opinion on the azurite – malachite transformation?
I assume that the phenomenon of pseudomorphosis is more common than we think.
Thanks.

[Daniel Garcia]

[James Catmur]

I tend to feel that when people add up all the terms ....morph they are fairly common. Epi, Pseudo, Poly, Para, Allo, etc.

I guess we are talking about the sub-type of an Alteration, not a Substitution or Incrustation?

[Peter Lemkin]

[Daniel Garcia]

Sorry, but Im not currently able to attach a pdf file to my personal messages....

[James Catmur]

One I collected back in 1983, from land being cleared so not a mine

[silvia]

[James Catmur]

It is the cast it made of the long-lost fluorite that I love - they were clearing land near the road so I had to stop and see what was turning up.

[Daniel Garcia]

[Roger Warin]

Hello Daniel,
Thank you very much for your nuanced answer.
Away from laboratories for a long time (which gives me the opportunity to reflect on the transformations of minerals and even their origin (since I love to interpret thin sections of meteorites) without future risks for my career!
I still meet professors of mineralogy or geochemistry, when I ask questions of this kind, only a vague idea is given, and often it is the thought of a guru which takes precedence. This has been written and recorded in scientific articles.
The advantage of age is that we no longer fear the wrath of the defenders of exact science.
To consider only the (irreversible) azurite-malachite transition, I only have my (optical) microscopes. Azurite is a sort of metastable chemical state that may or may not break depending on the fortuitous arrival of external agents, as anions.
As a collector of Tsumeb minerals, I have seen perfect azurites and other specimens completely transformed into malachite.
These are obviously not perimorphoses.
The perfect azurite thus undergoes chemical degradation.
In a 1st stage, I thought of a diffusion of atoms, but these processes are extremely slow (seen in meteorites).
For the azurite-malachite pseudomorphosis, it is indeed a chemical degradation (anionic) of probable origin of hydrothermal nature. It seems to me that we have to accept a temperature-pressure anomaly.
This transformation takes place in situ, in the host crystal: the copper remains in place and the anions are exchanged and replaced.
As only proof, I have only what I see. On fine specimens, it can be seen that the fine textures of the crystals (like streaks) of azurite can be preserved.
We are very far from the “pseudomorphosis of a nail into a totally rusty nail”. This trivial example shows the importance of the nature of the new molecule. Also, rust is an ill-defined mineral occupying a larger volume than the nail. At the limit, the initial shape of the nail disappears.
The volume occupied by the replacement mineral therefore affects this metamorphosis.
If the morphology of the azurite crystal is preserved, it is because the malachite can remain confined in the host crystal in the first approximation.
Here is my first thought. I can abandon it in favor of another scenario.

[Daniel Garcia]

Here is a thin section of a porous rock (mostly quartz) with pore filling azurite (blue) being partly converted to malachite (fibers). Looks similar to some of the Tsumeb specimens previously posted on this forum.