FMF - Friends of Minerals Forum, discussion and message board

[RayStraw]

The attached picture shows botryoidal malachite nucleated on the side of a 7cm azurite crystal from the Tsumeb mine, Otavi Highlands, Namibia (Ray Straw specimen, Jeff Scovil photograph).
In a recent paper Smallwood and Cairncross suggested that the overall reaction is as follows:
Two azurite portions plus water yield three malachite portions plus carbon dioxide (for the same amount of copper).
.
2*Cu₃(CO₃)₂(OH)₂(c)+ H₂O(l)
= 3*Cu₂(CO₃)(OH)₂(c)+ CO₂(g)

(c stands for crystal, l for liquid, and g for gas)

I believe there may be some intermediate reactions that would allow local movement of copper or copper ions. A crystal to crystal reaction would probably be very slow
Any ideas?

[James Catmur]

Ray

I quick search on azurite+malachite shows your interest in this area. Let's see if this question helps. Not my area, so I cannot help

James

[lluis]

Hi, Ray

It is the reaction, known from before I was still young.
Or better said, more or less... Water, that you place as liquid, works also as humidity in air... So, no necessary liquid.
Azurite is only stable at, if I remember well, 5 atmospheres CO₂ pressure. Taking in account that we are at 1 atmosphere pressure, total, not only CO₂, well, it is unstable in atmospheric conditions...
So, the reaction you place occurs, at the rate it is possible.

For crystal to crystal, just take in mind that any place that is more sensitive in structure would begin the change.... that will go till it could depending on humidity available...

Maybe not too clear, but just it is as it go... More stable phases coming for less stable phases... Thermodynamics... :-)

With best wishes

Lluís

[Roger Warin]

Totally agree, Lluis,
The attack is done at the level of the unit cell and the surrounding gas.
I think that the writing of equations remains purely formal.
The degradation reactions take place step by step, from molecule to molecule under the influence of acid H₂O (gas) molecules (with CO₂), in the solid state like rust devours iron. The active agent could be a very small wet reactive aggregate (superficially absorbed liquid) which would make sense of the chemical equation.
Copper is a transition element that readily forms organometallic complexes. In a 1st stage, H₂CO₃ can behave like a copper ligand.
As with viruses, humidity and temperature must be important in the kinetics of the process.
I think so. It's just a suggestion.
Roger.

[James Catmur]

So it is similar to the way rust spreads across steel? Interesting

[lluis]

Hi, James, Roger, List

Well, iron is sensitive to humidity (it is under hydrogen, so, in water, it reduces cation hydrogen and goes to Fe²⁺, that is later oxidized to Fe³⁺)

Not the same mechanism as in azurite to malachite (azurite is just metastable in atmospheric conditions; Iron is less noble than hydrogen. Similar, but not the same (to my mind... and I think it is what is said in chemistry. At least to me. But taking in account that there are as many schools as you wish.... :-( )
Steel is a generic. There so many steels as you wish. Inoxidable steel is also iron, so in principle sensitive to water, but there is a mechanism due to composition that makes corrosion take much more slowly (unless you place there a complexant like chlorides, that accelerate decomposition.... Except for maritime iron, that is high in Titanium, and titanium, if memory serves me well, is the only metal that resists corrosion by chlorides.

Sorry for long wording

With best wishes

Lluís

[Roger Warin]

Thanks Luis,
I looked at the unit cell structures of azurite and malachite (AMS Data Base).
If the unit cell of azurite seems more symmetrical to me (both minerals are monoclinic), the unit cell of malachite surprised me, in particular at the level of the CO₃ groups. Azurite crystals are also better formed with a rich morphology.
In malachite, each Cu atom is surrounded by 2 nearest oxygens and 2 (OH).
Cu is square planar. Thus plane Cu alignments are formed and the Cu octahedra are very deformed.
I would like to understand this pseudomorphosis which always begins on the surface of the crystal.
The comparison with rust stops there because it is not a mineral.

[lluis]

Hi, Roger

Transformation of azurite into malachite always should take place in surface..just because water is needed, and water reaching insides without touching surfaces, well, impossible...
In the remote case that water could be inside the azurite, then CO₂ should be released, and pressure would be over the 5 atmospheres and azurite would be stable....
Yo my mind, maybe too simple, is easy: simply possibilities (and think that azurite is formed in water... Just at pressures of CO₂ over the 5 atmospheres.....

I hope it may serve...

For structure., it is like diamond and graphite. One is cubic, the other hexagonal/trigonal.
symmetries are different... Nevertheless, graphite is metastable in ambient conditions and when done what is needed, it becomes graphite (I think to remember that heating till a temperature...)
Or the tin, forms Beta and Alfa. Form beta is malleable, and is how is usually known. Its crystalline structure are tetrahedrons. But under 13,2ºC, it becomes tin alfa, with structure based in cubes... and is pulverulent....
In Austria it is said that there is a big trouble with some monuments done in tin (as far as I remember the burial of Empress Maria Theresia...)

With best wishes

Lluís

P.D.: I would like to know as many crystallography as you know! Chemistry is so easy in comparison....

[RayStraw]

Thanks for all the insight.

I trying to understand how the pseudomorph grows with the azurite crystal.

Perhaps the first step is dissolution of the azurite near the pseudorph with acid. Then the malachite forms next. This would presume that the chemistry at the azujrite-psudomorph is different than the chemistry on the outside surface of the azurite.

Does this make any sense?

[Bob Morgan]

This discussion is largely beyond my knowledge particularly of chemistry. The acicular growth of malachite within a euhedral azurite crystal seems to me to require dissolution of the azurite. Perhaps that's already recognized in the previous discussion about the role of water. But somewhere there has to be a transfer of material to a dispersion fed growth of malachite.