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Stability of Azurite vs Malachite
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bob kerr




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PostPosted: Jun 10, 2019 09:30    Post subject: Re: Stability of Azurite vs Malachite  

John Betts wrote:

I believe these are actually "malachite ps. after azurite" that altered from the inside leaving the outer azurite surface unaltered.

Microscopic analysis of the thin azurite skin fails to reveal any growth faces/features along the thin edges where an azurite skin would be growing.
.


I too have a problem with the "inside out" interpretation - I posted the following photo of what I think is clearly azurite covering (partially here) mal after azurite. I can't see how the inside out argument fits this example.

it may indeed be the case that some milpillas pieces are formed "inside out" but from the many I've looked at this just doesn't seem to fit.

and - I continue to think that the final thin layer of azurite is so easily chipped off and examples are so common - many are sold as natural - see examples in the thread mentioned above. caveat emptor

bob



t_interface_1_of_example_3_152.jpg
 Mineral: Azurite and Malachite - natural interface
 Locality:
Milpillas Mine, Cuitaca, Municipio Santa Cruz, Sonora, Mexico
 Description:
 Viewed:  261 Time(s)

t_interface_1_of_example_3_152.jpg


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RayStraw




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PostPosted: Jun 12, 2019 11:31    Post subject: Re: Stability of Azurite vs Malachite  

Bob:

I believe that the reflections in your picture are indicative of the original azurite surface but portions removed due to the pseudo morphing process. I believe it is very unlikely that isolated azurites can grow on a rough, malachite pseudomorph surface in identical orientations to each other.

I use widespread reflections on partial pseudomorphs of azurite to malachite to identify the extent of the original azurite crystal(s).

Does anyone see how this type of oriented azurite growth on a rough malachite surface can happen?

Best regards,
Ray Straw
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bob kerr




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PostPosted: Jun 13, 2019 10:33    Post subject: Re: Stability of Azurite vs Malachite  

RayStraw wrote:
Bob:

I believe that the reflections in your picture are indicative of the original azurite surface but portions removed due to the pseudo morphing process. I believe it is very unlikely that isolated azurites can grow on a rough, malachite pseudomorph surface in identical orientations to each other.

Ray Straw


Well, I will certainly defer to more experienced and knowledgeable people - a few items though:

- I personally don't see why there is a problem with azurite reforming on a rough malachite pseudo surface. as a matter of fact, it seems the rough surface provides more nucleation points

- in the upper middle of the photo above, you can see the one azurite face that is perpendicular to both the rough pseudo surface below and the upper surface of the azurite above. how could the "inside out" argument explain this. to me this simply looks like an azurite xl formed right on top of the malachite.

- how can the inside out argument explain the consistent thickness of the thin azurite coatings over the entire specimen? wouldn't "inside out growth" result in much more variable thicknesses?

- what are your thoughts on those specimens that I think show the thin azurite coating chipped of exposing the malachite pseudo? damaged or natural? (irrespective of formation process.)

thanks,
bob
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Peter Megaw
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PostPosted: Jun 13, 2019 11:46    Post subject: Re: Stability of Azurite vs Malachite  

I regret I did not have a chance to see Ray's talk at Rochester, although he has shared a copy of his Powerpoint with me and his argument seems coherent for the examples where there are side-by side external domains of malachite and azurite. However, there are electric azurites from Milpillas that have complete outer layers of azurite, with malachite only showing on the detachment surface. This leaves me struggling with a mechanism whereby the malachite-pseudomorphing fluids get into the core of a solid, coherent azurite crystal. The likeliest would be fluids moving through the wallrocks (porous gossan?), or along the interface between the wallrocks and the base of the azurite crystals and eating them out from the center.

It is hard to reconcile this model with what we know of the environment of azurite growth at Milpillas. First, the azurite/malachite lines the walls of narrow open fractures. In this case any supergene fluids would most likely fill and move within the open space. Second, the wallrock matrix at Milpillas is mostly clay-altered volcanic rocks, with no sign of spongy porosity to allow fluids to migrate through them, or along the interface with the base of the crystals. On the contrary, given the amount of clay alteration, which we can tell is pre-azurite because it commonly forms inclusions in the azurite, it seems the matrix would be largely impermeable. This makes it hard to see how the malachite-stage fluids could get to the cores of the azurite without affecting the lustrous outer layer. I think that the mechanism has to fit both the situations where the outer layer is complete and lustrous, partially "breached" (ie the tips at Milpillas) or side by side partial.

I think the KISS principle suggest that post-malachite azurite growth is more likely than an inside-out process that coincidentally leaves little oriented domains of the original species untouched. This does not invalidate the mechanism where the malachite altering process proceeds from one side to the other (outside in) but in those cases I am not convinced there is evidence for a final lustrous outside shell of untouched azurite).

Finally, I have to agree with Bob...I see no reason why azurite can't crystallize in an "organized" oriented manner on a rough malachite surface. I have a number of fluorites with an early growth stage is covered by a thick (and rough surfaced) layer of a different species (quartz or calcite or a sulfide) with a later growth stage of fluorite top of that coating...in crystallographic co-orientation with the first fluorite stage. Despite the thick coating the later stage of fluorite is somehow connected to the earlier one enough to share the same crystallographic orientation. Is it possible that there may still be an azurite core to these malachite crystals and the last layer may be grown in continuity with that?

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