FMF - Friends of Minerals Forum, discussion and message board

[Peter Megaw]

Especially if you can give it 20 or 30 million years to oscillate in...

[Peter Lemkin]

I fully realize this is focused on Azurite to Malachite and if the reverse it possible...but looking in my Photo Atlas of Mineral Pseudomorphism [Kloprogge & Lavinsky], I note there are also ps of cuprite, gypsum, smithsonite and of course malachite after azurite. Also listed are ps of cobaltian calcite, baryte and azurite after malachite. The first half of the book is about chemical mechanisms, but sadly doesn't have an index to that portion of the book.

[bob kerr]

[John Betts]

The interpretation of the thin-skinned azurite over malachite is interpreted as "azurite over malachite ps. after azurite."

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.

This was presented by Ray Straw two years ago at the technical sessions at the Rochester Mineralogical Symposium.

[Peter Megaw]

There may be more than one process going on here.

If you look at the picture of the large group I posted previously ( you can get there readily by following the link in Bob's comment) the tips of the crystals show malachite ps azurite peeking out from under the azurite skin. These are not broken tips, the azurite is crystallized fully with little faces pointing down towards the underlying stage. I do not think this relationship is compatible with an inside out process that left the outer azurite sline.

[bob kerr]

[RayStraw]

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

[bob kerr]

[Peter Megaw]

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?