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[dontgogreen]

Please help me understand this specimen from Milpillas. Malachite can be seen underneath a thin azurite coating around some of the edges of the specimen. As I comprehend it (please correct me) the sequence of events is:

1. Formation of azurite crystals under relatively low-pH & high carbonate activity conditions.

2. Replacement of azurite by malachite as conditions shifted to higher pH.

3. Deposition of a final layer of azurite at a point when conditions reverted to those of step 1.

Is this the correct that azurite was the original mineral? Were these crystals completely replaced by malachite throughout their entire volume before being coated by azurite again?

The information about pH and carbonate activity was obtained from Vink, 1986, Stability relations of azurite and malachite, Mineralogical Magazine vol. 50, pp. 41-7
( https://www.minersoc.org/pages/Archive-MM/Volume_50/50-355-41.pdf )

To be honest the chemistry discussion in this paper goes over my head. What exactly is meant by the "activity" of the carbonate ion?

Is the reversal of conditions to produce azurite>malachite>azurite very rare? There are plenty of malachite pseudomorph after azurite specimens, but do we see this sequence anywhere besides Milpillas?

Lastly, any suggestions for photographing this specimen?

[Les Presmyk]

This series of 2 and 3 levels of pseudomorphism in azurite and malachite is not unusual or even uncommon. There are many examples from Ajo and Bisbee, Arizona as well. I cannot respond to the chemistry and environment questions but through observation it is generally when one sees larger crystals of azurite displaying a recognizable blue (and not almost black) that you can assume it has gone through this series of alternating layers of malachite and azurite pseudomorphing.
Then when you see chrysocolla replacing azurite, all of the specimens I have seen have first been replaced by malachite and then the change to chrysocolla.
Hope this helps.