FMF - Friends of Minerals Forum, discussion and message board

[bob kerr]

In Tucson this year I acquired a couple of cuprites from Milpillas that are nicely iridescent - so what causes the iridescence? I hear that it may be a light coating of Malachite?

thoughts?

thanks,
bob

[lluis]

I do not think so....

I would think in a very thin layer of tenorite, the cupric oxide, over cuprite, the cuprous oxide... Just some temperature rising....

With best wishes

Lluís

[Bob Carnein]

I would guess a thin layer of goethite, which often causes iridescence. Iridescent specimens of botryoidal goethite are available from Milpillas at present.

[bob kerr]

take a look at:
https://www.its.caltech.edu/~chima/publications/Secret_Lives_of_Minerals.pdf
(link normalized by FMF)

this points to a thin iron oxide coating also and like Bob Carnein mentioned, iron oxides are present at Milpillas.

bob

[Peter Megaw]

It may also reflect residue from a cleaning agent...

[lluis]

Hi, Bob Carnein, Bob Kerr, Peter

It could be a thin coating of iron oxide, as well as residues of a cleaning agent, of course. Also it could be tenorite.

Just take a piece of item (small), place it in clorhidric acid and test for iron with ammonium sulfociante. If a red color is developed, iron is there. If not, well, no iron.
Just more copper or residues.
Take some clorhidric acid solution, add some water and shake. If it bubbles, maybe it is a residue of a cleaning agent.
If not, then copper, so, tenorite....

By the way, cuprite use to darken in surface, due to further oxidation. If oxidation is very thin, then iridescence. And is just more copper (or same copper in another state of oxidation)

With best wishes

Lluís