Coquimbite

Recently I acquired several specimens from my contacts in Peru. Knowing that coquimbite is associated with calcanthite it was apparent that this mineral would be in some degree water soluble. Copper sulfate (calcanthite) dehydrates in dry air to a white powder. Coquimbite does something similar - the beautiful blue crystals developing a frosting of brownish tinged material. Does anyone have any experience in practical methods of preserving coquimbite?

Coquimbite is violet, not blue, and it is relatively stable compared to other water-soluble iron sulphates, so no special treatment needed. Are you sure your piece is really coquimbite?

If the coquimbite gets damp it can turn brown due to oxidation of the iron. Other than keeping it in a sealed container, I like to put mineral oil on my water soluble minerals just in case.

I think the iron in coquimbite can't oxidize any more than it already is, because it's already ferric iron, so if a brown coating develops then some other alteration process must be going on, other than oxidation. Perhaps sulphate loss is possible, with gradual replacement by hydroxyl? I don't know. But if anything is changing, I suspect it's because either the "coquimbite" isn't coquimbite, or else it's impure coquimbite mixed with other, less stable, sulphates. Coquimbite is a species I rather like and I have violet coquimbites from Japan, Utah and Peru, and never had any change on me. But anyway, Reiner's suggestion of a film of mineral oil is a good idea.

Hello Alfredo,

By oxidize I mean combines with oxygen to form iron oxides, probably through some intermediate hydroxide when it dissolves in the water. I imagine it forming Al sulphate and Iron hydroxide.

Alfredo

Actually +3 iron in coquimbite can oxidize up to +6 oxidation state, but it requires heating mineral sample in hypochlorite solution (bleach) :D

I know I had coquimbite from Utah in a display cabinet and it turned a very ugly brown after a short time (months-ish). I have several that I have kept in bags inside flats and they are fine and have been for decades.

I did put a bottle of small xls in a bottle and put it in the window seal. A year later there was no change. So it must me the air/humidity that makes it change.

Rick
I know I am in my own little world, but everyone knows me here.

Thanks!! This is exactly what I am experiencing - but not with all specimens. I thought of putting specimens in a perky box to shelter pieces from extremes in humidity.

Marshall

Might perhaps be a combination of light plus atmospheric changes that hurts them? I keep mine in the dark in the refrigerator, so that is maybe why they stay fresh forever.

Thanks to all that responded to my inquiry!
Marshall

Does anyone have experience in cleaning coquimbites from Peru? Some specimens develop a white/brown coating when exposed to air. Knowing that this evaporate is water soluble increases the challenge.
Thanks,
Marshall

Use a spray bottle filled with pure alcohol. Methyl Hydrate will do and is inexpensive.

I've also had Coquimbite turn on me in a very short period of time in the summer months when the swamp cooler was running. A specimen in a sealed box but otherwise stored similarly was fine.