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How to prevent chalcanthite from fading

Posted by Ashley Wise  
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Ashley Wise July 17, 2017 03:17AM
I've had a couple of man-made chalcanthite samples, and recently acquired a "real" nature-made one.

My oldest man-made chalcanthite is now completely matt light blue instead of translucent deep blue. I got that one probably 12 years ago

My newer man-made is starting to lighten around the edges. I got that one around 4 years ago.

My newest "real" one is about a year old and hasn't faded yet.

I've read that chalcanthite is readily soluble in water and humid environments cause it to degrade.

What, if anything, can I do to preserve it? To prevent it from fading over time?

Can I restore the older specimins that have already faded?
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Doug Daniels July 17, 2017 03:41AM
Actually, dry environments cause the degradation - it dehydrates (loses its loosly bound water). As far as the easy answer - you can't restore those that have degraded. Nope, no way, no how. As to how to keep them from degrading - that gets into some things that "purists" will fight you over. I have a few natural ones, and I have sprayed them with acrylic. And I know I'm going to be vilified for that.....it is what it is. The same would work with man-made samples. I have one that I grew, kept it in a zip-loc bag, it's about 15 yrs old (maybe more), and it is showing signs of dehydration (I did not spray it with the acrylic).

As to your "real" one.....what do you mean? Is there a location associated with it? (It does help) Can you attach a photo of it, so we can make an assessment? Until we can figure it out, at least keep it in a zip-loc.
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Joel Dyer July 17, 2017 06:28AM
I grew "chalcanthite" in around 1980 - that's 37 years ago - and still have 2 nice crystals left, see below for a quick photo of one. This crystal is a bit damaged, due to rough accidental treatment. It's now in my open mineral shelf in a mostly heated office.




Many years ago, I decided to protect the crystals by spraying from afar a very thin, misty layer of lacquer onto the crystals. A layer which, BTW, doesn't affect the Raman spectrum due to it's thinness, my default used objective and the focusing method.

The crystals had before their protection been stored in alternating dryish and dampish conditions varying from -30 to perhaps +35 degrees, before they were "re-discovered" with a "mess" of other mineral samples in a cupboard in my parents garage. I guess the survival of the crystals were made possible due to them being left in peace in the darkness of the drawer, which was not fully air tight, though.

Cheers,

Joel



Edited 2 time(s). Last edit at 07/17/2017 07:21AM by Joel Dyer.
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Alfredo Petrov July 17, 2017 06:38AM
Doug is right, Ashley. Your chalcanthites need humid air (but of course no direct contact with water).
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Thomas Lühr July 17, 2017 12:03PM
If your specimen is only coated with a thin dehydrated layer it is possible to restore it (partly). Perhaps you will have to live with a bit loss of quality, such as rounded edges, though.
Hold it for a few seconds in the steam over boiling water, to generate a thin(!) film of water on the surface. Then store it in a sealed jar for some time. When the blue colour is back (it takes a while), then remove the lid and cover the jar with a sheet of paper, to let dry the rest of the water SLOWLY.
This procedure has worked fine for me with a natural specimen.
I'm storing it in a box together with a smaller box filled with wet cotton, that is also used (upside down) as a stand for the specimen.
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Ashley Wise July 20, 2017 12:55AM
I attached pictures of the three chalcanthite sample




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Doug Daniels July 20, 2017 02:46AM
The first one may be beyond help, although you might try Thomas' suggestion - it may work. The other two don't look so bad. It's up to you whether to spray them with an acrylic, or just try keeping them in a more humid environment. The strange thing with natural specimens is that some seem to dehydrate quickly, some only slowly, and some don't seem to give a hoot and stay the way they were found (unless put into 100+ degrees with no humidity). So, for the second and third, I'd say keep an eye on them (every few months), and if it looks like they are changing, maybe consider the acrylic (or other method, if anyone knows of one).
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Alfredo Petrov July 20, 2017 04:31AM
Chalcanthite is a quite stable mineral unless its environment is much too dry. Natural samples that appear to be altering rapidly may turn out to be Pisanite (Cu-rich Melanterite) rather than true Chalcanthite.
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Donald B Peck July 20, 2017 02:59PM
I have had artificial crystals turn from very blue (much like the 2nd photo) to the whitish - greenish - blue (as in the first photo) when kept in a somewhat damp basement environment.
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Lawrie Berthelsen (2) July 21, 2017 05:08AM
I live in the tropics where humidity is often over 80%, and we have monsoonal wet seasons. Artificially grown crystals that I have had have gone white within 10 years, however I have had a specimen of supposedly natural Chalcanthite crystals in my collection for over 45 years which has shown absolutely no deterioration in all that time. It consists of centimetre sized crystals that have grown on a piece of mine timber, and it shows no signs of having been sprayed or coated with lacquer.

I have often wondered whether the commercial copper sulphate might have impurities that accelerate the decomposition, or whether the natural crystals might have impurities that have arrested the deterioration. I have noticed when growing crystals many years ago that a saturated solution of copper sulphate often has a greenish sludge in the bottom of the jar.
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Tim Jokela Jr August 21, 2017 03:04PM
So, aside from a lacquer spray, nobody knows how to stabilize manmade CuSO4?

How about soaking in highly thinned epoxy, under vacuum?

Casting in resin?

Soaking in oil?

A material called PEG is used to stabilize green wood, it infiltrates and replaces the water... hrmmm?
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Alfredo Petrov August 21, 2017 05:02PM
Doing anything with chalcanthite under vacuum would be the worst! Suck the water right out of the crystal structure. Even putting it in pure alcohol sucks the water out of the crystal structure.
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Doug Daniels August 22, 2017 02:24AM
To follow up with Alfredo's comment....anything that replaces or removes the "loose" water (those 5 , or fewer, waters) from the structure will change the color of chalcanthite. Something about how the water bonds around the copper that causes that particular blue color; danged quantum physics and such.
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Alan Pribula August 22, 2017 12:40PM
Alfredo and Doug are correct. Removing the water from chalcanthite or replacing it with something else will (probably) change its color. The same is true for many other hydrated materials. These materials are not merely "wet" like a sponge--the water occupies a definite and essential spot in the crystal lattice. Remove the water or change it to something else and the structure is very likely to change as well. Since the color is related to the structure, a change in structure will likely lead to a change in color.

In hydrated materials, the water molecules most often are bound to a metal ion in the lattice (in this case, a copper ion, but the same is true of many minerals containing iron, manganese, nickel, cobalt, aluminum, and many others), but sometimes it is bound in other ways. In chalcanthite, four of the five waters of hydration are bound to the copper ion, while the fifth is bound with the sulfate anion.

The reason that many materials are colored is because the electrons in the material can absorb energy and jump to a higher energy level. If the energy absorbed corresponds to light in the visible region of the spectrum, then we perceive this as color. The light that we see transmitted or reflected by such a material is the light which is not absorbed (that is, the complementary color). So, for example, if red/orange/yellow is absorbed, the material will look blue or blue-violet.

The energies of the electrons in a metal ion are affected by their surroundings. Change their surroundings and you change their energy levels. Change their energy levels and you change the energy they can absorb. Change that and you will probably change the color of the sample. (I say "probably" because there will be no change in perceived color if the energies absorbed are outside the range of visible light, in the ultraviolet or infrared.) The change in surroundings could be due to different molecules or ions surrounding the metal ion in question, such as removing the water molecules from chalcanthite, leaving only sulfate ions surrounding the copper ion. When the copper ion is surrounded by four chloride ions, it gives a yellow-green color; when surrounded by four ammonia molecules, the color is a deep violet-blue. The change in surroundings could be as simple as having the surrounding molecules or ions be closer or farther away from the metal ion. That's the cause of the difference in color between azurite and malachite, and between ruby and alexandrite.

So, while replacing the water with something else (such as your idea of replacing the water with polyethyene glycol (PEG)) might sound appealing at first, it would be extremely unlikely that doing that would keep the chalcanthite its original color. Coating the crystals with something that prevented the water loss would have a better chance of success. However, any treatment method involving vacuum or heat would run the risk of removing the water molecules and changing the color, as Alfredo said.

The theory explaining the colors produced by metal ions is called Ligand-Field Theory. (A simpler version, called Crystal-Field Theory, was originally developed to explain the colors exhibited by mineral crystals, and Ligand-Field Theory grew out of that.) The basics of it can be found in most introductory college-level chemistry textbooks. The details of it can get pretty hairy, though. I taught college-level chemistry for 38 years and there are plenty of aspects of it that I don't fully understand.
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