FMF - Friends of Minerals Forum, discussion and message board

[Tracy]

I thought I'd start a thread on the subject of sulfide minerals because I have a nice pyrite specimen that recently started degrading. I am sad, though I understand from previous posts (and through conversations) that it's easy for pyrites to become unstable. However, Jordi brought to my attention the fact that all sulfide minerals are sensitive and need to be carefully protected. This bit of info concerns me greatly because a large part of my collection is made up of sulfides (I don't have very many other pyrites though). Also, the house I live in is not climate-controlled, and we have humid summers and dry winters (not to mention the temperature changes with the seasons). So I was wondering whether there are other sulfide minerals that I have to be particularly careful about, and why - I know about realgar and i think proustite is sensitive to (light? moisture?) but am wondering: is there a list I can refer to of other "sensitive" sulfides, or can the FMFers help me compile one as we did with fluorites? To help keep the thread from going all over the place I'll add that in addition to the sulfides mentioned above, my collection includes galena, stephanite, stibnite, enargite, pyrargyrite, orpiment, cinnabar, polybasite, hutchinsonite...I'll add other species in the morning as they come to me, the ones I named were off the top of my head.

Secondary question, does the degree of this "sulfide sensitivity" vary with locality (as with pyrites)?

Thanks in advance, I hope I won't have to reconsider the focus of my collection! :-/

- Tracy

[Matt_Zukowski]

Chapter 5 of Howie, The Care and Conservation of Geological Materials, is all about sulfides and related species. Below are some items from this chapter, but before I begin, I have found Howie perhaps too concerned about bad things that can happen to mineral specimens – taking Howie too literally makes one too paranoid.

The chapter begins explaining that sulfides are a huge and quite chemically diverse group of minerals and so one would expect a wide range of sensitivities. Also, many apparently monomineralic specimens actually have inclusions of other species (I suspect this would be one reason why stability would vary with locality). The chapter then moves on the various mechanisms of degradation including simple oxidation, photo-oxidation, tarnishing in air, and oxidation with water vapor.

I would like to add that some of the things people have said about pyrite (e.g. nice clean cubes are more stable than microxtaline aggregates) should also hold for other susceptible sulfides because surface area enhances reactivity (and possibly also because the free energy on the surface of a bunch of micro xtals may be higher and thus more prone to change).

Susceptible Sulfides – some from the book:
* Acanthite – tarnishes quickly with light or at 150F with water vapor. Tarnish becomes a friable crust. Keep from light.

* Chalcocite – probably stable in light. Tarnish is self-limiting. Overly acidic or alkaline environments could be a problem, avoid polluted environments.

* Covellite, Bornite – pretty darn stable, tarnish self-limiting.

* Chalcopyrite – pretty stable but since crushed chalcopyrite reacts with water vapor and thiobacillus, Howie recommends keeping chaclo specimens below 50% RH. Perhaps Howie too conservative?

* Cinnabar – keep away from light.

* Galena – very stable, only problem is intimate mixtures of micro xtaline pyrite+galena.

* Molybdenite – pretty stable but avoid high humidity.

* Pyrrhotite – pretty stable, especially macroxtals. RH may enhance degredation.

* Linnaeite, Carrollite – Howie says that these species tarnish rapidly in air from metallic grey to red and thus recommends a low (<50>30C) and humid environments. . I have had many carrollite specimens that have undergone no such degradation, so perhaps Howie is too cautious.

* Realgar and Orpiment – very susceptible to light.

* Stibnite – tarnish is probably photochemically induced but self-limiting.

Susceptible Sulfosalts (say that with a lisp) – some from the book:
* Many silver sulfosalts (Argyrodite, canfieldite, pyrargyrite, andorite, and proustite) are light sensitive.

* Some silver sulfosalts and other sulfosalts (Polybasite, polyargyrodite, pyrostilpnite, xanthoconite, Samsonite, diaphorite, freieslebenite, miagyrite, aramayoite, matildite, trechmannite, sartonite, vrbaite, fizelyite, jamesonite, and Hutchinsonite) experience light-induced but SELF-LIMITING surface changes.

* Enargite – undergoes rapid oxidation to copper arsenate under “mine conditions.” Recommends less than 50% RH.

* Boulangerite – probably most reactive sulfosalt besides the silver sulfosalts. Humidity accelerates degradation.

Other similar minerals (antimonides) – some from the book
* Arsenopyrite – pretty darn stable. Possible problems in hot (>30C) and humid environments.

*Dyscrasite – tarnishes from sliver white to dull grey, yellow, or black, probably by conversion to antimony oxide.

There was nothing specific in Howie on Stephanite.

[Carles Millan]

And what about marcasite? As far as I know about this topic, that is not much, some marcasites are very unstable and degrade quickly while keeping them in a showcase.

[Paul S]

Carles has a valid point. I have two marcasite specimens in my collection. They are both spherical and radiating on the inside. One of them is in good condition, but the other one has pulverized completely! I have kept the powder, but it can't be called a specimen anymore. I do have the photos of how it looked before it completely crumbled (it was already transforming back then). I was surprised at how fast it decomposed and I didn't put it on an open shelve or in direct sunlight.

Will my other specimen end up the same way? Or will it remain stable and is decomposition just a case of bad luck?

[John S. White]

Marcasite, very definitely. But I would not be overly concerned about this problem. In my collection I have only one pyrite that is degrading, and that was beginning to do so when I acquired it but I wanted it just the same because it was a rare twin. I have lots of other sulfides, none of which is altering.

While at the Smithsonian I noted that the specimens of pyrite that degraded were random. For example, there were perhaps several drawers of pyrites from Elba, but only some of them had gone to powder while others looked as fresh as the day they were collected, and they were in the same drawer with the degraded ones.

[Paul S]

I have read that there is the possibility of 'contamination' where one degrading specimen affects specimens that lie next to it. I guess that John's observations tell a different story. Although direct contact might still have an effect, specimens close to each other will not 'contaminate' each other when one of them is degrading.

But what conditions then cause degradation and how can they be controlled in the case of stored specimen?

[GneissWare]

A rule of thumb I've used successfully, is to place the specimen in a box. A few weeks later, if I open the box and smell sulfur, then I presume the specimen is going to have a higher likelihood of instability. If I really want to keep it, I segregate it from any other specimen that will not react favorably to an H2SO4 vapor.

[chris]

Hi folks,

Sphalerite is quite stable too, so no worry about it. As mentioned earlier suldides' sensitivities also depends on locality (inclusions ?). for instance I know a mine in France which produced "explosive" tetrahedrite (and I really mean it) triggered by heat exposure.

All my collection is based on combinations between sulfide/sulfosalts and other mineral elements. Until now I never experienced decaying, just tarnishing over time. However I stay away from minerals such as realgar & orpiment which are not only light sensitive but moisture senstive too, and remain careful with coloured ones. Except sphalerite and unless been kept away from light in a box inside a drawer, I try not too spend too much on such specimens knowing they'll change of aspect over time.

The stability can also depends on treatment(s) used to clean the specimens prioir to marketing them. For instance, Bulgarian galena can be found with calcite crusts. A good trick to remove calcite is to use chlorhydric acid. Unfortunately this exposure also trigger a tarnishing of the galena over time. So if you want to buy one, better to focus on specimens having calcite as well, the chance of it having been clean with acid will be lower with better chance of keeping the lustrous galena over time.

Be careful while cleaning them too. Avoid caustic chemicals and more generally avoid cleaning them. Of course they are exceptions. I know a dealer who cleans Pyrrotite with copper cleaning chemical. According to him it is quite efficient, but I never dared to try on mine.

For marcasite, this very same dealer told me that crystallized marcasite tended to be more stable than massive marcasite. Here too it depends on localites. Correct me if I'm wrong Jordi, but in Panasqueira, Portugal, marcasite is mostly mixed with arsenopyrite which helps stabilizing it. As a result it is more stable than from some other localities.

One trick with both pyrite & marcasite : if your specimen starts to have a white crust with acidic smell on it, it might reveal an alteration into melanterite. In this case remove it of your collection as the alteration will also produce sulfuric acid which could damage surrouding specimens.

Hope it helps

Christophe

[alfredo]

I too have mostly sulphides and sulphosalts in my collection. Have mainly had problems with marcasite and pyrrhotite, less frequently pyrite and chalcocite. Never had a problem with any others, not even arsenopyrite.

[Jordi Fabre]

[Tracy]

Thanks all for the feedback. Matt, you'd mentioned the Howie book before and I forgot you had it - it's an excellent reference! Pretty much covered all the sulfides in my collection. I'm relieved to see that more minerals are sensitive to light than to moisture, as it's easier for me to control light exposure than humidity! For the moisture-sensitive species, maybe I should get hold of a dessicator?

Gneissware, when you say that you segregate unstable pieces from H2SO4-sensitive specimens, how do you do this? Move to a separate location and/or keep in a particular type of container of box? I have one rather stinky specimen that probably deserves its own space.

- Tracy

[GneissWare]

[bugrock]

May seem odd to add calcite to the list but will relate a danger I was told by another collector and dealer. I actually own one such susceptible piece in my estimation. It is a plate of calcite xls on dark colored matrix from the Shullsberg, Wisconsin Pb/Zn mining district, SW Wisconsin. Actually the calcite xls are stained in an unattractive yellow orange and I really should get rid of the piece but it is a locality thing.

Anyway the collector I refer to had quite a stash of Shullsberg pieces in a room of his house and would sometimes here a "pop" coming from that room. Turns out it was the sound of calcite xls actually being lifted off the matrix, he figured due to the build up of gas (HS?) between the matrix and the calcite.

Again, I've have not personally observed this, just offering it as a second hand observation from someone I trust.

I wonder if anyone else has seen crystals separated from matrix secondary to deterioration of high sulfide matrix and if that deterioration was ever "explosive"?
Less dramatic would be the simple deterioration of matrix with the supported
xls left intact.

[Tracy]

Very helpful. Thanks Bob!

- Tracy

[Jordi Fabre]

[Paul S]

[Tracy]

[alfredo]

Tracy, An electric dehumidifier in the mineral room will work wonders. I bought one on amazon dot com for about $200. Not only protects the minerals, but prevents mold growth and makes you feel more comfortable in summer too, so I consider it one of the best investments I've ever made. Just remember to empty the water bucket every day in summer, or else it switches itself off when the bucket is full.
Regards,
Alfredo

[Tracy]

Thanks Alfredo. I do have a dehumidifier which is connected by hose to the outside (so it never gets full and I don't have to empty the water all the time). The pyrites aren't close to it, though. Maybe I'll relocate them.

- Tracy

[chris]

Hi Tracy,

If your specimens are inside a display, you can also buy a small cartridge dehumidifier as I did and put it inside your display or just beside it (like I did). It will create a small area around the display with much less moisture in the air as a result.

Don't know if it is the consequence or if I'm just lucky, but none of my sulfides every decayed until now.

Christophe