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Sensitivity of sulfides
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Pete Modreski
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PostPosted: May 25, 2010 12:26    Post subject: Re: Sensitivity of sulfides  

Tracy,
Out of curiosity, what was the locality of your starting-to-decompose pyrite specimen that led you to begin this thread? As people have noted, pyrite from some localities pretty much "never" decomposes, some occasionally, and some, often.

There have been a number of research studies recently that also show that bacteria play a role in the decomposition--some like to "feed" on minerals, and that introduction of organic materials (skin oil from handling?) can accelerate decomposition, and different types of sterilization may inhibit it.

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PostPosted: May 25, 2010 12:59    Post subject: Re: Sensitivity of sulfides  

I've treated some samples of Catalan Baryte specimens from an old mine. These specimens where partially degraded because in the matrix there was sulfides (galena, marcassite, pyrite?) and possible bacteria. I've used Sulphanilamide (CAS 63-74-1), a sulphamide very soluble in acetone. I will inform about results.
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PostPosted: May 25, 2010 19:27    Post subject: Re: Sensitivity of sulfides  

Pete Modreski wrote:
Tracy,
Out of curiosity, what was the locality of your starting-to-decompose pyrite specimen that led you to begin this thread? As people have noted, pyrite from some localities pretty much "never" decomposes, some occasionally, and some, often.


Hi Pete -

It's a "pinecone" pyrite nodule from Ambassaguas, La Rioja (did I spelll that right?), acquired about 5 years ago. I picked it up to clean the shelf and felt a "crunch" beneath my fingers. Looking more closely I noticed a number of the small crystals had come loose from the specimen. I have isolated it inside a box and tonight I see that even more crystals have fallen off. The label upon which it was originally resting shows no signs of staining.

I wrote to tell Jordi of this (for informational purposes) and he was surprised because he had kept these pieces for years and they did show any signs of instability. He recommended I try to repair it with Loctite but this is too delicate for me to do on my own. I regret that all I can do is watch it fall apart.

I freely admit that I might not have handled the pyrite properly, and more importantly, have kept my small pyrite suite in the worst possible location in this house. They are all about to move to a better place.

Random event, humidity, bacteria, skin oils, all of the above...alas.

- Tracy

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PostPosted: Jun 05, 2010 12:22    Post subject: Re: Sensitivity of sulfides  

Here's another related question: what about light-sensitive sulfides (or any other minerals) that exist as inclusions - e.g., realgar in quartz or orpiment in calcite? Will they degrade unless kept out of the light?

- Tracy

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PostPosted: Jun 06, 2010 02:24    Post subject: Re: Sensitivity of sulfides  

Hi Tracy,

Just wondered the same as you. Unfortunately I have no answer. But I assume it is closely related to the lightwave(s) that trigger the decay. if the surrounding mineral acts as a filter the mineral could be protected.

Does anybody has realgar included inside calcite in his/her collection which has been exposed to light for some years ?

Thanks

Christophe
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PostPosted: Jun 06, 2010 04:08    Post subject: Re: Sensitivity of sulfides  

Tracy wrote:
Here's another related question: what about light-sensitive sulfides (or any other minerals) that exist as inclusions - e.g., realgar in quartz or orpiment in calcite? Will they degrade unless kept out of the light?

Hi, Tracy!

Since the transit realgar to orpiment is a chemical oxidation, in addition to the light as a catalyst it also needs an external agent taking the electrons, so I think realgar in quartz or calcite inclusions should be stable. Does everybody agree?
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PostPosted: Jun 06, 2010 07:43    Post subject: Re: Sensitivity of sulfides  

A bit more about realgar becoming orpiment...

Not only orpiment has arsenic with a higher oxidation state, as said in my previous message. It has more sulfur (or less arsenic) than realgar. So I can't see how in a tightly closed environment like a quartz inclusion can such process be performed. No matter you irradiate it at any wavelength, it's not possible to increase the sulfur content neither make the arsenic disappear.
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PostPosted: Jun 06, 2010 09:04    Post subject: Re: Sensitivity of sulfides  

Carles, That was actually an old mineralogical mistake, that realgar alters into orpiment. In reality realgar alters into pararealgar, which has the same As-S ratio, so I don't think any oxidation is involved, just a change in the bonding and crystallography, so it would probably happen even if the realgar were completely isolated as an inclusion in quartz, Baryte, etc.
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PostPosted: Jun 06, 2010 10:25    Post subject: Re: Sensitivity of sulfides  

alfredo wrote:
Carles, That was actually an old mineralogical mistake, that realgar alters into orpiment. In reality realgar alters into pararealgar

Alfredo,

Many thanks for your correction.

I remember having read in several books that realgar may become orpiment. Don't laugh, but my main reference book is still the "Lehrbuch der Mineralogie", 14. Edition, by Friedrich Klockmann and Paul Ramdohr, published in 1954. I fear it is time to get an update.
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PostPosted: Jun 06, 2010 12:22    Post subject: Re: Sensitivity of sulfides  

Carles Millan wrote:
alfredo wrote:
Carles, That was actually an old mineralogical mistake, that realgar alters into orpiment. In reality realgar alters into pararealgar

Alfredo,

Many thanks for your correction.

I remember having read in several books that realgar may become orpiment. Don't laugh, but my main reference book is still the "Lehrbuch der Mineralogie", 14. Edition, by Friedrich Klockmann and Paul Ramdohr, published in 1954. I fear it is time to get an update.


...OK, I did a bit of searching. Do I have it right that realgar alters to pararealgar when subjected to light, at about 500 nM wavelength (Douglass and Shing, American Mineralogist, 1992) but is oxidized to arsenic acid in the presence of water under acidic conditions (Lengke and Tempel, Geochimica et Cosmocihima Acta, 2003)? I was trying to figure out where the excess sulfur was coming from to lower the As:S ratio following alteration to orpiment, if the only catalyst was light (before I read Alfredo's post).

According to Lengke, the oxidization of realgar in water occurs acccording to the equations

AS2S2 + 14 H2O -> 2H3AsO3 + 2HSO4(-) + 2OH(+) + 18e(-)
and
H3AsO3 + H2O -> H3AsO4 + 2H(+) + 2e(-)

...so there is no actual As:S ratio shift with either light or water. But I'm still confused. Will a transparent/translucent crystal which has inclusions filter out enough radiant energy to prevent the alteration process, or can included crystals alter or degrade anyway? Alfredo's comments suggest that this is not likely to occur, but I have this nagging uncertainty.

Maybe realgar is a bad example, perhaps another sulfide that is light-sensitive only would be a better subject for my question. And it is purely theorectical so not limited to quartz. Meanwhile, I'm off to find a new home for my realgar specimen, if it is sensitive to BOTH light and water! :-) (right after I am finished relocating the pyrites) It's a muggy day today, which adds incentive.

Chris, thanks for your comments and suggestion for the carrtridge dehumidifier (which I will look into). I meant to respond sooner, you gave me an idea for how to protect my pyrites and other sulfides that I have. Have fun at Ste Marie, we look forward to the show reports from you and others.

- Tracy

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PostPosted: Jun 06, 2010 12:25    Post subject: Re: Sensitivity of sulfides  

By the way, I forgot to mention that the Geochimica issue has a number of interesting articles about oxidation of sulfides. I downloaded several. Volume 67 No. 5, 2003.

- Tracy

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PostPosted: Jun 06, 2010 12:36    Post subject: Re: Sensitivity of sulfides  

By my own experience I never saw specimens of sensitive minerals included in others minerals decayed (of course if the "others minerals" aren't sensitive), including very old specimens from historic collections, so I believe that when them are included, for some reason, them are preserved and them don't decay.

About the Realgar-Pararealgar is very interesting the fact that according Douglass, a Pararealgar return to Realgar if you hot it to a very high temperature (about 300 – 400 Celsius I believe). Unfortunately it seems that although you recover the Realgar species, its previous beauty never return ;-)

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PostPosted: Jun 06, 2010 14:26    Post subject: Re: Sensitivity of sulfides  

Tracy wrote:
Do I have it right that realgar alters to pararealgar when subjected to light, at about 500 nM wavelength (...). But I'm still confused. Will a transparent/translucent crystal which has inclusions filter out enough radiant energy to prevent the alteration process, or can included crystals alter or degrade anyway?

Since the visible light spectrum is in the range from 360 nm to about 700 nm, starting with purple color and ending with red, the 500 nm light you are talking about might approximately match the green color, I guess. So a colorless and transparent quartz (or calcite) crystal does NOT filter that wavelength radiation. Otherwise you wouldn't be able to see a green object through it. Indeed, such crystals do not absorb any wavelength radiation in the visible range just because they are transparent and colorless, the opposite being also true.

That said, getting the conclusion that quartz included realgar is not safe might be too much. But there is an argument to think so, provided that the above statement saying that 500 nm radiation is the responsible for the decay of realgar is true.
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PostPosted: Jun 06, 2010 18:39    Post subject: Re: Sensitivity of sulfides  

It also take a fair amount of exposure to complete a transformation. I probably would not keep pyrargyrites and realgars in a sun-filled display case, but if kept in a dimly lit room, and exposed to light occasionally, I suspect little appreciable degradation will occur. I fact, with respect to pyrargyrite, I had a Sampson Mine piece, circa 1850, which still displayed good red color, with minor silvery tarnish.
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PostPosted: Jun 07, 2010 09:32    Post subject: Re: Sensitivity of sulfides  

To complete that, I used to have an orpiment specimen many years ago. It was keep in daylight, but with no direct sunlight. After a few years, it turned yellowish & dust...

Christophe
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