[Nhcoll-l] [External] Storing and or displaying pyrite

Wed Apr 19 04:14:53 EDT 2023

Hi,

I would like to add to this discussion that once oxidation starts, there is no way to stop it. You can slow down the process by controlling the environmental conditions and doing some treatments, but the destruction of the specimen will be a reality.
You should always have good records of those specimens and, if you can, scan them.
The oxidation process of one specimen will contaminate the others by the emission of sulfuric acid, destroying or damaging labels and all paper materials associated which are near the specimens.

Please have a look to all the helpful bibliography sent previously.

Cheers,

Vicen

Ms Vicen Carrió ACR
Geological Conservator/ Preparator
National Museums Scotland
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From: Nhcoll-l <nhcoll-l-bounces at mailman.yale.edu> On Behalf Of Tacker, Christopher
Sent: Tuesday, April 18, 2023 8:49 PM
To: Leslie L Skibinski <lls94 at cornell.edu>; Nhcoll-l at mailman.yale.edu
Subject: Re: [Nhcoll-l] [External] Storing and or displaying pyrite

Hi,  Leslie,

I have an open source paper you might find to be of interest, a literature review of the process of pyrite oxidation and the growth of efflorescent minerals. I'm a mineralogist, so I've always thought that if you didn't understand the minerals involved in a geologic process, you'd never understand the process.

A review of "pyrite disease" for paleontologists, with potential focused interventions. Palaeontologia Electronica, 23(3):a45.
https://doi.org/10.26879/1044<https://linkprotect.cudasvc.com/url?a=https%3a%2f%2fdoi.org%2f10.26879%2f1044&c=E,1,Nw22Ize5CipGsm5qrdz1nqn2Gln3Kpqt7W1ZpS5lFtSyIq5uWBTwkEkzlnCu8Md_ZVqLmUQaIIFLdIWHfEE6BtVsZKxN1Dh3JQzpj_g64_PoFfG1TiY2dOsH&typo=1&ancr_add=1>
//palaeo-electronica.org/content/2020/3159-pyrite-disease<https://linkprotect.cudasvc.com/url?a=https%3a%2f%2f%2f%2fpalaeo-electronica.org%2fcontent%2f2020%2f3159-pyrite-disease&c=E,1,bEyaZU2OIOALxBC7WXJH3Zt88KbiU4zApstFGdH_1n7b1cK12MnHm1Gpkx-zLuBytgt4VNh4D_n05fodYtz5m5HgaT56-1Tbcyocd6qb&typo=1&ancr_add=1>

To summarize for those without time to track it down.

  1.  Oxidation, reduction and hydration are reactions that involve movement of electrons. Pyrite is a semiconductor, so it moves electrons easily. In vacuum, for XPS spectroscopy, pyrite will pick up oxygen within minutes to seconds. The race against oxidation may be lost once the pyrite is exposed to air.
  2.  Oxygen and water work quickly to oxidize the pyrite surface. Together, they work even more quickly. The third major oxidant is Fe3+.
  3.  Movement of electrons, for pyrite surface oxidation, involves the oxidant, the entire body of the pyrite, and the pyrite surface. If it's happenin' at the surface, all the other electrons come to the party. Electrons move though the body of the pyrite and across the surface, from Fe to Fe as well as from Fe to S.
  4.  Electron conductivity of pyrite covers four orders of magnitude. This is why some pyrites decay quickly and others don't. Conductivity is enhanced by trace elements and vacancies in the pyrite. Trace element concentrations for pyrite also covers four orders of magnitude.
  5.  Efflorescent Fe2+ sulfate minerals will change hydration state quickly with changes of humidity. Fe3+ sulfates do not lose the water once it's there. This is why humidity control alone is ineffectual. You must control oxygen as well. Dry nitrogen is probably cheapest. If you use expensive oxygen "getters", I think you need to show that they work faster and have a higher buffer capacity than your pyrite does. I would bet on the pyrite.
  6.  And BTW, Fe3+ sulfate minerals develop quickly from the initial Fe2+ sulfate minerals. In my experiments, with a wekk or so.
  7.  The pyrite and the sulfate minerals all gather a film of water as time progresses. This enhances the movement of electrons to the reaction site. It's uncertain if the pyrite will lose that film under lower humidity conditions, but it is certain that the Fe3+ minerals will not.
  8.  Various coatings are ineffectual at preserving the pyrite or the fossil. They do not block oxygen or water or electron movement.
  9.  "Neutralizing" acid is ineffectual. Strong acids don't just hang around waiting for a base. They react, unfortunately, with nearly anything. My experiments on the reaction between sulfuric acid and apatite (a bone analog) went so fast that I couldn't turn around and put the specimen on the spectrometer stage. You can watch the calcium sulfate crystals grow. Also, cleaning pyrite with carbonate minerals actually enhances the pyrite surface reactions.
  10. Macro does not equal micro. The actual humidity at the pyrite surface reflects the hydrous minerals present, not the ambient humidity of the air. The actual humidity depends on the film of water on the pyrite surface.
Cheers,
Chris

Chris Tacker  (he, him, his)
Research Curator, Geology |  Ph.D.
North Carolina Museum of Natural Sciences |  11 West Jones Street | Raleigh, NC  27601-1029
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From: Nhcoll-l <nhcoll-l-bounces at mailman.yale.edu<mailto:nhcoll-l-bounces at mailman.yale.edu>> on behalf of Leslie L Skibinski <lls94 at cornell.edu<mailto:lls94 at cornell.edu>>
Sent: Tuesday, April 18, 2023 12:27 PM
To: NHCOLL-L (nhcoll-l at mailman.yale.edu<mailto:nhcoll-l at mailman.yale.edu>) <nhcoll-l at mailman.yale.edu<mailto:nhcoll-l at mailman.yale.edu>>
Subject: [External] [Nhcoll-l] Storing and or displaying pyrite

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Hello everyone,

We have just gotten some fossils where the organism has been replaced with pyrite (FeS2).  Because of this, they are inherently unstable and very prone to pyrite disease.  Just in the short time we have had them, they have started to become less shiny (oxidize).

Has anyone had experience with storing pyrite?  Are there ways of packaging or storing the specimens that reduce oxygen and help maintain a stable low relative humidity?  Should we use a molecular sieve or is silica gel OK. Has anyone worked with a fabricator to design and produce a storage or exhibit case?  Would storing the specimens in an anoxic environment such as storing them in nitrogen or some inert gas be feasible and reasonably cost effective?

Any information or suggestions would be greatly appreciated.

--Leslie

Leslie L. Skibinski

Collection Manager

Paleontological Research Institution

1259 Trumansburg Road

Ithaca, New York  14850

Phone:  (607) 273-6623  ext. 128

Fax:  (607) 273-6620

National Museums Scotland, Scottish Charity, No. SC 011130

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