[NHCOLL-L:4161] FW: Cleaning oxidized pyrite mineral specimens

[Joy Irving]

From: joyirving at hotmail.comTo: hodgkins at chem.ucla.eduSubject: RE: [NHCOLL-L:4140] Cleaning oxidized pyrite mineral specimensDate: Thu, 15 Jan 2009 14:21:22 +0000

Dear Robyn,  As Greg has already pointed out, oxidation is a legitimate part of the specimen, and is not necessarily damaging, unless you know for certain that it is pyrite decay.  The way you describe the minerals containing iron sulphides as turning brown, suggests that the pyrite has a 'passive' coating of goethite (an iron oxide), which can sometimes under the right conditions, replace pyrite, and will appear as a normal hard coating.  If this is so, then the pyrite is stable and should not deteriorate further.  It would not be wise to remove such a coating, as then the newly-exposed iron sulphide will potentially be exposed to high humidity levels, which together with oxygen, are the causes of pyrite decay.   However, pyrite decay is immediately recognizable as white, yellow or grey powdery oxidation products, sometimes even looking 'furry' if in an advanced state of decay.  This often leads to the specimen becoming unstable and crumbly, due to expansion caused by the oxidation products.    It would be a good idea also to check which mineral species are present before treating with ammonia, as many species will react with ammonia, sometimes with spectacular results - for instance, chalcopyrite (a copper iron sulphide) will turn bright blue (due to the formation of a cupro-ammonium complex), if there are any oxidation products present.  I would also recommend that you read "The Care and Conservation of Geological Material - Minerals, Rocks, Meteorites and Lunar Finds" by Frank M. Howie, published by Butterworth-Heinemann - though I think that this may have already been mentioned - it really will tell you a great deal that you need to know about conservation of minerals. Correct storage of minerals is also covered in the Howie book - this is highly necessary if you want to prevent further deterioration of any pyritic specimens.  I note that you've had a communication from Chris Collins - he should give you any information that you need to know about which papers to read concerning oxygen-free microclimates, which are not covered in Howie.   I hope that this information is useful. Joy IrvingMineralogy CollectionsOxford University Museum of Natural HistoryOxfordU.K.> Date: Thu, 8 Jan 2009 16:53:35 -0800> From: hodgkins at chem.ucla.edu> To: NHCOLL-L at lists.yale.edu> Subject: [NHCOLL-L:4140] Cleaning oxidized pyrite mineral specimens> > I am a UCLA chemistry graduate student helping the Natural History > Museum of Los Angeles County Mineral Science department. They have > several minerals containing iron sulfides that have become brown in > color. The corrosion has not been completely identified, but we believe > it is mainly oxidized pyrite corrosion. The Museum is interested in > cleaning these minerals. I have found literature about using > ethanolamine thioglycollate to clean. Has anyone used this or has an > opinion of how this would compare to Super Iron Out? thanks!> > Robyn Hodgkins> Graduate Student> Garrell Group> UCLA Chemistry and Biochemistry> hodgkins at chem.ucla.edu> 310-206-9434> 

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