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Techniques for CollectorsCleaning pyrite / marcasite samples found in black shale...

23rd Nov 2015 17:11 UTCScott DeLano

Hello everyone...


Quick question...I have found several nice little pyrite and marcasite samples within black shale deposits in northeastern Oklahoma. After picking at them with a dental pick, I wasn't very happy with the results and the time it takes. So I had someone that I know, who etches glass with a blaster take a shot at a couple of the samples. They use Black Silicon Carbide (180 grit) material to blast with. It took the black shale right off and left the crystals but now those three samples are dull gray color. I thought I would just need to do my usual 6% vinegar and then soapy wash to get them to shine and clean off the gray. Instead, the samples are staying that dull deep gray color. Is that normal or did the blasting material somehow recolor my samples (I can't believe that would happen)? If it recolored my samples, what blasting material would be better to use in the future? Thank you!

23rd Nov 2015 17:16 UTCAlfredo Petrov Manager

Silicon carbide is much harder than pyrite, so it has left the surface of your pyrite covered in millions of micro-scratches, changing the luster. Blasting with glass beads is better for pyrite.

23rd Nov 2015 17:36 UTCRob Woodside 🌟 Manager

Glass beads can chip pyrite. I'd use something a little softer. Walnut shells?

23rd Nov 2015 18:43 UTCMichael Sommers

If the pyrite/marcasite is relatively stout (no thin extensions of crystals) you could try a dry-cleaning (water) gun.

23rd Nov 2015 19:39 UTCBob Harman

I like MICHAEL's idea of trying a hi pressure cleaning gun. Also if your Oklahoma shale has a lot of carbonate as part of it, you might give the dilute acid, which ever you use, a bit more time. I found that using most any type of dry blasting grit, even the gentlest, will tend to dull and pit the surface of the pyrite crystals. This will probably be irreversible.


Several years ago I found a large number of silvery finely striated pyrite cubes up to 2cm in a nearby quarry. They had never been reported there before. Anyway they all cleaned up quite nicely as they were in a carbonate rich matrix. Several of the nicest are now at the museum with a new locality report. With a bit of luck, maybe yours will come out as nice as mine did! CHEERS.......BOB

23rd Nov 2015 19:48 UTCScott DeLano

Thank you everyone for your input. I'll check and see if I can find someone using the softer blasting material or someone who has a dry cleaning gun.

23rd Nov 2015 19:50 UTCDonald Lapham 🌟

Here is WNY we use Sani Flush toilet cleaner followed by a scrub with a paste of baking soda to clean the Alden, NY pyrite nodules. Sani Flush is a sodium bisulfate powder with soaps and perfumes. Soak overnight in a dilute solution of the acid, then make a paste of baking soda on a toothbrush to scrub for a few minutes and then finally soak in fresh water for a few days to remove residual chemical.

23rd Nov 2015 22:15 UTCScott DeLano

Thanks!

24th Nov 2015 21:53 UTCJohn Oostenryk

Using air abrasive with too hard a material and too high a pressure certainly fries whatever it hits. I always think of it as being "burnt" not in the fire sense~This is just a conceptual term, and indicates trashed.

But actually, yes, do consider the frictional 'heat' generated by striking and abrading. In the micro view picture- similar to a grinder being used!


I have used baking soda as air abrasive on pyrite and found even that can leave a matte luster on the surface of some. It is worse on marcasite and especially on

prior chemical altered surfaces( like when colored red or yellow by prior alteration.)


It is certainly something you have to experiment with on lesser pieces before utilizing on your best.

SAME thing with chemical prep- test crap stuff first!


Are you wanting the pyrite free of the matrix or just to clean up the pyrite? Just thought I would ask that for clarity.

I find that many shales tend to disintegrate when wetted after being dug up. This can vary from repeated rains to literally just dipping in water for 2 seconds.

Be careful about your matrix being immersed in a chem bath, test first...


~JO:)

30th Nov 2015 21:04 UTCScott DeLano

Thanks! I'll see if I can find someone around here with a "softer" abrasive and have them try it for me. I tried muriatic acid at one point and...nothing. So the shale doesn't contain carbonate materials. To answer another question, I am hopeful that I can get the black shale off the entire crystal so that I can shine it up but still leave it attached to the matrix on one side for presentation purposes.

30th Nov 2015 21:34 UTCBob Harman

03832280016018616056788.jpg
Pyrite nodules in shale and a 6.5 cm liberated nodule. Harding St. Quarry, Marion County Indiana. These nodules can easily be liberated from the shale matrix by soaking in soapy water for several days and then gently teasing the shale layers away from the pyrite with a sharp knife point. The nodules then are cleaned and shined up with a soft toothbrush. CHEERS.....BOB
 
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