Removing shale matrix from pyrite?

Hello Everyone,
I have some pyrite specimens from SE Missouri, USA. They were found in a road cut through a shale layer. The shale seems to be an oil shale (it smells like oil when struck with a hammer). Some of the specimens still have the shale embedded among the crystal terminations. I have been able to flake off most of it with a dental pick but I cannot get it out of the smaller crevices. Does anyone have any tips or tricks to remove the shale and expose the smaller crystals?

Thanks & Regards,
Thomas Morrow
Bentonville, Arkansas

Hello,
Thought I would post an update. I am making slow but good progress with a heavy duty ultra-sonic cleaner.

Regards.

One possible addition to consider: use salt solution (NaCl) to assist in the ultrasonic dispersal of the clay. Changing any swelling clay layers to the sodium form should allow them to expand and soften. This won't work with illite, but may help if some of the layers are still smectite (esp. montmorillonite or beidellite).

Thanks, I will give that a try.

Regards.

hydrating and allowing the specimens to dry several times should aid in the removal of the shale encountered on you pyrite specimens. Have you tried soaking the specimens in HCL? Often pyrite is found in carbonate rich shales and the acid can degrade if not totally dissolve the shale.

Update:
1. The salt solution doesn't seem to affect this particular shale. :(
2. I have tried soaking for up to 2 hours in HCL. This doesn't affect
the shale either. It does remove the tarnish from the pyrite.
3. As part of initial cleaning I have been soaking the specimens in
a solution of dish soap and water and then drying. This seems to help
loosen the shale. I have been repeating this several times.
4. Here is the cleaning regimen I am currently using:
a. Soak specimens in solution of dish soap and water.
b. Dry specimens.
c. Flake off shale with dental pick.
d. Rinse and brush with nylon brush.
e. Repeat steps a-d several times.
f. Ultra sonic cleaner using solution of laundry detergent and water.
I run the cleaner continuously on a timer 30 min. on, 45 min. off to give the transducer a break.
g. When solution gets dirty. Change solution.
h. Dry specimens.
i. Flake off shale with dental pick.
j. Repeat steps f-i until specimen is cleaned.
h. Soak specimens in oxalic acid solution for 30 min. to 1 hour to
remove tarnish.
i. Rinse with water.
j. Nylon brush and clean with soap and water.
k. Brush wet specimen with old toothbrush and whitening toothpaste to
help shine it up.
l. Dry specimen. I now have a sparkly clean piece of pyrite.

This procedure is averaging about 2-3 days. A stronger ultra sonic unit might speed up the process but I am using a fairly heavy duty unit (the 1.5 quart heavy duty model from Harbor Freight). Any additional suggestions,
especially about ultra sonic cleaning solutions would be most welcome.

Thanks and Regards,
Tom

Tom, I call it "pickin' and grinnin'"....with some matrix, thats all you can do....there ain't no panacea!!!

A Low Tech Method
1) Place the pyrite in a small amount of water
2) Freeze
3) Thaw and lightly pick loose shale off.
4) Repeat freeze process as many times as necessary

Hi there!

Pyrite is dissolved(as far as I know...)only using Nitric (Azotic) Acid- HNO3.Feel free to use HCL (I think it only cleans the specimen). and try H2SO4-Sulfuric Acid.This awful "monster" solution is very dangerous,so be carefull!I have cleaned Quartz with Pyrite and no (visible) damage done to the Pyrite,and according to my knowledge the Pyrite is not affected Chemically by the Acid.


You'll need a specialist's opinion on what I'm gonna say now,but,probably,the tarnish effect by the HCL is because your speciments have a light film of rust upon them(if so,they should look more golden-red than golden-silver like Pyrite is).If you leave the acid act one day or so 4 water to 1 acid for one day or so,it may clean the rust AND the matrix.Two hours are short tim!

If you have specimens to sacrifce,try Aceton,tooth paste or even Nitro-Cellular Dissolver(you can find these in a wall-paint store).

To be more effective mechanically,use a tooth brush.Use scissors to cut the brush hair and shorten them to half their normal lenth.This will make them less elastic and harder.Fell also free to use a needle,Pyrite Hardness is 6,kitchen knive hardness is 5.5,a needle has to be less,so you don't damage the specimen.

Be careful with these nasty acid and use masks!
-Kostas.

Tom, I know you don't want to hear this but any contact with pyrite and acid (any type) is not good. It may look great when you are done but wait it will decompose. I have had the same problem with pyrite nodules fron New York. Sonication is the only thing that works. I use Alconox in the water instead of your dish soap, it is on the basic side. In any case I can not stress enough the use of acetone to dry the pyrite when you are done. Certain pyrites like those fron Spain or Peru are stable but most others will break down with time in a moist atmosphere. Since we are on the subject of pyrites, do you have any from your location you would like to trade? Al

Hi all!

Al,are you sure any acid will harm the Pyrite?I've used even HNO3(that is supposed to dissolve the Pyrite)and no damage done(ok,with the Nitric HNO3 I saw some troubles like change in color after a day in the acid,but nothing more.)HCL and H2So4 caused no (visible) damage to my Pyrites.If there is any sign that I could find as a clue my Pyrites are damaged,please let me know(an examination under a loope for example!)

By the ways,yes,I have some Pyrites for trade from Madan,Bulgaria.I will trade for intersting specimens of Quartz(since I'm building this kind of collection right now),but after 2 weeks or so,since now I'm very busy with my exams.Al,please contact me at daus_fr@hotmail.com,if you are interested :)


Cheers!
-Kostas.

hi,
just be careful of what lies underneath the shale...sometimes you dont want exposed what the shale is hiding...

cheers, joe

The main decomposion pathway for pyrite is via acid (acid run off from mines is caused by pyrite). Some pyrite doesn't even need the acid it will generate it itself (H2SO4) on contact with moisture. Any acid will cause the pyrite to break down from a thermodynamic point of view but kineticly this may be slow (years depending on the pyrite and its structure). You may not see any damage now but time is not on your side. If you use it let the specimen sit in bicarbonate solution to neutralize the acid then pure water. But again use acetone to dry it as good as possible. Al

It is a common misconception that neutralizing the acid emanations from pyrite will prevent or impede further decomposition. This is mistaken; neutralizing the acid will speed up the decomposition! If this sounds counterintuitive, consider the following: The sulfuric acid oozing out of the cracks in your pyrite is a byproduct of bacterial metabolism; neutralizing it does not kill the bacteria, it merely removes their waste products so that they can then flourish and generate more acid. A nice soak in HCl, on the other hand, is quite uncomfortable for the Thiobacillus critters in your pyrite. I soak my pyrites and marcasites in strong HCl and then give them only a brief rinse before mounting the newly shiny sulphides in a plastic box. Not a permanent solution, but they stay brighter a lot longer than the ones I tried to neutralize.

Alfredo, what you are saying is true, however, the bacteria is not always present. Thermodynamicly pyrite is not stable to acid. The bottom line is many pyrites in collection have decomposed because of moisture that generates acid even if the bacteria is not present. The key to this is moisture.

Hi all!

May I suppose a solution?First kill any (probably existing)bacteria using Antibiotics,then use a concetrated solution of Sodium Bicarbonate to neutralize any Acid.

Is that right?Could this be done?


Cheers!
-Kostas.

I believe the use of antibacterials have been tried. The sad thing is once decomposion starts on pyrite there is little you can do except remove it from the rest of your specimens. This is an age old problem, if you do a search you will find many things have been tried. This is a real problem for those who collect fossils since many of them have been replaced by pyrite. It is interesting that some pyrite as I have mentioned is very stable (like the spanish cubes). In the case of fossils, some museums use a weak solution of super glue after cleaning not that I am suggesting that for a mineral specimen. You can buy it on the internet its called PaleoBond. I have tried it on a very specimens that have already started to break down just to experiment. I first cleaned the specimens then treated with triethyl amine to remove acid then spoaked in acetone, that way water did not come in contact with the specimen. Then soaked it in PaleoBond. Now I have to wait to see if this stops the decomp.

Thomas,
The solution to your pyrite cleaning problem is not a chemical one. You need to get a sand blaster that is using glass beads and not quartz sand. You will be able to clean your specimens much faster with this technique. I have cleaned thousands of pyrite specimens with air abrasive techniques and it works quickly and well. The pyrites you are cleaning are not very valuable, so you don't even have to exercise much care in their cleaning. Find a metal finishing shop or a shop that has a sand blaster for cleaning sparkplugs and see if you can use one of their sand blasters. You might have to buy some glass beads though and change out their garnet or sand abrasive media for your work, but it will save you many many hours of work.
Rock

Is acetone completely safe to use on Silurian era (New York) Rochester Shale fossil fauna without harm? I am concerned that acetone submersion may dissolve the shale or damage the very thin calcitic outer shell material of a fossil.

As you can see from the attached image I have figured out how to clean pyritized fossils in shale.

I find these in western new york. Using the method below i have cleaned hundreds of them, and they still look nice and shiny.

What you need:

- 5 gallon bucket
- strainer
- rubber gloves
- face mask
- well ventilated space
- towel you can ruin
- nail brush
- throw away electric toothbrush $5
- Iron out ($14/bottle (dry powder)
- CLR $10-$20/bottle depending on size.

Cleaning method for round nodules of different sizes when you don't want to preserve the matrix:

1) Soak them for a day in water (soften up the shale)
2) Emerge them in a extremely concentrated solution of Iron out. fill a 5 gallon bucket with 3 inches of water and pour in at least 2 cups worth. once you stir it in, the water needs to become clear, or you need more water. soak the nodules in the Iron out for a few hours. Pour them into a strainer. Wearing gloves, cover them in dish soap and massage them together under running warm water. your want them to lightly grind against each other, this will cause most of the shale to fall off and not damage the pyrite. once the soap is all gone, dry them off, and rub each one with a towel (takes time but makes a big difference)
3) Repeat step 2 once more.
4) Soak them in CLR for a few hours. Wash them off good, and than buff with a towel. Shale might still be present in the cracks, if so use the pointy end of a tack to remove it, then soak in CLR again.

Method for cleaning odd shaped nodules when you don't want to preserve the matrix:

1) Soak them for a day in water (soften up the shale)
2) Emerge them in a extremely concentrated solution of Iron out. fill a 5 gallon bucket with 3 inches of water and pour in at least 2 cups worth. once you stir it in, the water needs to become clear, or you need more water. soak the nodules in the Iron out for a few hours.
3) using a nail brush or a electric toothbrush scrub the bejesus out of them to remove the shale, rinse constantly under water to see progress. use the end of a tack to remove shale from cracks/ around the edge of exposed fossils. You will need to repeat step 2 many many times until you have removed the shale.
4) Soak them in CLR for a few hours. Wash them off good, and than buff with a towel.

Method for cleaning pyritized fossils you WANT to leave in the matrix.

1) dip toothbrush in iron out and scrub. rinse briefly under water, dry and repeat, over and over and over. if possible set up your piece in a dish so that only the exposed pyrite is exposed to the iron-out solution, then scrub it every hour or so. change out the iron-out solution ever few hours. Once you have most of the tarnish removed, use CLR on it instead of iron-out.