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cleaning millerite

Posted by Mark J. Sigouin  
Mark J. Sigouin January 17, 2012 11:04PM
A question for the chemists.

Last Fall, I participated in a collecting trip to the Gap Nickel Mine, Bart Township, Lancaster County. The trip leader and pretty much all the participants were convinced that millerite specimens would not exist because chemical weathering would have eliminated all the millerite by now. After a day of picking up metal containing rocks, I toted a large yard rock home. It was a crumbly cobble. I placed it in my garden mainly to use the acid drainage from the rock to discourage grass invasion of a flower bed. It worked well.

After some snaps of freezing weather the cobble began to disintegrate. Much to my surprise, my granddaughter pointed to a shiny vein coating the outside edge of a part of the cobble. Upon close inspection, I saw the acicular crystals of millerite smiling up at me. See the picture.

Here is the question;. This vein is sandwiched with brown/red rusty crusts that only come off a little bit when soaked in super iron out. You can see a bit of the coating on the left portion of the mineral. Any ideas on how to get that coating off? Soaking it in Muriatic Acid or Wood Bleach?
open | download - millerite 2.jpg (49.8 KB)
Rock Currier January 18, 2012 08:34AM
I think you are going to have to experiment using different acids. You of course will need to find a small piece to experiment on.

Rock Currier
Crystals not pistols.
Harjo Neutkens January 18, 2012 12:29PM
It looks like it's a bit of calcite vein with limonite.
If this is the case, dilute (15-20%) HCl (from the drugstore) will do the trick, but make sure you thoroughly rinse and the specimen with water before putting it in the acid. Also make sure to thoroughly rinse it afterwards, and soak it in water about three times the amount of time it has spend in the HCl.
Reiner Mielke January 18, 2012 06:39PM
If you try to remove the crust on the top and bottom the whole thing is likely to fall apart on you. I would leave it as is.
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