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?
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.