FMF - Friends of Minerals Forum, discussion and message board

[bugrock]

Hello,

Recently acquired a thumbnail specimen from the Laurium Mine, Keweenaw Peninsula,
Michigan that has small bright silver crystals on matrix with xls of other more common minerals such as quartz and epidote. Under the microscope there are a few little pockets of very fine blue-black flattened xls. These xls are mostly in a few pockets between quartz xls and not on the surface of the silver.

Without testing it may not be possible to pin down the identity of the black mineral. First thought of chalcocite (known from the location per mindat photos) but few images on mindat match what I am seeing. Acanthite seems quite possible and there are a number of images in mindat that are similar but many are labeled as "secondary" or "typical secondary" acanthite. I assume that means the mineral forms as microxls after collection of the piece.

The specimens I have is recently collected and the silver is very bright. Likely the piece was acid treated, very common for specimens like this, and often sulfamic acid is used. This raises the next question, could acid etching in sulfamic acid lead to a reaction with the silver to deposit acanthite? But if so, seems odd that there is no black tarnish on the silver xls.

Looking for forum members who have experience preparing silver specimens and also those who might know the chemistry involved when sulfamic acid encounters native silver.


Thanks,

George

[Peter Megaw]

Bugrock: There is substantial evidence that what people have long thought to be "secondary" or supergene acanthite is not that at all (See Sillitoe, 2008 or 2010...in Economic Geology) as silver is relatively immobile in the secondary environment...especially a low-sulfide system like the Keeweenaw.

I have limited experience with sulfamic acid, but I doubt sizeable crystals grow from it rapidly enough to be what you describe. I would venture that for starters if the silver is shiny, if this is a "cleaning product" the black species should be dull and pitted. If not, you're probably dealing with primary species.

Flat black crystals sounds more like chalcocite than "secondary" acanthite. If they are hexagonal plates that should eliminate acanthite as a possibility since acanthite is monoclinic...unless we're talking pseudohexagonal trillings. There are other platy possibilities like luzonite. Since you don't provide images we're speculating here, but it's worth more work