FMF - Friends of Minerals Forum, discussion and message board

[Niels Brouwer]

Interesting locality indeed, I don't think I've come across one of these before.

[kakov]

This is a fragment of the famous Ivigtut pegmatite.
We have an unusual fluorite: the red-brown concentric grains. It has a low content of the radioactive element thorium (Th).
The cryolite is normally white but darkens under the impact of radioactivity. Consequently, the color of the cryolite we see here is darker, practically black, close to the radioactive fluorite grains. It is lighter (only dark gray) in the center of the grains.
All the white in this piece is quartz.
Like in many Ivigtut pieces, we also see siderite (greenish brown rhombs, most obvious on the top).
The sulfides galena and chalcopyrite, also typical of the site, complete the panorama.

I´ve had this piece for over 20 years. Recently somebody has been so kind to cut it for me, whereby the constituting minerals got much clearer. I will soon get it polished. For this photo I simply put it under water.

Another piece with same paragenesis on Mindat: https://www.mindat.org/photo-324583.html