Gamma Spectrometry Forum

I've read some time while ago that black sands are composed of the heavier mineral fractions of vulcanic and/or intrusive rock and may sometimes have elevated levels of radioactivity. And a specific place where I often encounter spots of black sands on the beach is on Sithonia peninsula, Greece (I fell in love with that place and I visit every summer since the first time I went there - it's really, really lovely).

Sithonia is a mountainous peninsula predominantly made of intrusive rocks - granites, diorites and granodiorite - except for the western parts. This means a lot of interesting samples to analyze - although most of the rock samples I've brought back and tested turned out to be rather low activity ones showing a typical thorium spectrum and (interestingly) low in K40 activity - guess the feldspar in granites has low potassium content.

This is the second summer I collect black sand samples from all around the peninsula and I've collected quite a few samples from there - basically every day there we drive to a different location, there are quite a lot of rocky beaches suitable for obtaining both rock and sand samples. It's basically combining the fun of going to a beach with the fun of doing some work for spectrometry purposes :) The previous year though, none of my black sand samples turned out particularily interesting - all of them were very low activity ones.

This summer it certainly got more interesting. That is - the first black sand sample gave out almost 60CPS from just a bit less than 50g of material inside the shield (room background being about 120-130CPS). Interestingly, I recently got some apatite from Brazil (about the same mass) - they were thorium-enriched and still had lower activity. So this black sand has higher activity than apatite, which was rather surprising. Ironically, the only photo I took of the black sand "in situ" was exactly from the location I took the sample - at a beach near the town of Sarti:



Normally, the black sand is localized at relatively small spots close to or in contact with seawater and forms thin layers, I guess the sea somewhat sorts the heavy minerals in a "mechanical" way. This one in particular was quite a large spot, with a relatively thick layer and had that dark-greenish color. I was suspecting that might be interesting, because....well, it looked somewhat different.

So the spectrum turned out to be a typical Thorium one:



Due to it's relatively high activity, it didn't require neither background extraction, nor a particularily long measurement time, just a couple of hours were quite enough.

Compared to the sand I previously tested from Vromos bay, Bulgaria (where a poorly recultivated copper mine was to blame), that one has like 4 times higher gamma activity. The Vromos sand though was Uranium-enriched, while this one is natural and Thorium and unlike Vromos where practically all the sand is like that, the Sithonian thorium sand is localized in a small spots near the water.

Apparently you don't need to go to India or Brazil for that, Europe has those too :) Well probably not as radioactive as them nevertheless.

Wondering what minerals in the sand are responsible for the thorium. Monazite? Zircon? Who knows...

Nice find, Milen. If you still go to the Black Sea (I also prefer the beaches in Greece and go only there the last few years), you may test some samples there too. I remember spots with black sand were quite often to see, but those early years I had no clue of gamma spectroscopy :)

Unfortunately, I rarely go to the Bulgarian seaside those days too. I did visit for a few days this year though. I guess though it would be difficult to find a "surprise" like that one there for one reason - our seaside is predominantly made of sedimentary rocks - limestone and sandstones and that goes all the way from Durankulak down to Rezovo. Even that part of the Balkan mountain that reaches the seaside is mostly sedimentary rocks. Igneous rocks are exposed to the seawater at relatively few places, mostly south of Burgas near Sozopol. So I don't expect to find dark sands with higher activity there, well at least that won't be easy. But well, we have a different kind of a "surprise" in the Vromos bay :)