Dunwich, North Stradbroke Is.

[Rej]

This will be a blog-style post but since I don't have a blog for this stuff, I'm posting here.  Hopefully this will be of interest to someone.

While looking into places to look for radioactive rocks, I came across information that the mineral sands at Straddie had monazite in them, which is a rare-earth-thorium phosphate mineral and is slightly radioactive, so I set out to investigate.

The car ferry to get across is a bit expensive for a trip like this but luckily one of the most concentrated areas is right next to the ferry terminal at Dunwich so I parked my car on the mainland at took a cheap pedestrian fare across for the day. For those who don't know, one of the main industries on the island (for another year or so anyway) is sand mining and they're after the titanium oxides (ilmenite + rutile). They ship the concentrates to Brisbane, remove the monazite and zircon and haul them back to the island to be mixed back into the sand for safe disposal. I guess that might explain the heavy concentration of black sand around the terminal, concentrates spilling from the trucks?

Here's what the beach next to the ferry ramp looks like, notice the black sand patches (and my little helper/explorer). The black stuff was about 1cm thick at the edge of the beach so easy to collect without too much silica contamination.



So is it radioactive? Well, first the baseline. On the boat ride over, I register 10 counts per minute on the SMB-20 probe or ~0.05 uSv/h, which is low but expected since there are no rocks around, just cosmic rays. On land it was more like 20 counts or 0.1uSv/h which is similar to what I get at my house.



Now, with the probe on the bag of collected heavy sands I get 61 counts or ~0.3uSv/h so about six times more than background radiation.



Now in case you're wondering if this amount of radiation is dangerous, should I really be putting it on my lap, the answer is No, it's not dangerous at all. Refer to this nice infographic to realise I've got the equivalent of 3 bananas there and it's considered normal background for many places on earth.... 

So while it is technically radioactive, why is it so low? Well first the monazite composition of the mineral sands is on average only 0.5%, AND thorium is a small part of the make-up of monazite AAAAND if you look at Thorium 232's decay chain, you'll see that for most of its transitions to other elements through radioactive decay, it gives off alpha radiation (the little "α" next to most of the arrows) and the geiger-muller tube I'm using only detects beta and gamma rays. (Don't worry I've got a alpha-beta-gamma probe in the mail )

Anyway with the main objective of the trip somewhat successful, here are some other things I observed. The rocks that are used as wave breaks are very interesting, with lots of pretty copper minerals.






I don't know if they're local (I assume not as most of the island is sand but I'm what do I know, maybe)

I also found this oxide-filled crack that gave a nice "high" reading on the geiger (120 counts or 0.55 uSv/h), maybe a bit of Uranium, maybe higher concentration of Thorium



That's about it. I still need to attempt a separation of components in the sand but it's going to be hard as specific gravities are very close for all the minerals except the silica.

silica 2.65
ilmenite 4.72
rutile 4.25
zircon 4.65
monazite avg 5.15  (range 4.8-5.5)

Maybe there's another method of concentration that doesn't require specialised industrial equipment. Anyone?

[FlashGP]

Hi Reg,

Good work.  Alpha radiation doesn't travel very far and is stopped by your skin.

That was s nice stone your little helper cut and brought to the meeting on Sunday.

Regards
Gordon

[Rej]

Thanks Gordon. Yeah once I get the alpha probe, I'll need to test without the bag as it would block alphas.

And the helper in the photo is my youngest (4), the one that cut his first stone is his older brother. He was so keen on coming to a guild meeting. I think he understood a bit of what was discussed, and did say he wanted to come back..

[Jimnyjerry]

Great blog.     Looks like you had a great time.

[Gemster]

Nice finds... I Spent a Year working over there and saw some nice rock Collections from there and Redlands area..
   While I was there I heard off some 'opal' in small seams somewhere inland from Dunwich.. I saw some sort of Blueish/Greenish Opal like seams in a Rock, a few years back and the owner swears it was found on the Island.. I Didn't have my Glasses with me . Perhaps it is/could be simular to Yours...?  And that there are Rocks on Straddie...

 I think  some of those Rocks came from the Mainland... Used to stop Erosion along the Foreshore in the 60's or early 70's... My Dad had something to do with the walls over there after the 1974 Brisbane floods...

The Rocks on the Seaway Wall here on the GC are Very Interesting Too...

  Gemster.....
 

[FlashGP]

Hi Rej,

Since he is keen, I came across a good design for Garnets called Lighthouse from Jeff Graham.  It doesn't have too many facets so cuts quickly.

if you like I can email you a copy.

[starsapphire78]

Very nice specimens, MMMMMM SHINNIES

[MrSydney]

StarSapphire you are driving me mad!  It is SHINIES!  Unless you are talking about the fronts of your lower legs!   

Really interesting blog and photo's!  Thanks!!

[starsapphire78]

oops I stand corrected

[Plutonium]

Looks like my post a couple of days ago fell off the internet.
Looks like some nice common opal.

Not everything you hear about alpha particles is true, it's more a good rule of thumb.
We were told how paper will stop alpha particles and aluminium foil will stop beta particles.
But I can demonstrate alpha particles passing through foil quite easily.

Your pancake tube won't be so good for fossicking but rather confirming a source.
You'll also want to consider calibrating it because they're typically ~5% efficient for alpha, then then also factor in the aperture of the tube.

If you're building your own counters, then you might want to consider building some other variations like a coincidence detector or a GM array which is much more useful for fossicking.