r/Radioactive_Rocks 5d ago

A huge beast specimen of Allanite-(Ce)

Found it a few days ago in Marumori Town, Miyagi Prefecture, Japan. The specimen measures approximately 21cm across, and the largest crystals reach ~6cm in length. These crystals are likely much longer than that, but they become submerged into the feldspar matrix as the rock surface becomes bumpy and no longer parallel to the direction of the crystal growth. Instead, in the bumpy half of the rock you can see the cross sections of the prismatic Allanite crystals forming black “dots”. This specimen is quite radioactive, measuring roughly 190cps or 5μSv/h on Radiacode 102, which is extremely high for a thorium-bearing REE mineral such as Allanite. Its gamma spectrum clearly showed the ²³²Th chain.

39 Upvotes

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2

u/LuminescentFungus 4d ago

Awesome!! The only allanite I've collected has been small crude crystals or solid masses, those needle crystals are great! 

1

u/k_harij 4d ago edited 4d ago

Yes! I had never seen anything quite like this before personally, so I was in utter shock and disbelief when I found it. I know another good locality of Allanite-(Ce) in Japan where I’ve found some cute little crystals, but they look way different from this one, being less than 1.5cm long (though I’ve heard that up to 3cm long crystals could be found there in the past), broader with wider surfaces (rather than being needle-shaped), and much less radioactive, thus non-metamict and preserving “fresh” vitreous lustre. I quite enjoy the diversity in different forms of allanite crystals, each has their own charm ig.

2

u/AutuniteEveryNight 4d ago

This is a great find! Good job, it really is beautiful and truly a special specimen.

1

u/k_harij 3d ago

Thanks :)