|Allanite-(Ce) - dark brown to black
crystal shapes, often with radial cracks around them
Dolomite - dull gray
Quartz - dull gray
Goethite and other iron oxides - brown, yellowish-brown
Allanite occurs most commonly as brittle "crystals" which actually have
very little organized crystal structure remaining. They retain the outward
shape and appearance of the original crystal, however. The word for
this is "metamict". At one time in geologic history the crystals were
slightly more radioactive, because they contained more uranium and / or thorium.
The radioactive decay of these destroyed the crystal structure. Today
the allanite crystals contain only small amounts of uranium / thorium.
The radioactivity is very weak. The word "weak" means what it says:
you may get a few clicks on a Geiger counter if you put the probe almost
directly against the crystals.
Allanite is an accessory mineral in many igneous rock environments.
Sometimes it occurs in larger amounts. Coarse-grained igneous rocks
can also house significant amounts of monazite and radioactive zircon ("cyrtolite").
If you live in New England, New York, northwestern NJ, or other igneous /
metamorphic rock environments, you probably have more radioactivity in and
under your house than there'd be in all the allanite specimens you'll ever
find on your trips to Franklin. (You cannot escape radioactivity;
it's even in your own body, from carbon-14 and potassium-40!)
Allanite is an interesting mineral that occurs primarily in the pegmatite
regions of the Franklin deposit. Allanite would not have occured
inside the actual orebody, only outside and along the contact zones.
Any time you find a rock with allanite crystals in it, you probably won't
find any good fluorescent minerals (though willemite and microcline do occasionally
occur with it... oh, and hydrozincite).
As a side note, willemite muscled its way into regions that weren't even
in the Franklin orebody. It found its way into the "pegmatite" zones.
Sometimes it even got into the dolomite! Willemite is so powerful that,
if it could, it would muscle its way out of this website and into your livingroom.
It would form exsolution structures with your decor. Everything would
fluoresce bright green.
Anyway, that's beside the point, since we're talking about allanite here.
Just saying, where you find allanite, you might also find willemite.
You just won't find certain things, like clinohedrite or hardystonite. Probably.
Allanite is appealing to micromounteres and species collectors. It
sometimes occurs with other rare-earth minerals, some of which are hard to
characterize because (like allanite) they lack any crystal structure that
can be analyzed with the typical methods.
By the way, the (Ce) after "allanite" refers to cerium. Allanite
can be dominant in cerium (Ce), yttrium (Y), or lanthanum (La).
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