FMF - Friends of Minerals Forum, discussion and message board

[pro_duo]

I found a pyrite specimen that has a shape of deltoidal icositetrahedron which is a typical shape of garnet crystal.
So, my question is......Duff Quarry, Logan Co., Ohio is the only origin has this kind of pyrite or not?

[Paul S]

According to the information from Mindat: https://www.mindat.org/loc-7910.html, pyrite can be found there, and nothing else that is close to pyrite chemically, nor in shape. I think that what you have there is genuine pyrite.

[pro_duo]

I did found the imformation from mindat.org before asking question here.
Thank you, Pual.S
In other words, Any places can find pyrite specimens that have deltoidal icositetrahedron shape?

[Paul S]

A deltoidal icositetrahedron is like a cube with all its edges bisected. Its not the most common form of pyrite, but I think its very well possible to find it that form, due to the cubic nature of pyrite. You do have a very nice specimen there, the pictures clearly show the deltoidal icositetrahedronly formed crystals.

[Pete Richards]

It's even more strange than these posts suggest, but it IS pyrite. There is some possible problem with nomenclature: I presume a deltoid icositetrahedron is the same thing that in the U.S is usually called a deltoid. If so, however, a deltoid icositetrahedron must have all of the cube edges modified by two symmetrically-equivalent faces, and in an uneven manner, which is to say that the edges of the new faces cannot be parallel to the edges of the former cube.

I have studied these Duff pyrite crystals in some detail, and there are many variants, but most of them have the diploid as their major faces. It is an icositetrahedron because it has 24 faces, but it is not equivalent to any common garnet form, because the faces are not symmetrically inclined to any of the axes. I'm sorry to do this, but without writing a 20-page treatise I cannot do otherwise. 24-faced forms of garnet have Miller indices of the form {hkk} where h and k are integers and h is greater than k. By far the most common of these is {211}. Pyrite has reduced symmetry, and the diploid is the so-called "general form", with 24 faces. It has Miller indices {hkl}, h, k, and l all different from each other; an example is {321}. The equivalent form in garnet, {321}, has 48 faces, and is an example of a hexoctahedron.

The dominant form on these pyrite crystals is, by repeated measurement, {22 10 7} (crystallographers are screaming impossible!), which is reasonably close to {311} but differs from it by about 6°. {311} is an icositetrahedron (or trapezohedron in American nomenclature) in the symmetry class of garnet, but the common trapezohedron for garnet is {211}. {311} is also a trapezohedron in the symmetry class of pyrite, and it is often seen as modifying faces on these Duff pyrite crystals, so it is clear that the dominant form is different from {311}.

These are remarkably strange crystals. They should not exist, by crystallographic theory, except in very strange circumstances. We don't know why they do. But they do.

If you want to dig into the past, I published on these crystals in the Mineralogical Record in 1987(!):
Pyrite Crystals from the Duff Quarry [Ohio] (by Steven C. Chamberlain & R. Peter Richards) 18: 391-398.

I'll append hereto a drawing of {211}, {311}, and a Duff pyrite {22 10 7) crystal, You'll be able to see the difference, but it will be subtle. Relative to {311}, the front set of four faces of {22 10 7} is a bit elongated in the horizontal direction. The side sets of four faces are "stretched out" vertically, and the top and bottom sets of four faces are stretched out front to back.

Crystals with this habit are found at several locations in Ohio near the Duff Quarry, but the Duff Quarry has been the most prolific source. Crystals with normal habits are found at the Duff Quarry only very rarely. I think there are a few other worldwide localities for such crystals as well.

[Matt_Zukowski]

Pete, I really like your thorough explanations. Thanks for taking the time to explain and illustrate.

[Antonio Alcaide]

Thanks to all of you for showing this kind of pyrite. I could not bear "the itch" so I managed to get a specimen. I have here, in front of my astonished eyes. It looks like dazzling golden garnets. Fantastic!!

Thank you again for your explanation -and questions-. Science -and pleasure to certain degree- begins in wonder. I will try to post a pic later.

Regards.

[Matt_Zukowski]

A bit of a tangent...sorry...but I just went to Pete's article in MinRec, and some of the pages are messed up. Pages 395 and 398 have been translated into and out of the binding, respectively, so that p395 is cut off on the right side and p398 is cut off on the left.

Anyone else have this defect in their set of MinRecs?

Just curious.

[Pete Richards]

[Matt_Zukowski]

In this case i think it might be better to have a complete, wonderful article than a rare defective copy of MinRec.

[Jason]

Dang pete..you da man...well done
Nice pics and spec. OP

[Screenname]

I can see your question has pretty much already been answered but I would like a little something to the topic. Pyrite is found in 4 different appearances, Striated, cubic, octahedral, and pyritohedral, some times occurring as iron cross twins. Even have been found as concretions, mammillated and stalactitic nodules and even fossils, or pseudomorphs would be the exact term, such as pyritized ammonites.