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[Sante Celiberti]

Hello.

I would like to receive the experts opinion about the specimen shown below.

The cubic pyrite from Gavorrano often shows octahedral and diploidal modifications on the corners.
Among thousands of specimens I handled I only found this one with these unusual modifications.
I can see the three diploidal faces (with scattered galena) but, instead of the central flat octahedral face, I find a trigonal pyramid (exagonal if we count also the three small folds) tipped by a "virtual" octahedral face composed by sub-parallel facets.
I supposed that the three big faces of the pyramid could be trisoctahedral ones. But what the three small folds are?
Perhaps I'm completely wrong.
May you help me, please?

Thank you very much.
Sante

[Bob Morgan]

The crystal is a cube with small pyritohedral faces. The larger 'wings' that have a sprinkling of other minerals are trisoctahedral. There are two sets of shiny faces. The smaller three are trapezohedral, alternating with larger diploid faces.

If you want to know how those identifications are made, I'll be glad to explain, or you could check out a presentation at the Rochester Academy of Science, Mineral Section website.

http://www.rasny.org/Pyrite/
(link normalized by FMF)

This is a special specimen. I've never seen trisoctahedral, trapezohedral and diploid displayed together so prominently.

[Sante Celiberti]

Hello, Bob.

Thank you very much for your kind answer with precious information.
Now, with my very basic knowledge of crystallography, my ideas are a little clearer.
I visited once again your interesting and pleasing article on Huanzala pyrite habits, and I had the confirmation that many of them are also present in Gavorrano pyrite. As soon as I can I will try to publish some, relying on your kind assistance and on that of other experts.

A warm greeting from Gavorrano.
Sante

[Bob Morgan]

I look forward to seeing more of your interesting pyrites.

Bob

[Pete Richards]

I will offer a different interpretation of this pyrite crystal. The first question is the nature of the large "wings" with a sprinkling of galena. I believe these faces are diploidal, as Sante said. If they were trisoctahedral, they would be symmetrically displaced relative to the cube edge they modify, but they are tilted off to one side, which makes them faces of a diploid. The diagram shows this with the forms {331} and {321}, but these may not be the exact forms. This analysis only establishes the kind of form, not its actual indices.

The pyramid sitting on these diploidal faces is unusual indeed. For one thing, it has concave angles with the main crystal, while crystals normally are wholly convex objects. This probably results from its being formed in a second stage of growth, as a kind of auto-epitaxy.

The small faces of the pyramid appear to be nearly in the same plane as the cube faces of the main crystal. If they are co-planar, they are of course cube faces. If they are inclined, they must be pyritohedral faces, because they share a common edge with the cube faces.

The larger faces of the pyramid are probably diploids again. They could be trapezohedral faces, but if so their bases should be at 45° to the cube faces, but that does not appear to be the case.

Finally, of course, the octahedron is present, sort of, as what Sante calls a "virtual" face, composed of various other facets.

[Sante Celiberti]

Hi, Pete. How are you?

Thank you for this new reading of the specimen and for your very instructive drawings where the difference between the trisoctahedral and diploidal modification is clearly distinguished.
Tomorrow I'll take a look at the pyrite to see if the small faces of the pyramid are co-planar with the cubic faces.

See you soon. Stay well.
Sante

[Bob Morgan]

I think Pete is right about the wings, the larger covered faces - that they are diploid. When he asked about the small set of shiny faces, whether or not they were in the same plane as the cube faces, I also became curious about the larger set of shiny faces - if they are in the same plane with the larger wings.

The smaller set of shiny faces clearly are not extensions of the cube faces. Their shape in relation to the diploid faces do not continue the cube boundaries with the larger diploid faces. They are trapezohedral though, being in the zone from cube faces to the rough tip of the shiny pyramid where an octahedral face is missing. Their offset orientation is the effect of the adjoining shiny diploid faces.

[James Catmur]

This is a great discussion about forms, faces, etc. It is a great primer and could even be turned into a nice article

[Sante Celiberti]

Hello.

First of all I apologize if, due to my inexperience, my photos and description may have misled the experts who are interpreting these modifications.

What in the photos (but even de visu!) seem to be concave angles in reality are not: it is an optical illusion due to the clear change in brightness.
In fact the three diploidal faces of the pyramid are co-planar to the diploidal ones covered by galena, to the point of seeming their natural continuation.
(Is it correct to call these two diploidal sets as of first and second order?).

The three smaller faces of the pyramid have the same inclination as the three larger ones (so they are not co-planar to the cube faces), but with a different orientation (I would say that they rotate about 15-20°).

In the light of these new data, I'm curious to know what the three small faces of the pyramid might be.

Thanks in advance. Warm greetings.
Sante

[Pete Richards]

Well, that certainly fooled me! The photography is quite good, actually, but as you say this is a compelling optical illusion that might fool the eye, even if one were holding the specimen, not just looking at a photo.

Below is a possible solution to the puzzle. The Miller indices document what is in the drawing, but they may not be exactly right. My solution builds from the assumption that the diploid is {312}; if the diploid has other indices, the other form would also be different.

In any case, the small faces appear to belong to a trapezohedron, {211} in this interpretation. The left drawing shows the crystal in standard orientation; the right drawing is approximately in the same orientation as the image taken from one of the photographs at the head of this conversation. The bottom drawing is an enlargement of the relevant corner. The grey lines represent the transition from dark to bright in the photographs. The relative sizes of the faces are not quite right, but the angles between the edges appear to be pretty accurate.

[Sante Celiberti]

Hello.

Finally all the faces of this pyrite have been identified.
It's surprising how a drawing can exemplify an already clear explanation.
I'm pleased to note that the Pete and Bob's identification coincide.
Thanks to both of you.

Greetings from Gavorrano.
Sante