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[prcantos]

Hi. I have just posted a pyritohedral-like pyrite crystal from Respine Mine (León, Spain), showing that it is in fact a diploid: 2014 - International year of crystallography: for example, pyrite.

But this specimen has another surprise. It has some pseudopentagonal faces, consisting of two diploid faces, that show another unidentified triangular faces between the diploid trapezoids. They are usually limited by parallel striae sets.

I can't find any crystallographic explanation for these triangles. None of the isometric system forms can produce this shape combining with the diploid nor the pyritohedron.

Can you identify this triangular face, or give some explanation of its origin? Thank you!

[Pete Richards]

Hi Pablo,

I think the triangular faces are those of the pyritohedron {210}. The diploid is a general form, so it can take any orientation that does not make it a special form with a reduced number of faces or a special relationship to the axes (e.g. trisoctahedron). In the drawing, I used the diploid {542}. It may not be exactly the right diploid, but it does make the pyritohedron have triangular faces and is approximately like what your photograph shows.

The main point is that a pyritohedron, when combined with a diploid, can have triangular faces.

[prcantos]

Thank you, Peter! That's right.

I couldn't understand it because my combined pictures didn't fit... but it was because of decimals.

[Pete Richards]

Incidentally, as far as I understand, the crystallographic wisdom is that the common pyritohedron on pyrite is {210}, and the complementary form {120} is rare or not found. The main evidence for this seems to be the direction of striations, but this is somewhat circular since the striations are oscillatory combinations of pyritohedron faces. I don't know if x-ray studies have been done to verify this conventional wisdom....

It is also true that at a structural level the {210} and {120} planes are different, so it is entirely possible that they would have different growth rates and therefore one would be favored over the other.

[prcantos]

I didn't consider before if the forms in this sample are positive or negative, but it is another interesting topic to study with these Respina pyrites.

Here you have another specimen from the same mine. It is a bigger crystal. One of its faces shows sharp striae which are orthogonal to the longest edge, so it looks like a negative-pyritohedron face. But a set of smooth oblique striae is also present... Anyway, may be it is unfinished...

The second picture shows another face of the same crystal. Here the smooth striae are marked in every 'subface' of the pentagon. Each diploid face has one set of oblique striae. But the 'pyritohedral face', though, presents both sets together!