FMF - Friends of Minerals Forum, discussion and message board

[Antonio Alcaide]

On the Spanish side, Sergio Pequeño posted several pictures of goethites pseudomorphs after pyrite from Girona, Spain.

They come from a small occurrence, not known until now. Those crystals attracted my attention due to their habit: mixed forms with cubes, octahedra and possibly rhombododecahedra.

Reading Sinkankas' Mineralogy for amateurs I thought the combinations were cube and octahedron besides as cube and rhombododecahedron.

Now, Sergio and I doubt it. We have found more information -for example Dana's textbook-. And this article on Mindat: https://www.mindat.org/article.php/1140/Crystal+Combinations+in+the+Isometric+System

Both drawings are very similar: equidimensional combination of cube and octahedron and equidimensional combination of cube and rhombododecahedron. I suppose a matter of interfacial angles is involved so one can distinguish the two forms, isn't it?

Please, have a look to the following pictures and help us with these simple questions:

1) The procedure to distinguish both combinations in general
2) What do you think about what combination the goethites from Girona display?

Regards

[Edelmin]

Antonio,

Sergio's goethite crystals from Gerona are clearly combination of cube and octahedra as you thought at first.

In the rhombododecahedra, faces join each other in groups of four faces ([100] axe or groups of three faces ([111] axe). If they combine with a cube, the points where 4 faces meet are replaced by the cube faces but the points corresponding to the join of three faces should be visible as they are in the picture you provide.

On the other side, octahedra are formed by 8 faces that should appear in the combination with the cube while rhombododecahedra have 12 faces (you can see them again at the picture above but not in the goethite pictures where only 8 faces are visible besides the cube faces).

I hope this can help you and Sergio,

Best regards,

Felix

[Antonio Alcaide]

The simpler, the better (Ockham's law again). Thank very much, Félix. It was only the number of faces. Oh!!!

If I understand correctly:

1) Combination of cube and dodecahedra has six cubes and twelve dodecahedra.

2) Combination of cube and octahedra has six cubes and only eight octahedra. This is the case of the Sergio's goethites, indeed.

To avoid future mistakes, I will look for -or count- the number of faces of the possible combinations.

Regards

[Pete Richards]

Another way to look at it is as follows:

Consider the crystal oriented so that one cube face points straight up. Look down onto that face. Surrounding it you will see four faces, equally inclined to the vertical, that belong either to the dodecahedron or the octahedron. They will be inclined slightly differently depending on which form is correct, but this difference is too small to assess by eye.

Below these will be a set of faces, all of which are vertically oriented. If this is a cube-octahedron, there will be four cube faces in this orientation and (if the cube faces are smaller than those of the octahedron) they will be separated by horizontal edges between octahedral faces. If this is a cube-dodecahedron, the vertical cube faces will be separated by vertical faces of the dodecahedron.

Again, this can be seen in the crystal drawing above.

[Antonio Alcaide]

Thanks, Pete. I have just checked your method and certainly it is as you describe.

I am going to tell it to Sergio and correct the Spanish thread.

Regards