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[Josele]

A few weeks ago I received a calcite crystal on dolomite from La Florida mining area (Spain). The crystal shows faces of a polyhedron combination: flat rhombohedron, scalenohedron, prism, “normal” rhombohedron and maybe any one more.

It's one of those specimens that you never get tired of turning around and around, identifying the faces of every polyhedron and discovering new details.

Besides evident large faces there are some small faces that I'm not able to recognize.

Can you help please?

[Bob Morgan]

For the thin face below the {012}, I suggest the {011}. The bowtie 2 faces below that are a scalenohedral pair perhaps the {321}. If you could draw the crystal using a crystal drawing program, you might be able to get matching angles. Peter Richards could confirm the faces much better than I. I hope he'll pick up on this.

[Josele]

Thank you Bob, you are always ready to help.

At the moment I have not a program to deal with crystals. It's long time ago that I miss this kind of application.

[jose luis jara jara]

Go precious Calcite crystal, congratulations

[Pete Richards]

A crystal like this presents an interesting challenge! Josele, your measurements are helpful, even if they may not be very precise, being made with a protractor. The assignment of the major forms to {012}, {211}, and {100} seems very likely to be right. Since the thin form lies between (012) and (010), it must be a negative rhombohedron. {011}, suggested by Bob Morgan, is a very uncommon form for calcite, unlike its positive equivalent {101}, the cleavage rhombohedron, though it would be in about the right place. Two common negative rhombohedra that are nearby (in terms of angles) are {045} and {054}, and {045} is in fact inclined to the c-axis 12°2' more than {012}, so this is most probably the identity of the thin face, entirely consistent with Josele's measurement.

The bow-tie shaped pair of faces are more difficult. Given their position on the crystal, they must belong to a negative scalenohedron, one with indices of the form {hkl}, with h<k. There are many of these, and here there are two angular degrees of freedom, not just one. Some experimentation with SHAPE produced a number of candidates, of which I chose {162} because it produced a bow-tie most nearly with the right shape to correspond to Josele's photos. It also fits with Josele's qualitative assessments of the angle between this form and {045}, and the angle between the two faces of the bow-tie.

So {162} is a reasonable guess, but it may well be wrong. A more certain solution would require goniometric study, which could not be done on this crystal.

The left drawing shows a crystal with these forms in the standard clinographic projection. The right drawing is oriented so that the face (045) is in the plane of the drawing. The red triangle indicates the position of the positive end of the three-fold c-axis.

[Josele]

Pete, thank you very much for your help. That's more than an elaborate response, it is a master class lesson! It's amazing your ability to draw conclusions from a few simple and imprecise data. A pleasure learning with teachers like you.

Drawings are very much like what I can see having the piece in hand. What a nice tool when one has the knowledge to run it properly! I have neither the one nor the other. That’s why I often use Smorf crystal database (thanks Mark Holtkamp!) to identify and/or check crystal features. It has a quite large collection of calcite forms.

Also thanks for the spell corrections of my very poor English.

[fena kuiperi]

I totaly agree with Josele: Pete thanks for such a great explanation about how to understand these crystal forms!

[Bob Morgan]

Thanks Pete. I knew you were the one to get this done.