Learning Center What is a mineral?The most common minerals on earth Information for Educators Mindat Articles The Elements The Rock H. Currier Digital Library Geologic Time

Minerals by Properties Minerals by Chemistry Advanced Locality Search Random Mineral Random Locality Search by minID Localities Near Me Search Articles Search Glossary More Search Options

The Mindat Manual Add a New Photo Rate Photos Locality Edit Report Coordinate Completion Report Add Glossary Item

Mining Companies Statistics Users Mineral Museums Clubs & Organizations Mineral Shows & Events The Mindat Directory Device Settings The Mineral Quiz

Photo Search Photo Galleries Search by Color New Photos Today New Photos Yesterday Members' Photo Galleries Past Photo of the Day Gallery Photography

Discussions

💬 Home 🔎 Search 📅 Latest

╳Discussions

💬 Home 🔎 Search 📅 Latest

Groups

Recent Images in Discussions

Improving Mindat.orgPOTD: crystal faces of this gold crystal

4th Oct 2017 08:23 UTCHerwig Pelckmans

Dear Edward,

Congrats with your photo being the POTD. https://www.mindat.org/photo-15432.html

It's a nice specimen of crystallized gold, no doubt about that.

Would love to have it in my collection. :-)

However, it does not show cubic and dodecahedral faces.

It is a combination of cubic and octahedral faces, and thus a cuboctahedron.

When the octahedral faces are dominant, they are six-sided.

That most likely confused you.

Cheers, Herwig

Herwig Pelckmans

President of the MKA (Belgium)

4th Oct 2017 13:18 UTCNorman King 🌟 Expert

In this specimen the cubic faces are small, and some may not be represented in this view. If the cubic faces include the face at the lower left and middle right, then this is indeed dodecahedron modified by cube. Sides of neighboring cubic faces would be parallel to each other, which is what I see here. Dodecahedral faces having little or no cubic modification are still rhombic in shape, also seen here. In octahedron modified by cube, faces of neighboring cube forms are not parallel, but are off by 90 degrees. Octahedral faces that are little affected by the cube will be largely triangular rather than rhombic. Both dodecahedral and octahedral faces that are modified by cube may be six-sided.

But maybe I have misidentified the faces visible in this view.

4th Oct 2017 14:49 UTCHarold Moritz 🌟 Expert

I have to agree with Herwig here, cube-modified octahedral crystal. In the center is a 6-sided octahedral face. The visible cube faces around it are top center and lower left and lower right. Thus every other edge around the central, 6-sided octahedral face contacts another 6-sided octahedral face. The corners of the cubic faces point toward each other rather than their sides being parallel. If the cube face sides were parallel, then it would be a cube-modified dodecahedron.

4th Oct 2017 14:56 UTCAlfred L. Ostrander

I have to agree with Norman. The faces of the dodecahedron in combination with the cube are also hexagonal in outline. A helpful determining factor is the position of the threefold axis that comes through the corner of a cube. It also comes through the center of the face of the octahedron. It comes through the joining of three faces of the dodecahedron. What this means in the combination of the cube and dodecahedron is you can see three faces of the hexagonal shape of the dodecahedron rotating about this axis of symmetry rather than one hexagonal shaped face of the octahedron rotating around the axis of three fold symmetry. It is all based on the proper orientation of the octahedron or the dodecahedron.

I know I am pushing at the boundaries of the oft used term cuboctahedron (or several spelling variants) but this form does not exist in classic crystallography. This particular form is the combination of the cube and the octahedron. By the geometric definition of a cubeoctahedron, the two forms would also have to be in complete equilibrium. That is, completely equal in development. It must also be considered that you cannot index the cubeoctahedron with only one set of indices. You need two, the cube {100} and the octahedron {111}. For you who may be sceptics, look up the list of accepted forms in classic crystallography. However, I do accept the use of cubeoctahedral as a habit as described by Dana. Note the difference in the usage of cube and octahedron as nouns and cubeoctahedral as an adjective.

Many photos of minerals, particularly fluorite, are misidentified as cubeoctahedrons and that mistake seems to just drive the error further and further in it's incorrect usage. This is a case where a misidentified picture is not worth a thousand words. As many collectors just use a photo or drawing to identify crystal forms, a misidentified photo is dangerous. You have to get the orientation of the forms correct to all the axes of symmetry or it just doesn't come out right. The octahedron and the dodecahedron seem to be a sore spot in relationship to the cube.

I could say something like I am going to double or triple check the photo one more time but I have already done that. I wish I could see the crystal from several other positions and clarify in my mind for certain just where the auxillary axis of three fold symmetry really is but from what I can see, I have to go with the combination of the cube and dodecahedron.

Good job Norman on your thoughts in explaining the combination of these forms.

4th Oct 2017 15:30 UTCLarry Maltby Expert

While recently researching the combination of the cube, octahedron and dodecahedron I came across this clever chart. It shows various combinations of how cube, octahedron and dodecahedron faces can become six sided. However, this crystal is so skeletal that I find it very difficult to understand.

It looks like the top face is a cube face and it is surrounded by dodecahedral faces.

4th Oct 2017 15:36 UTCErik Vercammen Expert

I think the gold crystal is on the upper row, second from right, rotated 45° clockwise.

4th Oct 2017 15:41 UTCHarold Moritz 🌟 Expert

We've been here before. See https://www.mindat.org/forum.php?read,62,409263,409453#msg-409453

In particular see this image posted there by Eric Briggs: https://www.mindat.org/forum.php?file,62,file=69825

Gold crystal matches the combined cube and octahedral forms, second from left in image.

And the one Erik V. calls out in the post above.

4th Oct 2017 15:43 UTCNorman King 🌟 Expert

I thought the face facing up and forward at top middle, and somewhat toward the right, is rhombic. As I said, it is difficult to get the right perspective with only one view. If the faces at lower left and the narrow rectangular face at right (and somewhat upward and forward) are cubic, then my analysis is correct.

4th Oct 2017 15:44 UTCJohan Kjellman Expert

the faces with black indices are not seen but "behind the edge"

cheers

4th Oct 2017 23:53 UTCNorman King 🌟 Expert

Johan,

OK, if the "face" I show with the arrow (below) does not exist, or is an isolated dodecahedral face between two octahedral faces, then your markings are good. I guess gold crystals are well-known for being indistinct and distorted. I was also thrown off by the 1-11 face seemingly not being hoppered.

5th Oct 2017 03:00 UTCKeith Compton 🌟 Manager

Guys

Great to see such discussion on a POTD despite not being the best quality photo.

Very informative stuff

Thanks for the insight

Cheers

5th Oct 2017 07:27 UTCJohan Kjellman Expert

Norman, that face may well be there. What we see or not see on such a "bad" crystal is partly subjective, if we want to (over)generalize or to what level of detail we want to go. The crystal itself must not be perfect in terms of outer symmetry.

cheers

5th Oct 2017 11:59 UTCHerwig Pelckmans

Johan,

Thanks for making such an educational illustration. You showed exactly what I was talking about.

Norman,

IF that is a (small, elongated) face, it would indeed be a dodecahedral face.

But then in theory (due to the symmetry operations) there should also be one "on the left" of the (111)- face, and that is not the case.

Of course it might be "rounded away" or not have been there from the start.

In any case, the large faces we can all see clearly, are only cube and octahedron faces.

I feel the caption of the photo should be adjusted accordingly.

Cheers, Herwig

MKA (Belgium)

5th Oct 2017 15:37 UTCAlfred L. Ostrander

Theoretical models based on symmetry are fine and dandy for purposes of illustration. However, if this was a theoretical model it would not be distorted and would clearly show all the elements of symmetry along with 6 faces of the hexahedron, and all 8 faces of the octahedron and/or all 12 faces of the dodecahedron. Nature is not that kind. Every possible face of every possible form need not have formed in a natural environment.

I indicated I would really like to see the crystal from other perspectives. This would indicate better what faces are present and where all the threefold axes of symmetry are located. In the photo Johan presented, at the lower left he indicates he believes this is the (100) face. If so he is most likely correct. The caveat is that this face and the 2 faces behind it may be so distorted or badly damaged to nullify any identification as this a broken surface and not a face at all. The same applies where in black you have indicated the presence of the (11-1) face. Theoretically, if your other faces are correct, this is correct. But a projection does not necessarily mean that face is truly there, it only projects what theoretically could be there. Johan, as you said, "The crystal itself must not be perfect in terms of outer symmetry." If evidence is removed by breakage it is not evidence at all. Along with breakage, if no other faces of the cube are truly present other than the (001), there is a problem. And as I said, there is no requirement that a theoretical face actually occours in nature. I see a lot of other faces and edges that are rather rounded and indistinct. But, if Norman has properly identified an isolated face of a dodecahedron, then the larger faces surrounding it are correct as Johan has marked them. But what happens if this is just a strangely rounded or damaged edge? That is why I am cautious until I can see the crystal from at least one, if not more perspectives.

I still am hesitant to speak about what is really going on here with a somewhat out of focus picture, all the hoppering, possible damage and rounding of many edges. Hedwig, I am not one who can "clearly" see the octahedral and cubic faces. One cubic face, yes, but that is all. I am not talking theory here as to what might be present, I am talking what is physically present and can be seen from the photo. Call me a sceptic if needs be. So much for trying to do crystallography from a neat specimen but done from a less than ideal photo and with less than theoretical faces all present and neatly aligned. Seems to me to be too much along the path of trying to identify what amphibole a specimen might be from a poor photo and absence much other physical evidence. Should prognostication take the place of identification based on hard facts?

5th Oct 2017 19:27 UTCJohan Kjellman Expert

I have been playing a bit in a free program called VESTA

not at all perfect, but I strongly recommend this software if you want to play

you can pick up crystallographic files (AMS or CIF) online aswell. The trick to do irregular cubic crystals is to default them as triclinic and do each face separately.

cheers

5th Oct 2017 21:15 UTCHerwig Pelckmans

Alfred,

* My first name is Herwig, not Hedwig.

* I agree that our observations are based on a single photo and not on angles measured on the specimen in question. It would be interesting if we could get the current owner of the specimen to measure the angles between the faces.

Johan,

Good job!

I agree with you: VESTA is a great (free) program to draw crystals.

It is also very handy and fairly easy to draw twins with VESTA.

I'm still learning how to use it ...

On the other hand, Smorf is faster and as such very handy to make a quick first draft.

Yiou can use the program online at http://www.smorf.nl/draw.php

Type in the first letters of the name of the mineral, and if it is in the list the name will show up.

Click or select the name and the program will provide the cell data and some basic forms

Add a form or delete a form and then hit the calculate button to see how it adapts the drawing.

In short: an excellent little free program, created and shared by Mark Holtkamp.

Thank you, Mark!!

Cheers, Herwig

MKA (Belgium)

6th Oct 2017 02:34 UTCNorman King 🌟 Expert

Hedwig is Harry Potter's pet owl.

6th Oct 2017 15:02 UTCAlfred L. Ostrander

Herwig,

Please accept my apologies. It certainly was not intentional or meant to insult. Herwig is quite a fine name to be proud of.

And thank you for understanding what I was saying. It wouldn't take much to clarify this issue. One more definite face or a better indication of a three fold axis in proper relationship to the major 4 fold axis of symmetry would settle it thoroughly in a manner that no theoretical drawings or any number of possible projections at any angle of rotation can do. You wouldn't even need any interfacial angles.

And if the one small face that Norman pointed out is a face of the dodecahedron a description should not be limited to a combination of the octahedron and cube but should include the dodecahedron indicated as a minor modifying face.

As to projections by several of the new programs, does anyone know if the vertical c axis is kept in a vertical position? This would be very close or the same as the projections that Larry Maltby posted. I like the ability to rotate a projection but after drawing literally hundreds of projections by the graphic method after Penfield and Naumann, I do prefer returning any drawing to the standard perspective of the c axis vertical in the plane of the drawing and the perspective raised upward. Too many of these computer generated images seem to be skewed in regard to the c axis. This is more than just a nod to a classic position of the axial cross and the projection seen in many older texts. The c axis was always kept vertical for several very good reasons. Naumann and Penfield along with others established this standard along with a graphic method that made drafting (by hand) a vertical axial cross in a standard rotation rather simple. The classic crystallographers could draft these projections with just a ruler, a pencil and a piece of paper.

6th Oct 2017 16:22 UTCJohan Kjellman Expert

Alfred,

In the program I used (VESTA) you can rotate the image as you like. There are several standard buttons on top and one of them goes by default back to the "standard position" - I assume the classic one you referred to.

cheers

Mindat.org is an outreach project of the Hudson Institute of Mineralogy, a 501(c)(3) not-for-profit organization.
Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: April 25, 2024 11:10:50

Go to top of page