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Identity HelpAn exercise in crystallography

14th Aug 2017 13:18 UTCLarry Maltby Expert

I have some photos that I took several years ago of a Tom Rosemeyer copper specimen from the Copper Falls Mine, Keweenaw Co. Michigan. It has an unusual diversity of free standing micro crystals of copper. I hope that you guys will help me describe the crystallography prior to up-loading to the data base. I plan to eventually post about five or six photos here for your comments. All of the photos are from one specimen about (4.0 x 5.0 x 2.0 cm).
Figure 1. (FOV 7.2 mm)

14th Aug 2017 13:24 UTCPavel Kartashov Manager

Here is combination of cube and rhombododecahedron in almost equal development with addition of very minor octahedron faces.

14th Aug 2017 15:03 UTCJohan Kjellman Expert

I agree,

However there is some variable size of the faces:

crystal 2: no octahedron, small cube, dominant dodecahedron

then crystal 1, 3 and 4 with continous growth (size of octahedron and cube), no 3 being intermediate and best fitting Pavels description.

crystal no 5, distorted with large dodecahedron and rectangular shape of the cube.

cheers

14th Aug 2017 17:11 UTCDonald B Peck Expert

I agree with both Pavel's description and Johan's. I would include #4 with #3 as fitting Pavel's description, well. There is another xl between and below #1 and #2 that shows all three forms also.

The Forms are:

100 . . . cube . . . . . . . . . . . . . . . . . . . . shown as square faces

101 . . . rhombohedral dodecahedron . beveling the edges if the cube

111 . . octahedral . . . . . . . . . . . . . . . . small triangles cutting the corners off the cube

Great photo of a superb specimen. I hope that when you post it, you will include both photos. The one with the numbers will be a help to anyone trying to learn crystallography. I am sure you know the Miller Indices of the forms, but a small table similar to the one I included could help the uninitiated. Your inclination to include a little crystallography with the photo could be emulated by others.

14th Aug 2017 21:31 UTCEd Clopton 🌟 Expert

Great photo, and great caption info. I second Don's hope that more contributors will include substantive information in their captions that both supports their decision to upload a given photo and repays our decision to look at it.

14th Aug 2017 21:57 UTCRob Woodside 🌟 Manager

Please upload these to the galleries. Great Xls!!!

15th Aug 2017 02:50 UTCLarry Maltby Expert

Thanks for all of the comments. I plan to up-load all of these photos and summarize your comments on the crystallography. You guys are much more advanced on this subject than I am. Here are three more photos from the same specimen. Please let me know what you think.
Figure 2. (FOV 3.6 mm)

Figure 3. (FOV 3.8 mm)

Figure 4. (FOV 10.5 mm)

27th Aug 2017 12:30 UTCLarry Maltby Expert

Hi guys,

I have started to work on figure 3. The crystal on the far right is straight forward and appears to be a “text book” example dominated by the cube and the dodecahedron. However, the other two crystals have a strange horizontal indentation. Is this a sign of twinning or just an imperfection?

27th Aug 2017 13:31 UTCKeith A. Peregrine

What a wonderful collection of crystals, Larry. Reminds me of a piece I saw recently at the Copper Country Mineral Show. Looked like the end of a femer bone with a collection of cubic copper crystals on top. Other shapes were present, but I couldn't begin to describe them.

27th Aug 2017 15:18 UTCJohan Kjellman Expert

Larry Maltby Wrote:

-------------------------------------------------------

> Is

> this a sign of twinning or just an imperfection?

These are certainly spinel twinned (rotated 60 degrees around 111-axis) of the cube-dodecahedrons. Compare for example Goldschmidt vol. 5 plate 36 fig 79. It is similar allthough NOT with dodecahedron (110) but with (210):

https://archive.org/stream/atlasderkrystalltaf05gold#page/36/mode/2up

cheers

27th Aug 2017 17:36 UTCLarry Maltby Expert

Johan,

Thanks for the info. I am fascinated by the diversity of the crystals on this specimen both in form and orientation. As you can see I am in a little over my head. I am pushing to go as far as I can with this project. I do have a hard copy of the Goldschmidt volumes but I did not realize that twins could vary so much in form.

I will post a working photo later today to further the discussion. If I cannot solve the problem I will only upload the crystal on the right into the data base.

27th Aug 2017 19:52 UTCLarry Maltby Expert

After much effort to colorize the twined crystals I came to the conclusion that I was hiding many of the clues needed to best see the dodecahedron faces such as the striations. I did colorize the cube faces to show the 60 degree angle that you describe and the similar orientation to the crystal on the right. When I upload the finished photo I will add a paragraph describing the twinning as you presented it.

It also seems apparent that the upper part of the twin crystals are showing some flattening. This is best seen in the crystal on the far left. This may help explain crystal No. 5 in figure 1.

Figure 5.

2nd Sep 2017 14:55 UTCLarry Maltby Expert

I have tried to interpret the crystal shown in figure 2.

Figure 6.

This crystal shows the cube-dodecahedron modified by the octahedron. On the left is the natural crystal. The center photo shows the theoretical intersection of the dodecahedron faces and the photo on the right shows the corners modified by the octahedron. The octahedral faces are not clearly visible in the photo of the natural crystal but they are inferred by the steep angle of the upper termination and the offset at the top of the predominate dodecahedron face.

2nd Sep 2017 15:38 UTCAndrew Debnam 🌟

Hello Larry, I like the colour highlighting you did. It helps following the thread.

Andrew

4th Sep 2017 12:37 UTCHarold Moritz 🌟 Expert

Larry,

This looks like a fun project, but doing it visually can be very frustrating, especially for distorted crystals.

I think you are finding out why the goniometer was invented. I think if you are going to pursue this and not be "over your head" you need to take measurements. There is nothing like data for making solid conclusions.

6th Sep 2017 02:20 UTCDennis McCoy

Larry, I am not experienced enough to say what these crystal forms are or if they are normal for copper, but one possibility is that these are psuedomorphs.

6th Sep 2017 19:07 UTCLarry Maltby Expert

Andrew,

Thanks for the comment. I also think that the side by side comparison is helpful.

Dennis,

I think it is safe to say that these crystals are not pseudomorphs. Copper crystals in the form of cubes, octahedrons and dodecahedrons and in combinations of all of these are well documented in most mineralogical texts.

Harold,

I agree with your comment 100%. Distorted crystals are very confusing and data is required for decision making. I don’t have any plans to go further with this project. The only reason that I tried this was because some of these crystals were near text book quality and the forms were obvious using the photos only. The photo above is one theory of what that crystal form might be but it is not conclusive. I don’t plan to upload it to the data base.

This project was also an effort to see what could be accomplished using common software that most people likely have in their computers. I will describe how the colorizing was done and some of the interesting reference material that I found during research.

6th Sep 2017 19:27 UTCDonald B Peck Expert

I believe the forms shown as 1. red are {100}, the cubic form; 2. yellow are {111}. the octahedral form which is usually quite small for copper xls; and 3. the green are probably {101}, the dodecahedral form. There are other forms that bevel the edges of the cube, but they exist in pairs and there seems to be no indication of such here. However. a confirmatory angle of the 101 on the 100 would be helpful. It should be 45^o ( over the edge of red on green). Or the angle of 101 on 011 should be 60^o (over the edge of green on green).

9th Sep 2017 14:03 UTCLarry Maltby Expert

Hi Don,

It would be nice to attempt to check the angles that you suggested but I no longer have the specimen in hand. I photographed it in 2011 and returned it to Tom Rosemeyer. I recently came across the photos and became interested in the crystal forms. Even though we are restricted to visual inspection only, some great comments by Mindaters have revealed useful information about the cube-dodecahedron-octahedron sequence in the isometric crystal system.

On the Goldschmidt pages that Johan referenced above, vol. 5 plate 37 fig 108 shows a crystal that has a large octahedron over the dodecahedron face. It appears that the interpretation of the crystal in Figure 6. above is feasible but hard to verify with data and, if there is a large octahedron, it does not show well in the photo.

Figure 7.

9th Sep 2017 17:20 UTCDonald B Peck Expert

Larry, The colored figure on the right (third from left) certainly looks like Goldshcmidt's Fig 108. After taking another close look at your uncolored photo (the one on the left), I am not so sure that there is not another form between the (100) faces of the cubic form, {100}, they look (sort of) like they are paired, and without interfacial angles, the most probable form would be {102} or perhaps slightly less probable, {103}.

Don

10th Sep 2017 15:53 UTCLarry Maltby Expert

Don,

I think I see what you are referring to. The crystal is more complex than the simplified version that I show in Figure 6.

With the help of Mindat experts I have learned a lot about these crystal combinations. Here are some excellent references that I encountered within Mindat:

This is a thread started by Saul Krotki in January of 2010 that is loaded with information on this subject including a discussion on software for illustrating crystals.

https://www.mindat.org/forum.php?read,6,166613,167124

This is a Mindat article by Dean Lagerwall on Crystal Combinations in the Isometric System, last update, October, 2011.

https://www.mindat.org/article.php/1140/Crystal+Combinations+in+the+Isometric+System

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