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GeneralHelp needed with crystallography

17th Nov 2014 16:10 UTCLarry Maltby Expert

I have attached a photo that I took some time ago for Tom Rosemeyer. It is an amazing grouping of copper crystals on copper. Please help me understand the crystallography. Numbering the crystals 1 thru 6 from left to right, here are my guesses.

1. Cube with octahedron and dodecahedron faces?

2. Twinned, what law?

3. Twinned, what law?

4. Almost a perfect dodecahedron?

5. Cube with octahedron and dodecahedron faces?

6. ?

For as long as I have fooled around with mineralogy, I am still weak on crystallography. Please confirm or correct. Thanks.

Copper (FOV 7.2 mm) Copper Falls Mine, Keweenaw Co. Michigan

17th Nov 2014 16:31 UTCJohan Kjellman Expert

1, 4 and 5: cube + dodecahedron + minor octahedron

2 and 3 looks like small cube faces connected through straight octahedron edges, i.e. "linear dodecahedrons", the octhaderal faces are hollow. (hope you understand)

6 is probably just like 1, 4 and 5 but istorted/incomplete

cheers

18th Nov 2014 00:33 UTCLarry Maltby Expert

Thanks for the info Johan.

Do you mean that 2 and 3 are twined but in a linear fashion?

Larry,

18th Nov 2014 10:58 UTCJohan Kjellman Expert

my impression is that they are not twinned.

better decription: skeletal crystals plain octahedra modified by small cube faces, however the major octahedral faces are gone or never formed so just their edges remain/stand out.

cheers

18th Nov 2014 16:52 UTCEd Clopton 🌟 Expert

Cubes and octahedrons are geometric duals: each has a face that corresponds to a corner of the other. When they combine, square cube faces replace corners of the octahedron or triangular octahedron faces replace corners of the cube, depending on which form is modifying which.

The dodecahedron has a face for every edge of a cube and a corner for every face. When they combine, long, narrow dodecahedron faces appear to shave the edges off of a cube, or little square cube faces blunt the corners of a dodecahedron.

These relationships explain all the forms seen in your copper crystals. I don't think these crystals are twinned, and I think the "missing" octahedron faces Johann mentions are present, just dimly lighted.

To me, crystal #4 appears to be mostly dodecahedral with a large cube face toward the camera and two smaller ones facing lower left and lower right, and triangular octahedron faces facing toward 3, 6, and 9 o'clock. #2 is in the same orientation but with much smaller cube faces and tiny or no octahedron faces. #5 is mostly a cube with rectangular dodecahedron faces along the edges and nice triangular octahedron faces at the corners. #1 is intermediate between #4 and #5, having all the same forms present but proportioned a little differently. #3 appears to be a distorted version of #2, and #6 is incomplete and difficult to orient from this single view.

Cool specimen--thanks for sharing it!

18th Nov 2014 19:53 UTCJohan Kjellman Expert

I agree fully with ELC.

I see now that 2 and 3 are also variants of the same theme as 1, 4 and 5.

I venture to say that 6 is too: a rectangular cube face "on top", surrounded by three large dodecahedra, and possibly a really small triangular octahedron at 3 o'clock, only these faces are present/distinct.

cheers

18th Nov 2014 22:54 UTCLarry Maltby Expert

Thanks Ed and Johan for the discussion. I learned a lot from this. Here is another photo showing more of the crystals on the same specimen

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