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[Jim Robison]

I recently acquired a Kalahari manganese field (KMF) calcite, exact location unknown. It appears to exhibit some type of unusual twinning faces, as the attached photographs illustrate. On each face of the normal rhomb is a small four sided "pyramid" perhaps 1-2 mm in thickness at most (at its center) and tapers to zero thickness at the corner of the rhombs. Each triangular face starts at two rhomb corners, and 'terminates' in a much smaller rhomb in the raised center. (size of central face varies from a small elongated face to a larger one, all exactly parallel to the normal rhomb face.

I suspect some type of twinning. The KMF generates some very unusual calcite morphologies, and I'd sure appreciate someone knowledgeable about such things to tell me what I have, and any suppositions about mode of formation.

In addition, if anyone can tie this to a specific mine that would be most helpful.

Thanks,, Jim

[Pete Richards]

Your description does not indicate twinning - the middle rhomb you refer to is probably the cleavage rhomb {10-11}. It is commonly modified at the top by shallower rhombohedra and scalenohedra, and at the bottom by steeper scalohedra, isolating it in an "island" as you describe, with its edges parallel to the main crystal edges.

However, your images appear to show a flange on the right side - a thick tabular projection that appears to mirror the morphology of the main crystal, but with distortion to be sure, t make it tabular. This suggests a twin on {0001} with the "bottom" half wrapping up to invade the "top" space. This is not uncommon with calcite.

If you can find it, have a look at my article on calcite twinning in Rocks & Minerals 74 (1999) and see the diagram and photo Figure 12. I think this is the same thing I see in your images.

[Jim Robison]

Pete

Thanks for your quick response. I looked back at the 1999 Rocks and Minerals. Figure 12 may describe in part what I am seeing on the piece. I can see what appears to be one flange on the back side of the main rhomb. The difficulty comes I am seeing in that there must be several thing happening on the rock. There is one big rhomb, which probably has a penetration twin.

But, there is also something happening on each of the rhomb faces on practically every clear calcite crystal on the specimen, even on crystals no more than 3-4 mm in size.

I've roughed together a sketch of a typical, though not to scale rhomb view (angles are undoubtedly off, but you will see what I am looking at). Each rhomb face has four truncated triangles tied to the four corners of a face, and the four come together in the center of the rhomb and terminate on a raised new rhomb parallel to the outside edge. The whole affair slopes up from the sides at a very slight angle, so that the feature is at most 1 or 2 mm high in the center.

[Pete Richards]

Jim,

If I understand your sketch correctly, I think it still fits the model I suggested, though the flattening is perhaps an indication of the "flange twin" I referred to previously. Attached is a SHAPE drawing showing a crystal of modified rhombohedral habit, with the "raised rhomb" that you describe. The four faces that surround and raise each rhombohedral face belong to two different scalenohedra, one shallower and one steeper than the rhombohedron.

No twinning was needed to describe and draw this crystal.

But the crucial question is: Does this drawing look like what you are describing?

[Tobi]

[Jim Robison]

Pete

Thanks for providing the SHAPE drawing. Yes, you have captured what I am seeing. I just spent about a half hour going over the piece with my scope. Aside from the major difficulty in focusing on a surface which is transparent, I was able to move the piece enough to get some clear face reflections. The dominant face is the one including the flat 'cleavage rhomb' and the two adjoining scalenohedra. As I looked closer, I did find in a few places the other numbered faces, and found several spots where the cleavage rhomb was absent and the scalenohedra came to a pyramidal point. Also found a number of unusual half terminations on some of the corners, and found the same pattern repeated on some of the golden yellow micro-crystal cluster.

In all, a very complicated rock, with I suspect some twinning of a few rhombs going on as well. I think what is needed is for me to bring the piece along to Tucson, and see if we can get together and let you have a hands on look.

I have never seen this pattern before, maybe because I didn't look close enough, or maybe because these were so obvious. Is it common? Thanks for your help to this crystallography challenged collector.


Tobias

Thanks for suggesting Wessels. That was my first guess, looks like I need to put the rock in front of a Kalahari expert to get a confirmation. The piece came out of the Ulrich Bahmann collection via Collector's Edge. I'm going to start there.

[Debbie Woolf]

If you have the Erongo edition of Mineral Record Uli Bahmann's email address is publicised, why not contact him direct ? I was going to suggest N'Chwaning II as I have seen similar specimens contributed to this mine.

[Jim Robison]

I received additional information from Collector's Edge. The calcite is from N'Chwanning II.