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Questionable Rhodostannite

Posted by Keith Compton  
Keith Compton March 26, 2010 08:46AM

First I would like to question the validity of the following Rhodostannite specimens:
262006; 247200; 261298; and 247201

In additon, I question the stated field of view in each - in one case (photo 262006) the stated field of view is 1.1 mm ..... now that is much smaller than the copyright symbol underneath the photo - Perhaps the field of view should be 1.1 CM. The photos do not look like they have been taken through a microscope etc, which is what would be needed for this field of view.

Similarly with photos 247200 (1.8mm) photo 261298 2.2. mm and photo 247201 (1.8mm)

Secondly the xl system appears to contradict the stated crystalography of Rhodostannite. As I have stated on other forums and in other messages - I am not a crystallographic expert by any means- but they just don't look correct to me, based on what I have read on this mineral. If they are then perhaps somone can explain it too me.

In each case I think that the Rhodostannites look more like micro Galena xls.

If one of these collectors could provide some form of positive ID (other than a visual id), or if some other mineral expert can explain how they are in fact Rhodostannite, then I am very happy to stand corrected - but visually they just don't seem right.

If I am wrong I apologise unreservedly in advance.

Over to the experts ...


Edited 1 time(s). Last edit at 03/26/2010 08:47AM by Keith Compton.
Lefteris Rantos March 26, 2010 10:58AM

Concerning the xl sizes and fields of view, I can confirm they ARE tiny and the FOV's (1.1mm, 1.8mm etc.) are probably correct - these are microphotographs. I have a similar specimen, and the xls are really that small, perched on large Andorite xls.

Concerning the ID, I can't say much as I have never seen a real analysis for any of these.
I only have to comment that I have seen such specimens labelled as "Ferrokesterite" at least once. Ferrokesterite and Rhodostannite have very close chemistry and should appear similar (if not almost identical) visually.
Is there any detailed and reliable analysis to prove this ID??

Keith Compton March 26, 2010 11:13AM

Surely field of view refers to the whole picture not the xls themselves

Jeff Weissman March 26, 2010 12:49PM
Appears completely consistant with specimens I have seen - minute octahedral-appearing crytals on stannite, crystals about 0.025 cm (0.25 mm) across
David K. Joyce March 26, 2010 12:58PM
I once had a specimen of tiny octahedra on andorite, with small faces truncating only some of the "octahedron" points. I was not convinced it was rhodostannite and so had one of the crystals analysed. It was Rhodostannite.
Alfredo Petrov March 26, 2010 05:38PM
I have 3 such specimens in my collection, probed, of slightly differing habits from simple pseudo-octahedra to complex, and they are rhodostannites. Some tiny crystals were also confirmed by PXRD (those no longer in my collection of course). They seem to have been found only a couple of times at the San Jose mine in the 1970s, on rather unusually flat andorite crystals, ranging from about 0.1 to 1 mm size. The more complex rougher andorite crystals found from the 1990s onwards don´t seem to have any rhodostannites on them, just pyrite, stannite, ferrokesterite, etc.
Marko Burkhardt March 26, 2010 08:02PM
Hello Keith!

The field of view in picture 262006 is really 1.1mm (+/-0.1mm) and not cm! Yes, the resolution is better than the ones of normal microscopes, because this photo is taken by using a bellows and very special objectives.

I contacted the seller of this specimen for more information about analysis. Perhaps he can tell more.
Maybe the photo is a little bit to blue. I will try to change this by using other lighting technologies in the future.

Keith Compton March 26, 2010 09:29PM
Hi Guys

Thanks for that

and thanks Marko for confiming the field of view - well done on the pics in that case.

Still not sure how tetragonal gets to look like octahedral but will do more research on this

David K. Joyce March 26, 2010 10:12PM
Hi Keith,

Rhodostannite crystallizes in the tetragonal system. There are simple forms in the tetragonal system that can look very much like isometric forms. Tetragonal prisms terminated by the pinacoid can look very much like cubes, if their length to width ratio is similar. As well, tetragonal pyramids with low angles can look very much like octahedra. Have a look in an old Dana Text or book on crystallography and you'll see lots of example of tetragonal crystals that appear isometric, at first glance.

The thing with the rhodostannites is that they look like octahedra but are really pyramids. Some of them even look like cube-octahedra because some of the points of the "octahedron" are truncated by the pinacoid. The thing is, though that only TWO points, at opposite ends are ever truncated by a pinacoid in the tetragonal system. That is what you see with rhodostannite. In the isometric system, all of the points would be or could be truncated by similar-looking faces. In the isometric system it would be the cube truncating the octahedral points.

Does that help, at all?

David K. Joyce
Gerhard Niklasch March 26, 2010 10:23PM
Could multiple twinning be involved, similar to (among others) Cumengeite?
Several examples in the Rhodostannite gallery appear to show basal pinacoids in more than one axis direction, just like the representative picture on the Cumengeite page shows intersecting dipyramids along all three axes. For comparison, an untwinned truncated dipyramid of Cumengeite looks like this.

Cheers, Gerhard
David K. Joyce March 26, 2010 10:54PM
It does look like some sort of twinning in some of the pictures.
Marko Burkhardt June 24, 2012 08:06PM
I get analysis data for my Rhodostannite sample:
S: 54%
Cu: 16,6%
Fe: 8%
Sn: 21%

I guess it really is Rhodostannite.
Any other opinion?
Rob Woodside June 24, 2012 09:04PM
Thanks Marco!!!
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