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[Elise]

My other find just unpacked from Springfield is that of a few slices of corundum crystal with overgrowth of spinel; I believe from Burma -- no notes, I was rushed buying these at the end of the show and found out what happens when you are caught not wearing a tag (Marty told me to SIT! until Jeff gave me one). Anyway, I read about these somewhere I can't recall at the moment, but what is the difference between this and a pseudomorph? Jeff Scovil showed me some pictures of pseudomorphs which I believe he said are not all the way through. I should probably check my facts before asking, but thought it might spark some interesting explanations never-the-less.

Best wishes,
Elise

[alfredo]

Looks like interesting stuff, elise!

Whether they are pseudomorphs or not depends on the timing of events. If it originally grew as all corundum, with the outer layer later replaced by spinel, then you would call it a "spinel pseudomorph after corundum", or the pedants might say "partial pseudomorph of spinel after corundum" (as you already noted), since the replacement doesn't go all the way through. If however a corundum grew first and was later overgrown by a layer of spinel (which is what I suspect happened here, because of the sharpness of the corundum boundary), then it's an overgrowth, and I personally wouldn't call it a pseudomorph, although some might. (What was the definition of "perimorph" again? i forget.)

[Elise]

[mmauthner]

Hi Elise,
Yogo Gulch sapphires, I believe, commonly have an overgrowth of spinel (pleonaste). I have a slide of that somewhere that I took for a friend of mine (Guylaine Gauthier) who did her thesis on these. I cannot remember what the interpretation was though, in terms of overgrowth vs reaction rim vs pseudomorphism. I also have a specimen in my collection.

There are also corundum pseudomorphs after spinel from Dewitt's Corners, Ontario. These probably are true pseudomorphs as they are brown octahedra. I used to have an example that was completely pseudomorphed on the outside and another that was partially pseudomorphed.

Mark

[crocoite]

These are really interesting. I haven't seen them before. Thanks for sharing Elise.

One other generally noted characteristic of pseudomorphs is that they are generally dull on the surface.

Regards
Steve

[Jordi Fabre]

[Elise]

Just a quick added note - the prism faces are still intact on the pink stone and apparently lightly sanded off on the brown one; both show even growth of spinel (the blue one is completely rounded and polished on the sides unfortunately).

I hate to bring up massive vs single crystal again, but is the spinel growth in this case multi-crystalline? It seems to be radial. I haven't digested all of the discussion in Jordi's link yet, so that question might be going down the same path already hashed out.

Elise

[Pete Richards]

If the spinel is replacing the corundum, the boundary between the two is unlikely to be sharp and uniform, at least at a fine scale. Pseudomorphs generally are polycrystalline, whereas overgrowths can be (but need not be) more nearly single-crystal, especially if epitaxy is involved. Epitactic overgrowths involve one to many crystals with the same (or equivalent) orientations. However, oriented replacements (topotaxy) also occur and are a special case of pseudomorphy.

In other words, there's no sure way to tell whether these are overgrowths or replacements! It's a judgement call based on geometry and texture.

[John S. White]

Well I will put in my two cents for what it is worth. To me this looks like an overgrowth of spinal on unaltered corundum. If we were seeing pseudomorphism then I feel that the corundum would not appear completely unaltered. I suspect that it would show some deterioration. Furthermore, the spinel appears quite transparent, a condition that one would not expect to see in a pseudomorph. The sharp boundary between the two phases also argues forcefully for overgrowth instead of replacement.

[Pete Richards]

I suppose it's worth asking whether it is known that the outer part is indeed spinel and not blue corundum?

In spite of my previous posting, I agree that it looks more like an overgrowth than a replacement.

[John S. White]

Good question from Pete, it would be nice to know how spinel was verified. I would also like to add that epitaxy is a good possibility if it is spinel. Nassau (Gems Made By Man) pointed out that spinel and corundum have very similar crystal structures.

[Elise]

[Elise]

I ran across an entry in Gems & Gemology Journal, Fall 1992 reporting a colorless corundum crystal with overgrowth of first a thin dark green spinel layer, followed by an outside layer of dark blue sapphirine (layers were confirmed by X-ray diffraction). Actually, it states "intergrown;" I assumed overgrowth. There is a photo of the 14.02x8.13x5.94 mm crystal which outwardly looks like corundum and is reportedly to be from Betroka, Madagascar. The same entry also mentions the "intergrowth" of sunstone feldspar and iolite (India) - similar to what my friend sent me from Norway - that actually seems to be the correct use of the term (?) - there is a picture I posted here of that: https://www.mineral-forum.com/message-board/viewtopic.php?p=10290&highlight=#10290
Best wishes,
Elise