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Two orphan spinels
  
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Pete Richards
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PostPosted: Apr 29, 2019 13:48    Post subject: Two orphan spinels  

Recently I acquired a spinel that came without identity or location from a collection from around the 1950s (is that now an “old” collection?). Its identity as a member of the spinel group was not in doubt. In the search for some idea about its locality, I also revisited another spinel I obtained several years ago, and came to doubt its locality as well. I hope some members may be able to help me with these two orphans.

The new one is a black 4 cm octahedron with dodecahedral and tetrahexahedral (OOPS I should have written TRAPEZOHEDRAL) {311} modifications. It is actually dark brown on very thin edges. It has a perhaps diagnostic pebbled appearance on the lower half, composed of parallel octahedral facets separated by what appears to be residual matrix. A few local zones are a medium-brown color, but most of it is macroscopically black. SEM-EDS shows it to be spinel (species) with significant Fe+2 content. Much to my surprise, EDS showed the presence of exsolution lamellae (bright lines in the SEM image below). EDS analyses provided the following:
Main mass: (Mg0.77Fe0.21Zn0.02)Σ1.0 (Al1.97Fe0.03)Σ2.0 O4
Exsolution lamellae: (Mg0.71Fe0.28Zn0.01)Σ1.0 (Al1.63Fe0.36)Σ1.99 O4
The lamellae are considerably richer in Fe, especially Fe+3, than the main mass, but both phases fit within the definition of spinel, the species.

The older spinel is labeled gahnite, Antsirabe, Madagascar. It is lusterous black with blue-green internal reflections. I did not do EDS work on it, but the species identity seems reasonable given the color. It has the same forms as the first one, but is strangely developed so as to appear to be tetragonal. Another crystal from the same purchase has more normal morphology, so this morphology is interesting but not diagnostic. Mindat does not provide much information about spinel from Antsirabe, much less gahnite, so the locality appears suspect.

I hope this will provoke some well-founded opinions about the localities of these lovely crystals.



IMG_5451.jpg
 Mineral: Spinel
 Description:
unknown
4 cm
 Viewed:  1176 Time(s)

IMG_5451.jpg



Spinel 1.jpg
 Description:
 Viewed:  1177 Time(s)

Spinel 1.jpg



Fragments.jpg
 Description:
Fragments viewed immersed in index oil at 100x with a petrographic scope, polarizers not crossed
 Viewed:  1172 Time(s)

Fragments.jpg



07224803.JPG
 Mineral: Spinel
 Description:
Parallel growth of octahedral facets on underside of crystal
 Viewed:  1176 Time(s)

07224803.JPG



Spinel 2.jpg
 Mineral: spinel
 Description:
Exsolution lamellae (bright lines at 120 angles) within main crystal. Both phases classify as spinel.
 Viewed:  1174 Time(s)

Spinel 2.jpg



IMG_5442.jpg
 Mineral: gahnite
 Locality:
Antsirabé, Vakinankaratra Region, Antananarivo Province, Madagascar
 Dimensions: 3 cm
 Description:
 Viewed:  1178 Time(s)

IMG_5442.jpg



Spinel 2.jpg
 Description:
 Viewed:  1177 Time(s)

Spinel 2.jpg



07224000.JPG
 Mineral: gahnite
 Description:
Blue-green internal reflections
 Viewed:  1174 Time(s)

07224000.JPG



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Tobi




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PostPosted: Apr 29, 2019 16:33    Post subject: Re: Two orphan spinels  

Hi Pete,

just a guess, but the first one reminds me of Russian spinels from Aldan: https://www.mindat.org/loc-20097.html

Tobi
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Bob Morgan




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PostPosted: Apr 29, 2019 21:12    Post subject: Re: Two orphan spinels  

After seeing your first spinel in Rochester, I came home and looked at mine. I purchased one from Jewell Tunnel identified as from the Neil Ydren collection. The only locality information was "Madascar?". It lacks dodecahedral faces, but has similar oriented grains making what looks like contact dissolution channels between them. It also has the spinel lighter brown material running between grains. Differing from yours some of the grains are relatively clear. The brown also seems to make thin layers between lighter grains and the larger crystal,

Another crystal from Wards has a locality "Betroska District, Anosy Region, Tulear Provence, Madagascar?" It has lighter coating similar to the first but all over. The grains on the bottom show even more oriented dissolution channels and completely covered by the lighter spinel coating.

Perhaps yours are from Madagascar.
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Kevin Conroy




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PostPosted: Apr 30, 2019 16:11    Post subject: Re: Two orphan spinels  

If you want to compare yours to some pictured on Mindat: https://www.mindat.org/gallery.php?loc=2247&min=3729
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Pete Richards
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PostPosted: Apr 30, 2019 20:26    Post subject: Re: Two orphan spinels  

Thanks to those who offered ideas about these spinels. The first one seems most likely to be from the Betroka area, Madagascar. I considered the Aldan area Tobi suggested, but illustrated specimens from there are much more lustrous than mine, and were collected more recently than I think mine was.

I have no real leads on the second one with the bluish internal reflections. I'm still watching....

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John S. White
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PostPosted: May 01, 2019 03:58    Post subject: Re: Two orphan spinels  

Bob Morgan - Your Neal Ydren is almost certainly Neal Yedlin the legendary collector of micromounts from Connecticut.
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PostPosted: May 01, 2019 07:27    Post subject: Re: Two orphan spinels  

Thanks John. It's nice to clear up these things.
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Clifford Trebilcock




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PostPosted: May 01, 2019 19:33    Post subject: Re: Two orphan spinels  

Hi Pete,

Just noticed your post about Gahnite specimens. Gahnite is a Zn Spinel Sub-Group of actual Spinel. The bluegreen Gahnite specimen looks exactly like Gahnite xls from the Davis Pyrite mine in Rowe,Ma, I have a matrix specimen from I got Neal Yedlin back in the 1950's.Neal collected there a lot. Most Gahnite from other New England locations is more greenish color. Check Mindat for photos and lots of info,

Cliff Trebilcock
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PostPosted: May 02, 2019 02:00    Post subject: Re: Two orphan spinels  

Pete Richards wrote:
I considered the Aldan area Tobi suggested, but illustrated specimens from there are much more lustrous than mine, and were collected more recently than I think mine was.
You are right, Pete. I also thought that the ones from Aldan are more lustrous and look "fresher" than yours, I suggested your crystal was an old one and a bit scratched. I didn't know the specimens from Betroka, but after comparing your crystal to photos of Betroka spinels, I agree that this could be most likely the locality.

Tobi
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