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GeneralInteresting brown spinel specimen

8th Dec 2019 18:02 UTCNick Gilly

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Hi all. I'd be interested in thoughts on this specimen, which arrived last week. It is a cluster of deep reddish-brown spinel crystals, consisting of a main larger crystal with many smaller ones attached. The main crystal and some of the others have stepped faces. This specimen is from Viet Nam, presumably the Luc Yen region.

What is interesting to me is that I don't think it grew in the usual marble matrix. Not only is the colour atypical, reminding me strongly of zircon, there is no sign of any marble on the specimen.  There are some areas featuring very small subhedral crystals of an unidentified dark greyish black mineral. What is also interesting is how sharply crystallised this specimen is, with none of the pitting, rough areas or slightly 'muted' appearance that often occurs with marble hosted crystals. There are lots of crystal faces and detail to be seen under a loupe! These crystals remind me of pegmatite-hosted growth, although I suspect the environment was different.

So, any ideas on the formation or host rock for this specimen? Has anyone else come across spinel like this from Viet Nam?

I have copied the pictures from the auction as they are excellent quality.

8th Dec 2019 18:03 UTCNick Gilly

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Picture 2.

8th Dec 2019 18:04 UTCNick Gilly

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Picture 3.

8th Dec 2019 18:05 UTCNick Gilly

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Picture 4 (shows the dark associated mineral).

8th Dec 2019 18:05 UTCNick Gilly

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Picture 5.

8th Dec 2019 18:06 UTCNick Gilly

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Picture 6.

8th Dec 2019 18:07 UTCNick Gilly

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Finally, picture 7, showing a larger area with smaller crystals.

Thanks for any thoughts on this.

8th Dec 2019 18:58 UTCAndrew Debnam

Although primarily  focussed on the blue spinel. This article covers some of the geologic setting questions you may have  for Luc Yen. 

8th Dec 2019 19:48 UTCNick Gilly

Thanks Andrew. Yes I'd read about the blue spinels forming from metamorphism of clay-containing limestones during the formation of the Himalayas. I wonder if those brown crystals are igneous in origin? I'm guessing the colour is from iron.

8th Dec 2019 23:02 UTCAndrew Debnam

Hello Nick, these deposits are marbles. Here is a good ref which concludes the Spinels are of a marble origin. Also not all marble hosted examples spinel or minerals are pitted or incomplete.  
cheers
Andrew

Garnier V., Giuliani G., Ohnenstetter D., Fallick A.E., Dubessy J., Banks D., Vinh H.Q., Lhomme T., Maluski H., Pêcher A., Bakhsh K.A., Long P.V., Trinh P.T., Schwarz D. (2008) Marble-hosted ruby deposits from Central and South-East Asia: Toward a new genetic model. Ore Geology Reviews, Vol. 34, pp. 169–191


8th Dec 2019 19:28 UTCGregg Little

Nick;
Oddly when you say "Not only is the colour atypical, reminding me strongly of zircon," What jumped to my mind is that it strongly reminds me of scheelite. Sorry I can't be of much help but good luck in your quest.

Cheers, Gregg

8th Dec 2019 19:39 UTCNick Gilly

Hi Gregg. Yes that as well! It's definitely not scheelite though as it scratches quartz fairly easily.

8th Dec 2019 22:29 UTCPeter Slootweg

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Hi Nick,

A few years ago i took the opportunity to visit the Luc Yen / An Phu area. One of my visits was with a farmer who had found a large batch of spinel on his land. On the photo you can see his stock. A few hundred kilo's of purple spinel with phlogopite and other minerals. None of these specimens did contain any residue of the marble in which they formed. The marble weathers easy and they usually only find nests of loose crystals and clusters in the mud/soil. This unfortunately gave iron oxides the chance to infiltrate the material producing a brown hue on most specimens.

In my experience the only marble matrix specimens available are those from the Sungate mine in An phu where the chondrodite/pargasite/ruby /spinel specimens are being actively mined.

This also became clear when visiting the many dealers in the village of Yen The where most of the dealers in area reside.

Therefor i think your specimens is pretty normal from that area and has certainly no pegmatite  origin although it may look like it.





8th Dec 2019 23:06 UTCNick Gilly

Thanks Peter. That is an impressive haul. I also have three larger purple specimens of spinel which would have been marble-hosted. Two of them show traces of matrix in the cavities but one of them only has iron staining in places with no marble left.

Just to be clear, my specimen above is not coloured by surface staining. It also has a different look to the other specimens I have, which is what makes me wonder if it had a different origin.
 
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