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Zircon or Spinel?
  
  Index -> FOR BEGINNERS: What is it? Where is it from?
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Nick_G




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PostPosted: Apr 07, 2018 14:17    Post subject: Zircon or Spinel?  

Hi all. I'm a new member here, so be gentle ;-)

Recently I bought from that well-known auction site a 'zircon' which arrived a couple of days ago. It's pink, about 4 cm across, and weighs 58 grams. It scratches quartz pretty easily, so I'd estimate the hardness to be 8. The seller described it as zircon from Rweko, Nkasi District, Rukwa Region, Tanzania. However, I'm almost certain this is a spinel, and if it is from Tanzania, it must be from Mahenge. It's a lovely specimen, whatever it is!

There are several photos attached. What do you think? The seller is adamant that it is zircon, even after I contacted him saying it looked like a spinel to me.

Thanks and regards,
Nick



Spinel Tanzania 1.JPG
 Mineral: Spinel
 Description:
Mahenge (presumed location)
40x36x30 mm
 Viewed:  2979 Time(s)

Spinel Tanzania 1.JPG



Spinel Tanzania 2.JPG
 Mineral: Spinel
 Description:
Mahenge (presumed location)
40x36x30 mm
 Viewed:  2980 Time(s)

Spinel Tanzania 2.JPG



Spinel Tanzania 3.JPG
 Mineral: Spinel
 Description:
Mahenge (presumed location)
40x36x30 mm
 Viewed:  2970 Time(s)

Spinel Tanzania 3.JPG



Spinel Tanzania 4.JPG
 Mineral: Spinel
 Description:
Mahenge (presumed location)
40x36x30 mm
 Viewed:  2969 Time(s)

Spinel Tanzania 4.JPG



Spinel Tanzania 5.JPG
 Mineral: Spinel
 Description:
Mahenge (presumed location)
40x36x30 mm
 Viewed:  2968 Time(s)

Spinel Tanzania 5.JPG



Spinel Tanzania 6.JPG
 Mineral: Spinel
 Description:
Mahenge (presumed location)
40x36x30 mm
 Viewed:  2965 Time(s)

Spinel Tanzania 6.JPG



Spinel Tanzania 8.JPG
 Mineral: Spinel
 Description:
Mahenge (presumed location)
40x36x30 mm
 Viewed:  2957 Time(s)

Spinel Tanzania 8.JPG



Spinel Tanzania 11.JPG
 Mineral: Spinel
 Description:
Mahenge (presumed location)
40x36x30 mm
 Viewed:  2959 Time(s)

Spinel Tanzania 11.JPG



Spinel Tanzania 14.JPG
 Mineral: Spinel
 Description:
Mahenge (presumed location)
40x36x30 mm
 Viewed:  2961 Time(s)

Spinel Tanzania 14.JPG



Spinel Tanzania 15.JPG
 Mineral: Spinel
 Description:
Mahenge (presumed location)
40x36x30 mm
 Viewed:  2974 Time(s)

Spinel Tanzania 15.JPG


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Bob Harman




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PostPosted: Apr 07, 2018 14:49    Post subject: Re: Zircon or Spinel?  

The same specimen with multiple photos was posted, apparently by the same individual, several days ago on MINDAT. The question there was also the same, is it spinel or zircon? There were a couple of responses favoring spinel from Tanzania. My posting here is to put this post into complete context.
BOB
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lluis




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PostPosted: Apr 07, 2018 14:55    Post subject: Re: Zircon or Spinel?  

To me also a spinel.
Place,could be the one you say. But maybe any other...
I had an opposite case: I bought a cinnamon spinel, that turned to be a zircon either Sri Lanka or Norwegian.... Who could say?
That is the trouble in any case, when a lie is found, how to get truth.... ?
Mineral, easy.
Location... just a guess always (and guess could be the true location! But how to be sure.?)

With best wishes (and congratulations for a nice specimen!)

Lluís
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Pete Richards
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PostPosted: Apr 07, 2018 15:03    Post subject: Re: Zircon or Spinel?  

Looks like a spinel to me. Zircons usually, but not always, have a prism as well as the termination faces. If it's a spinel, the faces should be equilateral triangles and should all meet at the same angles. If it's a zircon with the usual faces, the triangular faces will be isosceles triangles, but the apical angle will be about 65°, not 60°. And, viewed from certain directions, the angles between the faces will be different than in other directions - typically the crystal will be flattened perpendicular to one four-fold symmetry axis (and the other axes emerging from the other points will not have true four-fold symmetry).

These distinctions may be tricky to make with a crystal as irregular as yours.

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Jordi Fabre
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PostPosted: Apr 07, 2018 15:19    Post subject: Re: Zircon or Spinel?  

It looks a lot as a Spinel from Myanmar (Burma). I'm adding an image of a similar one.


Spinel - Pein-Pyit_Mogok Township_Pyin-Oo-Lwin District_Mandalay Region_Myanmar.jpg
 Mineral: Spinel
 Locality:
Pein-Pyit, Mogok Township, Pyin-Oo-Lwin District, Mandalay Division, Burma
 Dimensions: 4.6 x 3.7 x 3.8 cm
 Description:
 Viewed:  2953 Time(s)

Spinel - Pein-Pyit_Mogok Township_Pyin-Oo-Lwin District_Mandalay Region_Myanmar.jpg


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Bob Carnein




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PostPosted: Apr 07, 2018 15:21    Post subject: Re: Zircon or Spinel?  

I had the same experience a couple of years ago. The dealer (on eRocks) said it was a zircon from Merelani; I bought it despite misgivings. It turned out to be a pink spinel, probably from Mahenge.
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lluis




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PostPosted: Apr 07, 2018 15:23    Post subject: Re: Zircon or Spinel?  

Hi, Pete

My "cinnamon spinel" zircon has no prism faces. Looks as an octahedron, just that it is not,
Just it is not spinel. I could not be sure about the difference of 5º, in meeting.... I am chemist, but unfortunately for me, I am not a crystallographer.... :-(

Anyway, a XRF and all would be clear...
Also, maybe some tests chemicals could help. Although for shape and color, I am pretty sure that it is spinel.....

With best wishes

Lluís
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Nick_G




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PostPosted: Apr 07, 2018 15:30    Post subject: Re: Zircon or Spinel?  

Thanks guys. I did post this on Mindat but thought I'd post it on here as well - the more opinions the better ;-)

The shape is distorted by multiple growths but I have measured several uninterrupted angles on the triangular faces and according to the protractor they are all 60°.

There's some images of spinels from Tanzania on Mindat and they seem to have the same sort of phlogopite mica associated with them.

Out of interest do larger spinels grow like this as 'compound crystals' where the main octahedron is often split into smaller ones on some faces, or is this a result of selective etching after millions of years?
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Nick_G




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PostPosted: Apr 07, 2018 15:40    Post subject: Re: Zircon or Spinel?  

As an aside the seller also pointed me to four other pictures of specimens to back up his zircon theory. Three of them definitely were zircons but the last one I think is a purplish-red spinel, not a zircon.

I've also seen it the other way round an eBay specimen described as a spinel from Pakistan, when it is clearly a reddish zircon, probably from Astor Valley!
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Pete Richards
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PostPosted: Apr 07, 2018 16:04    Post subject: Re: Zircon or Spinel?  

lluis wrote:
Hi, Pete

My "cinnamon spinel" zircon has no prism faces. Looks as an octahedron, just that it is not,
Just it is not spinel. I could not be sure about the difference of 5º, in meeting.... I am chemist, but unfortunately for me, I am not a crystallographer.... :-(

Anyway, a XRF and all would be clear...
Also, maybe some tests chemicals could help. Although for shape and color, I am pretty sure that it is spinel.....

With best wishes

Lluís


Yes, it is true that not all zircons have prism faces. And the difference of 5° is not easy to detect, though apparently Nick was able to get some good measurements. I actually thought it was going to be a bigger difference when I wrote that, then I did the calculation....

Without a doubt, XRF, XRD, or EDS would solve the question, as would some qualitative chemical tests. However, most folks don't have access to these tools, sad to say.

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Nick_G




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PostPosted: Apr 08, 2018 03:39    Post subject: Re: Zircon or Spinel?  

Thanks again guys. Your help has been much appreciated. So spinel it is then.
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Pete Modreski
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PostPosted: Apr 09, 2018 18:54    Post subject: Re: Zircon or Spinel?  

Nick, here is a suggestion, if you have access to ultraviolet lights.

Under longwave UV, pink or red spinel USUALLY shows a noticeable red fluorescence (like ruby). In contrast, most light-colored zircons USUALLY have a yellow fluorescence, but this time under shortwave UV. Neither behavior is 100% universal, but, most likely, this could confirm the identity.

(I'm also betting on spinel, that is certainly what the crystal most looks like.)

Pete Modreski
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Nick_G




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PostPosted: Apr 10, 2018 12:17    Post subject: Re: Zircon or Spinel?  

Thanks Pete.

I did have a portable LW UV lamp but last time I tried it it was dead. I did try putting the spinel under the bug-killer light at work, but no obvious fluorescence was observed. It's far from ideal though - the room has plenty of natural daylight, and the bug-killer light gives off a lot of visible light too, so any weak fluorescence would have been masked.
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Nick_G




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PostPosted: Apr 12, 2018 11:30    Post subject: Re: Zircon or Spinel?  

Well, I tried a ruby sample, which I know from previous testing to be fluorescent, with the bug-killer light at work and nothing was seen with this either, so this is obviously not a viable test.

I'll have to get a new lamp, preferably one that does LW and SW UV.
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Nick_G




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PostPosted: May 15, 2018 14:41    Post subject: Re: Zircon or Spinel?  

An update. My wife & I visited a gemmologist recently and we tested the specimen under his ultraviolet lamp. In LW UV there was some magenta/red fluorescence although it wasn't very strong, which shows that chromium is the cause of the pink colour.. He also confirmed the spinel ID.
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Paul Bordovsky




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PostPosted: May 15, 2018 19:37    Post subject: Re: Zircon or Spinel?  

Here are a couple pics of my spinel from Tanzania. Looks similar to yours, but deeper color and very fluorescent.


P1017988a.jpg
 Mineral: Spinel
 Locality:
Tanzania
 Dimensions: 48mm tip to tip in the longest dimension
 Description:
 Viewed:  1992 Time(s)

P1017988a.jpg



P1017989a.jpg
 Mineral: Spinel
 Locality:
Tanzania
 Dimensions: 48mm tip to tip in the longest dimension
 Description:
 Viewed:  1991 Time(s)

P1017989a.jpg



P1017991a.jpg
 Mineral: Spinel
 Locality:
Tanzania
 Dimensions: 48mm tip to tip in the longest dimension
 Description:
 Viewed:  1987 Time(s)

P1017991a.jpg



P1017993a.jpg
 Mineral: Spinel
 Locality:
Tanzania
 Dimensions: 48mm
 Description:
 Viewed:  1987 Time(s)

P1017993a.jpg


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Nick_G




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PostPosted: May 16, 2018 08:42    Post subject: Re: Zircon or Spinel?  

^That's a lovely specimen Paul. A really nice raspberry red and excellent fluorescence.

Spinel is definitely one of my favourite minerals.
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