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Identity HelpAfghan mystery mineral.

30th Aug 2019 00:17 BSTLance Tannahill

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I recent purchased this specimen as an "unknown" mineral along with some others (sodalite, afghanite, phlogopite, spinel, and vorobyevite) on ebay from Afghanistan. The Crystal is 16mm x 12mm and is not florescent under UV. I could use some help with the ID or at least some possibilities. Thank you.

30th Aug 2019 01:48 BSTKevin Conroy Expert

I'm not sure what the blue mineral is.   It may just be the angle of the photo, or the way the crystal was exposed/prepped, but it looks like it may have been reaffixed to the matrix.   Does it look genuine in person?   I'm having a hard time convincing myself that someone would go through this kind of work for a fairly small specimen, but...

30th Aug 2019 05:03 BSTLance Tannahill

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I see what you mean but it is just because of the lighting. The ctystal is almost entirely embedded within the matrix. Here is a photo looking down from a short end.

30th Aug 2019 10:49 BSTKevin Hean

Lapis Lazuli,? They normally do looked re-affixed because the matrix is carved away to expose the crystal.

30th Aug 2019 12:47 BSTJosé Zendrera Expert

Blue, not fluorescent, on marble matrix and with dodecahedral form, to me it looks like lazurite (formally a sulfide rich variety of Hauyne) from Sar-e-Sang, Badakhshan.
Color is little light for lazurite from this locality but it could be due to acidic treatment to extract marble matrix and expose the lazurite crystal.

30th Aug 2019 14:54 BSTRobert Nowakowski

I agree with kevin.  It looks like it was glued into the matrix.  There is white rim around the xl that is significantly lighter in color and finer grained than the surrounding matrix.
Bob

30th Aug 2019 17:02 BSTTama Higuchi

Another vote for glued-on Lazurite.  The color is a bit lighter than the one I have in my collection, but has the same form.  I wonder what kinds of tests you can do to see if it is glued?

31st Aug 2019 16:01 BSTKevin Conroy Expert

Testing for glue can range in difficulty depending on the glue.   The easiest is just soaking the specimen in water.   Stuff like Elmer's glue will turn white and soften.   Soaking a specimen in acetone will soften "super glue".   Some glues fluoresce, so that's an easy non-chemical test.

Although the following link is about ceramic repair, some of the information is relevant for minerals: http://cool.conservation-us.org/jaic/articles/jaic37-01-005_4.html

1st Sep 2019 17:00 BSTSteve Hardinger Expert

Lazurite may be discolored when treated with 20% acetic acid, a chemical which has been used to etch (or attempt to etch) marble.

1st Sep 2019 18:24 BSTAymeric Longi

I've seen a couple of these one ebay, indeed labelled as "unknown". In the present case, I can't see what would make it seem to be glued, and even so, the question is about identification not wether it's repaired/glued or.
It doesn't look like Lazurite to me, even acid "cleaned" one.

What about Blue Diopside? That would be a large crystal though...

Cheers!

1st Sep 2019 21:19 BSTFrank K. Mazdab Manager

I too had wondered "blue diopside" based on the apparent monoclinic-like crystal shape (at least interpreting from the top photo) and that central Asia, at least on the Russian side, has several occurrences of bluish diopside in marble, but I also agree with Aymeric that a large single crystal would be much different from the usual fine-grained material I've seen photos of.  But alternatively, if a lazurite were bleached (whether naturally or by careless acid treatment) and grossly malformed, I guess it might look a bit like that too?  There really aren't a lot of blue minerals in marble to choose from.

3rd Sep 2019 04:59 BSTLance Tannahill

Aymeric, thank you for getting this thread back on topic. I was getting frustrated.

2nd Sep 2019 02:40 BSTDonald B Peck Expert

When I first saw the photo, my instant reaction was that the crystal is monoclinic.  Then I thought "Microcline, but the color is wrong." Then my mind blanked on it . . . a common occurrence at my age!

4th Sep 2019 14:58 BSTV. Stingl

What About lazurite replacing microcline? As far as I know, there are such pseudomorphs.

4th Sep 2019 20:41 BSTFrank K. Mazdab Manager

But that would beg a petrologic question... why would a marble originally contain a large sharp crystal of microcline?

If the crystal is indeed a lazurite pseudomorph after something, then that something ought to have been a mineral that would be expected to have reasonably crystallized during the development of the marble (a large sharp crystal like that, of practically any mineral, would almost certainly not be a holdover from the marble's protolith days). And if if is a pseudomorph, then why not afghanite or another blue mineral replacing the original mystery mineral?  By evoking the possibility of a pseudomorph, we've discounted shape as a clue and then have only color to go on.

I think Kevin's suggestion that we've moved beyond an "eyeball" ID is a good one.

3rd Sep 2019 14:43 BSTKevin Conroy Expert

Since this specimen is proving to be difficult to ID just by "eyeballing", lab testing is the way to get a definitive answer.  Who knows, you may have something new!
 
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