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Stannoan Grossular
July 05, 2012 11:50AM
Formula does not indicate how charge balance is maintained when Sn substitutes for Ca.
avatar Re: Stannoan Grossular
July 05, 2012 12:46PM
Are we sure that it's actually incorporated in the crystal structure and not just inclusions of SnO2 ?
Re: Stannoan Grossular
July 05, 2012 12:50PM
Well, there is certainly andradite with structurally incorporated Sn; I don't know about grossular.

If it is indeed only present as inclusions, then it shouldn't be in the formula at all.

If the Sn is structural, then I guess garnets would compensate the charge as they do for Ti: It would be in the second formula position, replacing Al, not Ca, with charge compensated either by a divalent metal in the same position, or by trivalent Fe or Al replacing Si. But that's just my guess.

As we do not know, it would perhaps be best to just erase the formula entirely?



Edited 1 time(s). Last edit at 07/05/2012 12:55PM by Alfredo Petrov.
avatar Re: Stannoan Grossular
July 05, 2012 02:56PM
    
If you look at the article by Alderton, D.H.M., and Jackson, N.J. (1978): Mineralogical Magazine 42, 427-434., the grossular contains a lot of Fe+3 and isn't particularly end member grossular. Also the max of SnO2 measured was 0.26%, much higher than normal, but really wouldn't ordinarily be designated as a variety.

http://www.minersoc.org/pages/Archive-MM/Volume_42/42-324-427.pdf

They didn't see any inclusions of cassiterite in their thin sections.
Re: Stannoan Grossular
July 05, 2012 07:05PM
Tin bearing skarns are not uncommon.

There is one 20 miles East of Sun Valley, Idaho. I don't think tin was the target element of the prospect, but rather copper and silver.

I collected it many years ago, but could not determine the host for the tin, the reason I visited the property.

The source of the tin was NOT cassiterite.

Diopside was prominent. Would that be a better mineral to contaminate the structure with tin than grossular or andradite ?

If I recall correctly, beryllium was also a prominent trace element.
Re: Stannoan Grossular
July 05, 2012 07:16PM
Did you check for fluorescence, Bart? Tin in skarns can be present as exotic stuff like malayaite or nordenskioldine, both of which fluoresce somewhat like scheelite, which is what the prospectors were expecting to find in the skarn smileys with beer
avatar Re: Stannoan Grossular
July 05, 2012 10:39PM
    
Hedenbergite formed in a slag contained up to 2.6% SnO2. Andradite was 26% for fast cooled crystals.

http://www.minersoc.org/pages/Archive-MM/Volume_42/42-324-487.pdf
Re: Stannoan Grossular
July 06, 2012 02:41PM
    
Several garnet-group end-members with essential Sn have been approved recently ( http://www.mindat.org/min-41898.html http://www.mindat.org/min-39570.html http://www.mindat.org/min-40023.html). Couldn't these provide clues for the incorporation of Sn in the Grossular structure, if it is indeed structural Sn?

Also, Sn-bearing Andradites are known from non-slag localities too. See http://www.mindat.org/loc-2388.html and http://www.mindat.org/photo-473126.html

Lefteris.
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