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MineralsStannoan Grossular

5th Jul 2012 12:50 UTCAlfredo Petrov Manager

Formula does not indicate how charge balance is maintained when Sn substitutes for Ca.

5th Jul 2012 13:46 UTCJolyon Ralph Founder

Are we sure that it's actually incorporated in the crystal structure and not just inclusions of SnO2 ?

5th Jul 2012 13:50 UTCAlfredo Petrov Manager

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?

5th Jul 2012 15:56 UTCDavid Von Bargen Manager

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.

5th Jul 2012 20:05 UTCBart Cannon

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.

5th Jul 2012 20:16 UTCAlfredo Petrov Manager

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 :)-D

5th Jul 2012 23:39 UTCDavid Von Bargen Manager

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

6th Jul 2012 15:41 UTCLefteris Rantos Expert

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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