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Verify native platinum

Posted by Larry Edelstein  
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Larry Edelstein December 24, 2011 06:49PM
There's a featured picture on Wikipedia which identifies itself as a native platinum nugget. At 35 x 23 x 14 mm, it's a biggie, and looks to my eye a little like the result of sintering rather than native, so I'm soliciting your opinions.

http://upload.wikimedia.org/wikipedia/commons/thumb/6/6a/Platinum-nugget.jpg/800px-Platinum-nugget.jpg

Here's a link to the page, which has more information:

Wikipedia's image page for the image



Edited 1 time(s). Last edit at 12/24/2011 06:52PM by Larry Edelstein.
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Don Saathoff December 24, 2011 08:09PM
The "wiki" article gives a mass of "around 112g" and a size of 35 X 23 X 14mm.....this gives a volume of ~11.27cm3 which, unless I'm making a gross error in either math or logic gives a D of ~9.93.....platinum has a D of ~21.....

Where am I going wrong???

Don
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Larry Edelstein December 24, 2011 10:15PM
The dimensions are supposed to delineate the smallest rectangular parallelepiped (er, box) in which the specimen could fit. But like all specimens, this one isn't a perfect rectangular parallelepiped, so much of the volume of that box isn't occupied by the specimen. And it's pitted, presumably somewhat porous, and not 100% pure platinum.

To my eyeballs it looks like it could occupy up to, I dunno, 4/5th of the box. Does that, plus the pits, porosity, and impurity make up the difference?

Looking at other platinum specimens in Mindat:

This one D ~ 7.27
This one D~ 9.09
This one D = 6
This one D = 10

So the density of the nugget in the picture in question (as calculated from the quoted dimensions and mass) is consistent with that of the specimens above.

Here is a big one, and here is another, both bigger than the quoted dimensions of the nugget in question.

I think I owe that guy an apology. Thanks for the research, Don!
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Don Saathoff December 24, 2011 11:26PM
Hello Larry....no research....I'd just never tried to estimate a density from mass & dimensions and didn't trust my results. I do think the nugget is natural and I thank you for the data on the other mindat nuggets. I use an old modified analytical balance for easy D determinations.

Don
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Alfredo Petrov December 25, 2011 12:23AM
It quite likely contains spinel-group mineral inclusions, which lower the density even further.
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Pavel Kartashov December 25, 2011 06:02PM
Alfredo is very right. As was shown by investigations of isoferroplatinum nuggets from Inagli massif placer, they contains up to 45 wol.% of chromite.
And of course 21 g/cm3 is density of pure metallic platinum. This nugget consist of Pt-Fe alloy with Fe content ~10-12 mas.%.
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