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Question, having not used an XRF before and only seen them used by other people - is this particular gun not set up to see non-metallic elements? One would expect some oxygen or sulfur in there, or some silicon maybe. Was your little target dot right on the silvery stuff and nothing else or did it overlap into the black stuff?

You and Dave may already have something like this, but here is a calculator to ID a mineral based on atomic percentages such as you get with an XRF. Adjusting the tolerance up to 10% or even 25% or so can be helpful since the XRF isn't always shooting one specific mineral. http://www.webmineral.com/chemical.shtml#.XDbzr1VKiUk

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Thats a pretty cool site jasong.   Never thought to look for a chemistry site..... thanks.

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On 1/10/2019 at 2:31 AM, jasong said:

Question, having not used an XRF before and only seen them used by other people - is this particular gun not set up to see non-metallic elements?

I haven't used one of those guns, but do have experience in the field of X-ray detection.  Hopefully I'm not getting into too much lingo here....

These devices excite the atoms in the sample and look for the de-excitation signatures, which are energies characteristic of the inner shell electrons of the atoms.  These are uniquely associated -- kind of like a fingerprint -- with each element.  As it turns out, the higher the atomic number of the element, the more likely the excitation leads to energy release of the characteristic energies.  So metals like lead and gold with very high atomic numbers and even copper and iron, with intermediate atomic numbers, are much easier to detect than oxygen, silicon, sulfur with their lower atomic numbers.

A secondary issue is that the excitation energies of the X-ray source (in the gun) vary in intensity depending upon multiple factors, and there isn't a "one size fits all" X-ray generator that works over the entire periodic table.  To excite the low atomic number elements efficiently you need a lower energy source, and vice versa for high atomic number elements.  My guess is that the guns are tuned for optimal operation somewhere between the iron group and gold and its neighbors, but well away from oxygen, etc.

There are other factors such as sensitivity of the X-ray detector itself as a function of X-ray energy (and thus as a function of atomic number) and absorption of neighboring materials (for example, the 'window' that protects the sensitive X-ray detector crystal) which make detection of the low atomic number elements difficult with X-ray devices.

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I'm not sure "alloy" is the right set up for mineral analysis - alloy will only say the metals in the specimen. It won't tell you the other elements in the mineral. So, at least you can say its a copper mineral with a fair amount of silver in it. Its not metallic silver or metallic copper. Probably Cuprite or Chalcocite. The copper mineral Covellite almost always shows a blue metallic sheen which I cant see on this specimen.

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