[Rockhounds] What is a mineral (GSA)

[DonH]

John Siebel wrote:

> Brother Don,
> 
> You're addressing the GSA? Do tell!


Oh, it was no big thing--a submitted talk on the use, and misuse, of EDS 
in forensics.  Here is the abstract:

http://gsa.confex.com/gsa/2007AM/finalprogram/abstract_129180.htm

It was neat because I was able to include a real-life situation where a 
collector from the Franklin NJ area sent out a sample to be identified 
using EDS (energy dispersive spectroscopy, an analytical system usually 
combined with an SEM).  The "closest match" was for svabite, a rare 
apatite that is in the same series as the ones known from that locality 
  but has never been identified from the Franklin/Sterling Hill 
district.  Using samples provided to me, I was able to demonstrate that 
EDS cannot identify these apatites with certainty.  In fact, I used a 
more accurate and precise instrument, the electron microprobe, and I'm 
still not certain.  The problem lies in the fact that there is a halogen 
site which contains one atom of either (F,Cl,OH), and possibly others, 
per formula unit.  The identity of the species is dependent upon one of 
these being >50% of the total of two others, and at such low quantities, 
the precision becomes a problem.  The bigger issue is that neither of 
these instruments can detect H, so with the probe, (OH) is calculated by 
difference.

I also did a segment on how you can't really tell gypsum and anhydrite 
apart using EDS, but you can with x-ray diffraction, and how this would 
be useful for people trying to determine whether residual dusts from New 
York city came from the World Trade Center incident (gypsum in wallboard 
becomes anhydrite when heated in a fire).

So anyway I was told it went very well.  Several crime lab and FBI folks 
seemed happy with my work so far, which suggests it is on the right track.

best,
Don