Hey All!
I've been analyzing some highly-variable silicate glasses (unknown stoichiometry), using USGS glass standards, and have been getting great oxygen numbers on both standards and unknowns. The person I'm getting the data for has asked to view the numbers in wt % oxide, as that's what she's used to dealing with. Ok fine, no problem.
I recalculate the data into oxides, and lo and behold, I now have anywhere from 7-15 wt% EXCESS oxygen, because the analyzed oxygen is less than the calculated oxygen. What are some explanations for this?
I'm using MPB for the oxygen, and getting good numbers of my glass standards, so I don't have a reason to suspect this is a background setting issue. I've also used the TDI function to minimize charging/damage, but there doesn't seem to be a huge oxygen change, regardless.
I've also been trying to find published data on possible water contents, but the record is sparse. In the few papers I have on this type of glass I'm looking at, no one has looked for water or OH. Also, these glasses were explosively formed so I'm not sure there's any water in them.
How does one deal with excess oxygen?
I've attached an unknown glass analysis, calculated as elemental and as oxide.
From John D.:
"Also why did you want to analyze for oxygen in the first place? Did you suspect excess oxygen from Fe2O3?"
Nothing so clever, John! I never thought about just doing stoichiometry; my default is usually to try and measure, and then when things go poorly, rely on stoichiometry. Maybe I should't have done it? The expected composition of the glass is broadly silicate, so I didn't see any big hurdles in measuring O.