Analysis of Boron

[Brian Joy]

Here are two scans collected using LDE2 under the stated conditions on the same tourmaline grain.  A further complication is that B Kα occurs at high Bragg angle on LDE2.  L ≈ 190 mm corresponds to sine(θ) around 0.68.

Thank you Brian, I appreciate it!

I attach my presumably-tourmaline raw and smoothed scans. The KLM database puts the peak at about 191 as you can see, while the BN (not attached) is peaking at about 193.X .

So what do you guys think....is that small bump at 191 is the B peak in tourmaline? And what is the big peak at about 195...is that a shifted B? O-III?

Deon.

Hi Deon,

What about optical properties?  Is it uniaxial negative, length fast?  What about the ED spectrum?  Tourmaline can vary greatly in composition, but here is a spectrum collected on a typical schorl from the Fairbanks schist:



If you want to verify the presence of boron using LDE2, then you need to increase the dwell time, perhaps to 1 s, and bump the beam current way up, maybe to 200 nA or so -- tourmaline can handle it, though I'd defocus the  beam to 10 microns.

Brian

[D.]

If you want to verify the presence of boron using LDE2, then you need to increase the dwell time, perhaps to 1 s, and bump the beam current way up, maybe to 200 nA or so -- tourmaline can handle it, though I'd defocus the  beam to 10 microns.

Brian

Thanks for the advice Brian. I'll try it.

I was brought an unknown mineral and asked to scan for Boron. So I first tried to see the B peak on a tourmaline (I assume the user who gave it to me correctly identified it optically; I don't have an EDS  ).

I'm trying to wrap my head around what's possible with LDE2. At 10nA there appears to be peaks at about 190-ish and 195 in various blanks, and the tourmaline (if those are in fact peaks and not noise). At 30nA some of the "peaks" begin to merge. I'll see what 200nA kicks out.

Have you tried to quantify B in tourmaline with LDE2?

Deon.

[Brian Joy]

If you want to verify the presence of boron using LDE2, then you need to increase the dwell time, perhaps to 1 s, and bump the beam current way up, maybe to 200 nA or so -- tourmaline can handle it, though I'd defocus the  beam to 10 microns.

Brian

Thanks for the advice Brian. I'll try it.

I was brought an unknown mineral and asked to scan for Boron. So I first tried to see the B peak on a tourmaline (I assume the user who gave it to me correctly identified it optically; I don't have an EDS  ).

I'm trying to wrap my head around what's possible with LDE2. At 10nA there appears to be peaks at about 190-ish and 195 in various blanks, and the tourmaline (if those are in fact peaks and not noise). At 30nA some of the "peaks" begin to merge. I'll see what 200nA kicks out.

Have you tried to quantify B in tourmaline with LDE2?

Deon.

Hi Deon,

I've never analyzed for boron in tourmaline because generally it's reasonable to assume wt% B₂O₃ that gives 3 B³⁺ per 29 anhydrous oxygens.  Like John pointed out, the absorption correction when using BN as a boron standard will be outrageously large and will likely lead to inaccurate wt% B₂O₃ (not to mention problems with variable peak position, variable peak shape, and poor counting statistics).  Many tourmalines fall at least roughly within the dravite-schorl solid solution.  For the dravite end-member (NaMg₃Al₆(BO₃)₃Si₆O₁₈(OH)₄), wt% B₂O₃ is 10.89, and wt% H₂O is 3.76.  For end-member schorl (NaFe₃Al₆(BO₃)₃Si₆O₁₈(OH)₄), wt% B₂O₃ is 9.91, and wt% H₂O is 3.42.

One approach is simply to analyze for Si, Al, Ti, Cr, Fe, Mn, Mg, Ca, Na, K, F, and maybe Cl (not likely to be present in measurable quantity) and adjust assumed wt% B₂O₃ and wt% H₂O (taking halogens into account if present) and see if you can produce a reasonable-looking tourmaline formula unit.  Keep in mind, though, that Al³⁺ is somewhat variable, as it can substitute for Si⁴⁺ in a coupled substitution that also replaces a divalent cation with ^VIAl³⁺.  Significant Fe₂O₃ may be present as well.  Also, Li⁺ can substitute on the octahedral sites, and this can complicate interpretation of the analysis -- it also requires adding octahedral Al³⁺ to maintain charge balance such that elbaite (Na(^VIAl,Li)₃Al₆(BO₃)₃Si₆O₁₈(OH)₄) may contain in excess of ~43 wt% Al₂O₃.

Maybe you already know about this, but a good on-line resource is the Handbook of Mineralogy, as it gives representative analyses of most of the listed minerals.

Brian

[cschwandt]

A couple of years ago I was experimenting with fully quantified EDS analyses of tourmaline measuring boron and oxygen, as the latest generation SDDs are nicely sensitive to lower energy than older SiLi detectors.  For analysis of tourmaline with a beam energy of 6 keV, I could get really good signal to noise, and fitting was pretty good, the values were not far off from an idealized stoichiometric result.  So, as with so many EPMA cases the issue is having an appropriate tourmaline standard to use. The interesting issue with this specific situation is that analyzing for boron in a silicate matrix like tourmaline, there doesn't seem to be a method capable of getting the Boron without some kind of method concern (use of borate fluxes for wet chemical analyses, neutron absorption issues with INAA, no measured standard for SIMS or LA-ICPMS).  My question to the group is, do you know of a method to analyze tourmaline that doesn't have a major complication?

In the end, I feel that one is probably left with selecting a particular tourmaline, analyzing excessively and determining Boron by difference, considering it stoichiometric and using it for your additional tourmaline analyses.  Not perfect, but at least one could generate a laboratory-internal consistent set of tourmaline analyses.