Al and Si Quantitation in Routine Silicate EPMA: Maybe not so routine. John H. Fournelle. Eugene Cameron Electron Microprobe Lab Department of Geology and Geophysics University of Wisconsin Madison, Wisconsin. Motivation.
Al and Si Quantitation in Routine Silicate EPMA:
Maybe not so routine
John H. Fournelle
Eugene Cameron Electron Microprobe Lab
Department of Geology and Geophysics University of Wisconsin
You mean your new automatic gizmo cant match the old manual one, from days of yore?
A veteran prober kept prodding me about problems with low totals his simple silicate analyses (olivine, pyroxene, garnet).
We narrowed down the problem to issues
related to peaks of standards and unknowns:
How wide are the peak tops?
Are there chemical peak shifts?
Is there a problem with our peaking procedure?
Ans 1. 5-10 sin theta units 2. Yes 3. Yes
Consider the Si Ka Peak
5 units = 0.42 eV
Precise Peak: 4 minutes of scanning
every 1 sin theta unit
TAP, 15 kev, 20 nA, 10 sec/channel, Si metal
7 units wide
11 units wide
13 units wide
Al Ka Peak Top Width: similar to Si except some wider at top
datum every 2 sin theta units
Si and Al Ka Peak Widths on TAP:
Chemical Peak Shifts
Have been recognized since the origins of x-ray spectroscopy in the 1920s, e.g., Cl and S Kb peaks (M shell electons = valence electrons)
Al Ka Chemical Shifts
White, McKinstry & Bates, 1959, Advan. X-ray Analysis
Al Ka Shift vs coordination
relative to Al metal:
Feldspar (IV): -0.07;
Sillimanite (IV+VI): -0.11;
Kyanite (VI): -0.12
Also Day, 1963, Nature; Wardle and Brindley, 1971, American Mineralogist
First attempt 2003
Al Ka Peak Shifts - UW SX51
But not reproducible...
Al Ka Peak Positions (on TAP) are very sensitive to stage Z position
A misfocus of 5 units in Z equals a peak shift of 3-4 sin theta units, not trivial.
Well adjusted autofocus critical for multiple automated repeated measurements (overnight)
So once autofocus was fixed and many repeat overnight scans run, we had consistent data for Al
Al Ka Peak Shifts
7 units or 0.6 eV
82%(NaAlSi3O8) - 18%(CaAl2Si2O8)
51%(NaAlSi3O8) - 49%(CaAl2Si2O8)
Al Ka Feldspars Only
White and Gibbs, 1969, Am. Min., noted that K-feldspar had the greatest Al Kb peak shift relative to Al metal (and sanidine more than microcline).
Al Ka Peak Shifts
Two independent measurements, very similar trends and not a simple function of Al coordination (e.g., consider the range in feldspars)
What about Si ?
Si Kb, Ka Chemical Shifts: Historical
White, McKinstry and Roy, 1962, GSA Abstract
Measured majorSi Kb shifts in SiO2 relative to Si metal:
Stishovite (IV): -0.010 ; Quartz, cristobalite (VI): -0.015
though no Si Ka Shift between IV and VI seen
Kaufman and Moll, 1966, Advances X-ray Analysis
Examined Si Ka1, Ka3, Ka4 and Kb for Si metal and 10 common silicate minerals; found differences between silicates
for all K lines but NOT Ka1
Si Ka Peak Shifts - UW SX51 - 2004
1 s Quartz
1 s Si metal
Microcline 1 s
1 s Olivine
Translating the above data as Ka shifts for quartz: Spectro1 = 0.5 0.1 eV; Spectro4 =0.60.1 eV
Compare Above With:
Further implication: there is a 0.7 - 0.9 eV shift for microcline Si Ka relative to Si metal
Si and Al Ka Peak Shifts:
How to locate peak center?
For many years I relied upon the ROM peaking built into the system.
but not anymore
Results of Automated Al Ka Peaking Options
ROM very reproducible: 10 measurements, s.d. of 1.2, range 32374-8
We now use John Donovans Probe for Windows software, which had 4 peak center methods.
Here is a post-scan on Al Ka, showing that the peak center returned by ROM was several units off the true peak center.
Operator now has final say over peaking
Results of other post-scans
and off a little
ROM was good
and off more
Accuracy in EPMA of Si and Al in silicate minerals requires attention to one of the first steps in calibration, defining the peak positions, because
On the basis for chemical shift in Al and Si Ka
Precision of EPMA peak measurements is much less than that possible
using XPS and AES, and those fields literature provide a basis for understanding the EPMA observations.
Streubel et al (1991 J. Electron Spectro & Related Phenom): Data on Si and P chemical shifts using XPS and AES
Figure 2 (top) plots relative Pauling
Electronegativity vs relative binding energy of L shell (2p)
DE(Ka) = DE(1s) - DE(2p)
Ka peak shift =
Difference (vs Si metal) in K binding energy minus
Difference in L binding energy