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Al and Si Quantitation in Routine Silicate EPMA: Maybe not so routine

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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.

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slide1

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

slide2

Motivation

You mean your new automatic gizmo can’t 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).

slide3

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

slide4

Peak Widths

±1s

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

slide5

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

slide6

Conclusion 1

Si and Al Ka Peak Widths on TAP:

  • Many peaks ~ 5 sin theta units wide
  • For correctly centered peak position, > 2-3 sin theta unit shift will lose counts, and
    • For 5-6 units shift, at least 1% error
    • For 7-8 units shift, at least 2% error
    • And these errors would be doubled for oxide values (oxygen by stoichometry)
slide7

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)

slide8

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

slide9

First attempt 2003…

Al Ka Peak Shifts - UW SX51

But not reproducible...

slide10

… 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)

slide12

Al Ka Peak Shifts

7 units or 0.6 eV

Al metal

TAP sp1

slide13

Al metal

NaAlSi3O8

KAlSi3O8

82%(NaAlSi3O8) - 18%(CaAl2Si2O8)

(Na,K)AlSi3O8

51%(NaAlSi3O8) - 49%(CaAl2Si2O8)

BaAl2Si2O8

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).

slide14

Coord

4

6

4

4

6

4

6

6

6

6

6+4

4

4

4

4

Coord

4

4

6

6

6

6

6

4

4

6

6+4

4

4

4

4

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)

slide16

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 Å; Quartz, cristobalite (VI): -0.015Å

˙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

slide18

1 s Quartz

1 s Si metal

Microcline 1 s

1 s Olivine

slide19

Translating the above data as Ka shifts for quartz: Spectro1 = 0.5 ±0.1 eV; Spectro4 =0.6±0.1 eV

A check

Compare Above With:

  • Si Ka shifts of Quartz by HRXFS (high resolution x-ray fluorescence spectroscopy)
  • Okura et al (1990 Spectrochimica Acta) a-quartz 0.655 eV
  • Liu et al (2004 Physical Review B) “SiO2” 0.62 eV

Further implication: there is a 0.7 - 0.9 eV shift for microcline Si Ka relative to Si metal

slide20

Conclusion 2

Si and Al Ka Peak Shifts:

  • Al: need pay special attention to which specific minerals are being analyzed, and use appropriate standard for peaking/counting (feldspars especially!)
  • Si: special attention to K, Na feldspars
slide21

How to locate peak center?

  • Automation
    • ROM routine
    • Computer data fit
  • Semi-automation
    • (operator assisted)
  • Manual
slide22

My experience

For many years I relied upon the ROM peaking built into the system.

…but not anymore

slide23

Results of Automated Al Ka Peaking Options

ROM very reproducible: 10 measurements, s.d. of 1.2, range 32374-8

slide25

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

slide26

Results of other post-scans

… and off a little

ROM was good

slide28

Conclusions

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

  • There are Si and Al Ka chemical peak shifts between some “common” silicate minerals.
  • Vertical stage drift can yield peak shifts.
  • Automated peak search routines should be used with a critical eye.
slide29

Electronegativity ---->

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

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