Stephen Seddio, Thermo Fisher Scientific, Madison, Wisconsin, USA
Key Words
Igneous Minerals, Low Beam Current, Principle Component Analysis,
Scanning Electron Microscope (SEM), Spectral Phase Mapping,
Wavelength-Dispersive Spectroscopy (WDS)
Introduction
The rigorous quantitative analysis of minerals by wavelength-dispersive spectroscopy (WDS) is typically done
using an electron microprobe, a WDS-specialized analytical
instrument consisting of as many as five wavelengthdispersive spectrometers. Routine mineralogical quantitative
analysis typically includes WDS analysis of ~10 elements,
which makes the multiple spectrometers of the microprobe
appealing. In this paper, the results of doing such analysis
in an scanning electron microscope (SEM) using a Thermo
Scientific™ MagnaRay™ WD spectrometer and a Thermo
Scientific™ NORAN™ System 7 X-ray microanalysis system
are reported. Typically, such results are accompanied by
bulk compositional and petrographic results from the
same sample. The scope of this paper is to investigate the
ability and practicality of doing WDS quantitative analysis
of common igneous rock-forming minerals using an SEM
with the quality of electron-probe microanalysis (EPMA).
Methods
Sample Preparation
A basaltic sample was mounted in epoxy as a petrographic
thick section. WDS quantitative analysis is sensitive to
surface roughness and flatness. The sample was iteratively
polished using 9, 3, and 1 µm diamond suspensions.
Finally, the sample was polished using 0.3 µm alumina.
After each polishing step, using both diamond suspension
and alumina, the sample was washed with soapy water
and sonicated in order to avoid carrying over coarser
diamond polishing compound to the next step and to
avoid contaminating the sample with alumina.
Analytical methods
Quantitative WDS microanalysis was done using the
NORAN System 7 analyzer. X-rays were counted using a
MagnaRay parallel beam WD spectrometer, which
contains a sealed Xe proportional counter and a hybrid
collimating X-ray optic.1 Analytical conditions were a
15 kV accelerating voltage and a 25 nA probe current.
Natural and synthetic minerals were used as primary and
secondary standards. The X-rays, diffracting crystals, and
on- and off-peak count times used for analysis are in
Table 1. X-ray maps were generated using NORAN
System 7, a MagnaRay WD spectrometer, and a 10 mm2
Thermo Scientific™ UltraDry™ EDS detector. A phase map
was generated using Thermo Scientific™ COMPASS™
spectral phase mapping, which identifies unique phases
based on the principle component analysis of the EDS
spectrum at each pixel.2,3
Appli cat i on N ote 5 2 6 1 7
SEM-Based WDS Analysis of Common
Igneous Rock-Forming Minerals
Results
Table 1: WDS quantitative analytical details
The quantitative results are in Table 2. The plagioclase
grains are zoned with core compositions that are relatively
more calcic (An69Ab30Or1) compared to the grain rims
(An45Ab52Or3; Figures 1 and 2). The K-feldspar grains are
not zoned with an average composition of An1Ab15Or84.
The pyroxene grains are exsolved into low and high Ca
lamellae (Figures 3 and 4). The low Ca pyroxene compositions are En46-54Fs39-50Wo2.5-11, and the high Ca pyroxene
compositions are En28-38Fs19-31Wo31-47. The olivine (Fo41-43)
grains are typically rounded, consistent with being
resorbed. The sample is ilmenite and magnetite rich and
trace phases include silica, sulfides, Fe oxides, baddeleyite,
and zirconolite (qualitatively identified by WDS energy scan).
Element
X-ray
Crystal
On-Peak
Time (s)
Off-Peak
Time (s)
Na
Kα
TAP
20
20
Mg
Kα
TAP
20
20
Al
Kα
TAP
20
20
Si
Kα
TAP
20
20
P
Kα
PET
20
20
K
Kα
PET
20
20
Ca
Kα
PET
20
20
Ti
Kα
LiF
20
20
Mn
Kα
LiF
20
20
Fe
Kα
LiF
20
20
Table 2: Average major mineral compositions in the basalt sample
K-feldspar
n
Plagioclase
5
SiO2
Al2O3
25
7
Olivine
7
5
σ
Wt%
σ*
Wt%
σ
Wt%
σ
Wt%
σ
68.3
0.96
54.9
2.0
50.6
0.77
50.1
1.41
33.4
0.35
0.55
0.20
<0.1
–
1.00
0.52
16.9
0.01
0.08
0.12
26.0
0.03
0.98
FeO
0.26
0.07
0.84
0.22
MnO
0.06
0.04
0.08
0.02
MgO
n.a.
CaO
Na2O
High Ca Pyroxene
Wt%
0.08
TiO2
Low Ca Pyroxene
–
n.a.
–
0.18
0.04
11.9
1.12
1.61
0.36
0.61
28.5
0.10
0.20
2.13
0.65
17.1
0.09
16.3
2.52
0.43
0.13
0.06
1.25
12.5
0.97
2.54
1.50
19.9
4.03
0.03
47.0
0.52
0.67
0.04
19.3
0.36
0.22
0.05
0.35
5.07
0.70
0.13
0.03
0.32
0.12
0.14
0.03
K 2O
13.2
0.75
0.33
0.07
0.08
0.01
<0.06
0.02
0.09
0.03
P 2 O5
<0.05
–
0.06
0.02
0.12
0.03
<0.1
–
0.11
0.01
Sum
100.6
99.3
100.7
101.1
101.1
“n” is the number of analyses averaged. “σ” is the standard deviation of the averaged oxides. “n.a.” refers to an oxide that was not included in the analysis.
* The plagioclase is compositionally zoned (Figures 1 and 2) so in that case the standard deviation of the oxide concentrations in plagioclase is not an accurate
representation of analytical precision.
Detection limits were calculated using the method of Scott and Love.4
Table 3: Maximum percent error for each analyzed mineral based on counting statistics
K-feldspar
SiO2
TiO2
Al2O3
0.25
37.5
0.47
Plagioclase
0.26
8.33
Low Ca Pyroxene
High Ca Pyroxene
0.28
0.26
0.36
7.27
BDL
11.1
0.38
4.92
3
7.14
1.02
1.41
FeO
19.2
MnO
66.7
50.0
MgO
n.a.
n.a.
0.70
0.80
CaO
22.2
1.18
3.94
1.06
9.23
14.0
Olivine
15.4
0.77
8.96
0.67
18.2
Na2O
3.11
2.17
30.8
12.5
28.6
K 2O
1.06
9.09
37.5
BDL
33.3
P 2 O5
BDL
16.7
BDL
18.2
33.3
This table reports the maximum percent error for each oxide from all analyses averaged in Table 2 (see “n”). The errors reported here are based solely on counting statistics.
“n.a.” refers to an oxide that was not included in the analysis. “BDL” refers to an oxide that was below the detection limit for the given analysis.
Figure 1: a) Backscattered electron image of a zoned plagioclase grain.
b) K (red; EDS), Na (green; EDS), and Ca (blue; WDS) X-ray maps. Silica and
fractures are black. K-feldspar is red. The relatively anorthitic plagioclase core
is blue. The relatively albitic plagioclase rim is green. c) A COMPASS phase
map (see text) in which silica is yellow, fractures are black, K-feldspar is green,
the anorthitic plagioclase core is blue, and the albitic plagioclase rim is red.
The scale bar in a) is 20 µm in b) and c).
Figure 4: Pyroxene quadrilateral diagram showing the pyroxene compositions
found in the basaltic sample. High Ca pyroxene analyses are red. Low Ca
pyroxene analyses are blue. Analyses with intermediate compositions are a
result of electron-probe interaction volume overlap and secondary fluorescence
because of the fine scale of the exsolution.
Conclusions
By using the MagnaRay parallel beam WD spectrometer,
WDS quantitative analysis can be done with the same
analytical rigor as with EPMA. A typical ten element
mineralogic analysis on a five WD spectrometer microprobe typically takes 3–4 minutes. Because this work was
done with a single WD spectrometer, one might expect
that each analysis took 15–20 minutes; however, because
the MagnaRay spectrometer is able to virtually eliminate
the time required to drive a Rowland circle spectrometer
from one analytical position to the next, the work in this
paper consists of analyses that required ~7 minutes each.1
Another practical consideration is that much EPMA work
is done using a defocused beam. An SEM may not be
capable of operating in this mode requiring that the analysis
of beam-sensitive material be done at low beam current.
References
Figure 2: Feldspar ternary diagram showing the K-feldspar and plagioclase
compositions found in the basaltic sample. K-feldspar analyses are red.
Plagioclase analyses are blue.
1. T
hermo Fisher Scientific (2014) Principles and Applications of Parallel
Beam Wavelength Dispersive X-ray Spectroscopy, White Paper 52608.
2. K
eenan, M.R. and Kotula, P.G. (2003) Apparatus and System for
Multivariate Spectral Analysis. Patent# US 6,584,413. 24 Jun. 2003.
3. K
eenan, M.R. and Kotula, P.G. (2004) Method of Multivariate Spectral
Analysis. Patent # US 6,675,106. 06 Jan. 2004.
4. S cott, V.D. and Love, G. (1983) Quantitative Electron-Probe
Microanalysis. Wiley & Sons, New York.
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Appli cat i on N ote 5 2 6 1 7
Figure 3: Ca (red),
Mg (green), and Fe (blue)
EDS X-ray maps of
exsolved pyroxene
merged into a single
RGB image.