SEM-Based WDS Analysis of Common Igneous Rock-Forming MineralsStephen Seddio, Thermo Fisher Scientific, Madison, Wisconsin, USA

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Key WordsIgneous Minerals, Low Beam Current, Principle Component Analysis, Scanning Electron Microscope (SEM), Spectral Phase Mapping, Wavelength-Dispersive Spectroscopy (WDS)

IntroductionThe rigorous quantitative analysis of minerals by wave-length-dispersive spectroscopy (WDS) is typically done using an electron microprobe, a WDS-specialized analytical instrument consisting of as many as five wavelength-dispersive 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

ResultsThe 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 compo-sitions 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

K-feldspar Plagioclase Low Ca Pyroxene High Ca Pyroxene Olivine

n 5 25 7 7 5

Wt% σ Wt% σ* Wt% σ Wt% σ Wt% σ

SiO2 68.3 0.96 54.9 2.0 50.6 0.77 50.1 1.41 33.4 0.35

TiO2 0.08 0.01 0.12 0.03 0.36 0.10 0.55 0.20 <0.1 –

Al2O3 16.9 0.08 26.0 0.98 0.61 0.20 1.00 0.52 0.13 0.03

FeO 0.26 0.07 0.84 0.22 28.5 2.13 16.3 2.52 47.0 0.52

MnO 0.06 0.04 0.08 0.02 0.65 0.09 0.43 0.06 0.67 0.04

MgO n.a. – n.a. – 17.1 1.25 12.5 0.97 19.3 0.36

CaO 0.18 0.04 11.9 1.12 2.54 1.50 19.9 4.03 0.22 0.05

Na2O 1.61 0.35 5.07 0.70 0.13 0.03 0.32 0.12 0.14 0.03

K2O 13.2 0.75 0.33 0.07 0.08 0.01 <0.06 0.02 0.09 0.03

P2O5 <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

K-feldspar Plagioclase Low Ca Pyroxene High Ca Pyroxene Olivine

SiO2 0.25 0.26 0.28 0.26 0.36

TiO2 37.5 8.33 11.1 7.27 BDL

Al2O3 0.47 0.38 4.92 3 15.4

FeO 19.2 7.14 1.02 1.41 0.77

MnO 66.7 50.0 9.23 14.0 8.96

MgO n.a. n.a. 0.70 0.80 0.67

CaO 22.2 1.18 3.94 1.06 18.2

Na2O 3.11 2.17 30.8 12.5 28.6

K2O 1.06 9.09 37.5 BDL 33.3

P2O5 BDL 33.3 16.7 BDL 18.2

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.

Table 1: WDS quantitative analytical details

Table 2: Average major mineral compositions in the basalt sample

Table 3: Maximum percent error for each analyzed mineral based on counting statistics

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 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.

Figure 3: Ca (red), Mg (green), and Fe (blue) EDS X-ray maps of exsolved pyroxene merged into a single RGB image.

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.

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ConclusionsBy 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 micro-probe 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.

References1. Thermo Fisher Scientific (2014) Principles and Applications of Parallel

Beam Wavelength Dispersive X-ray Spectroscopy, White Paper 52608.2. Keenan, M.R. and Kotula, P.G. (2003) Apparatus and System for

Multivariate Spectral Analysis. Patent# US 6,584,413. 24 Jun. 2003.3. Keenan, M.R. and Kotula, P.G. (2004) Method of Multivariate Spectral

Analysis. Patent # US 6,675,106. 06 Jan. 2004.4. Scott, V.D. and Love, G. (1983) Quantitative Electron-Probe

Microanalysis. Wiley & Sons, New York.