Analysis of a Contact Metamorphosed Calc-Silicate Rock Using Quantitative WDS

Rocks are an agglomeration of various minerals or chemical compounds. The shape, type and distribution of minerals in rocks are determined by the formation mechanism and environment.

Gaining insights into the chemical compositions of the minerals in these agglomerates is crucial to identify the types of mineral and their textural evolution.

This article discusses the application of Wavelength Dispersive Spectroscopy (WDS) to quantitatively determine the elemental composition of each of the identified minerals.

Materials and Instrumentation

This experiment used a thin section of contact metamorphosed Leadville Limestone. After sectioning and polishing the limestone, it was placed in a glass slide and final polished to optical transparency.

The Thermo Scientific™ NORAN™ System 7 (Figure 1) with seamless integration of Thermo Scientific™ UltraDry™ EDS and Thermo Scientific™ MagnaRay™ WDS detectors (Figure 2) was used to analyze this contact metamorphosed calc sample in a FESEM without carbon coating.

NORAN System 7 X-ray microanalysis system

Figure 1. NORAN System 7 X-ray microanalysis system

UltraDry EDS and MagnaRay WDS

Figure 2. UltraDry EDS and MagnaRay WDS

The analytical conditions are summarized in Table 1:

Table 1. Analytical conditions

Sample Contact Metamorphosed Calc-Silicate
EDS Analyzer NORAN System 7
EDS Detector UltraDry 10 mm2 Silicon drift detector
WDS Detector MagnaRay spectrometer
Accelerating Voltage 15 kV
Beam Current 10 nA

Quantitative Analysis

Using known composition standards, NORAN System 7 can offer the option to acquire EDS-only quantitative data, quantitative EDS plus WDS data, or quantitative WDS-only data. For this sample, the MagnaRay WDS-only quantitative analyses were carried out on the five major mineral phases determined by EDS phase analysis.

Using commercially available known mineral standards, elemental X-ray intensity reference measurements were obtained. Out of the 53 potential standards that could be selected, standards were chosen based on the similarity of the measured EDS compositions and the tabulated standards compositions of the desired elements.

The final standards employed for quantification were (1) Diposide for O, Mg, Ca and Si, (2) Biotite for K and Al, (3) Hematite for Fe, (4) Marcasite for S, and (5) Barite for Ba. Furthremore, it is necessary to select a diffractor in the WDS spectrometer for each elemental line.

Only a single diffractor available (Mg-K on TAP, S-K on PET, Fe-K on LIF) for most lines, but for other lines, especially very high and low energy X-ray lines (O-K on NiC or TAP, Al-K on TAP or PET, Si-K on TAP or PET, K-K on PET or LIF, Ca-K on PET or LIF, and Ba-L on PET or LIF), more options are available.

Using the diffractor with the lowest energy range (O-K on NiC, Al-K on TAP, Si-K on TAP, K-K on PET, Ca-K on PET, and Ba-L on PET) is the typical diffraction selection for these lines (Figure 3).

Figure 3

The net X-ray intensities for each of these standards need to be measured as part of the quantification routine. Therefore, both peak intensity and the background intensity have to be measured.

The background intensity at both a lower and a higher energy location is acquired and interpolated for the peak energy by the NORAN System 7. By this way, equal spacing of the background energies away from the peak intensity can be avoided, thus providing more options to the operators.

After selecting the diffractors for the elements, it is necessary to evaluate the background energy locations of each element to confirm that there is no coincidence or overlapping of energy between the peaks.

The default energy table under the Standards radio button on the WDS Setup tab in the software performs energy selection. However, analyst has to perform the slight energy adjustments typically required for high precision quantification work (Figure 4).

Figure 4

During the use of multi-elemental standards for quantification, it is necessary to setup the standard in the software by keying in the composition of each element in the standard. The stage location is also entered for unattended acquisition to facilitate the acquisition of the WDS intensities (Figure 5).

After inputting all standard compositions, it is necessary to select an appropriate standard from the potential list of standards containing the element for each elemental line (Figure 6).

Figure 5

Figure 6

For each standard, auto alignment of the spectrometer is the recommended method of acquisition as the highest precision intensities are required by the highest precision compositions. The standards “Start” button is pressed to begin the acquisition of all of the standards.

This moves the stage to the stored stage location for the first standard in the list, followed by reading the beam current from the Faraday cup (either stage mounted or column mounted) and aligning the WDS spectrometer for the desired elemental line to the optimum intensity.

A precise alignment is obtained by adjusting the alignment acquisition time. The peak intensity measurement is performed until termination depending on the time or precision desired, followed by the acquisition of the intensity at each of the low and high energy background intensities for half of the peak collection time.

Once completed, all of the WDS information will be displayed by the WDS Acquisition Status tab and the values will be saved in the standard file. All of the elemental lines for each standard were collected by the system before shifting the stage to the next standard in the list and repeating the collection procedure (Figure 7).

Figure 7

Acquisition of WDS quantification values is not much more involved when compared to the acquisition for EDS full-standards quantification. It is necessary to select the standards in the Standards tab (Figure 8 and 9), followed by enabling the Auto Quant On option on the Processing tab (Figure 10).

The next step is changing the matrix correction method to “… with Standards” on the Analysis Setup tab (Figure 11), followed by the selection of the “Acquire WDS” option on the EDS tab of the Acquisition Properties (Figure 12).

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

The WDS quantification process will also begin with a simple Start of the EDS acquisition if the material present in the SEM image is unknown. The seamless integration of the WDS control in the EDS control software facilitates WDS acquisitions. The beam current measurement will be performed first after the selection of the alignment option.

The peak and background intensities will be acquired to termination defined by either the time or precision for all of the elements in the standards list. The WDS Acquisition Status tab will show the WDS intensities and the Quant Results tab will display the quantification values. The WDS intensities and the resultant quantification values will be stored in the EDS EMSA file.

The stage will be shifted to each point for data collection in succession without user input in the case of stage automation to collect acquisitions from different points on the sample. It is possible to turn off the beam voltage of the SEM if desired at the end of the complete Automation acquisition.

The WDS quantitative data acquired from the minerals in the calc-silicate sample are summarized in Table 2. The weight percent and atomic percent elemental data are in line with the values predicted by mineral stoichiometries. The phlogopite wt% totals are low as predicted for hydrous mineral as it is not possible to measure H content with X-rays.

Table 2

Wt% Phlogopite Phlogopite Phlogopite Phlogopite Diopside Diopside Diopside Diopside Pyrite Pyrite Barite Barite Hematite
O 45.97 45.92 45.46 46.17 45.89 45.39 45.72 45.35 0.00 0.00 27.46 27.52 32.39
Mg 13.83 14.50 13.84 13.92 9.02 9.17 9.12 8.93 0.00 0.00 0.00 0.00 0.01
Al 10.62 10.80 10.17 10.26 3.46 3.68 3.33 3.73 0.00 0.00 0.00 0.00 0.01
Si 17.85 1 7.41 17.17 17.10 22.91 22.81 22.12 22.64 0.00 0.00 0.00 0.00 0
S 0.01 0.00 0.02 0.00 0.00 0.02 0.01 0.00 53.19 52.57 13.67 13.11 0.1
K 8.69 8.89 8.59 8.54 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01
Ca 0.02 0.02 0.01 0.01 18.25 18.31 17.78 18.37 0.00 0.00 0.00 0.00 0
Fe 1.43 1.58 1.50 1.50 0.93 1.01 0.88 1.36 48.91 49.28 0.00 0.00 66.58
Ba 0.26 0.46 0.31 0.36 0.15 0.11 0.14 0.22 0.00 0.00 58.75 58.23 0.01
Total 98.68 99.58 97.07 97.86 100.61 100.50 99.10 100.60 102.10 101.85 99.88 98.86 99.11
Atom% Phlogopite Phlogopite Phlogopite Phlogopite Diopside Diopside Diopside Diopside Pyrite Pyrite Barite Barite Hematite
O 60.85 60.46 61.13 61.45 61.60 61.15 62.06 61.20 0.00 0.00 66.75 67.36 62.79
Mg 12.05 12.57 12.25 12.20 7.97 8.13 8.15 7.94 0.00 0.00 0.00 0.00 0.01
Al 8.34 8.44 8.11 8.10 2.76 2.94 2.68 2.99 0.00 0.00 0.00 0.00 0.06
Si 13.46 13.06 13.15 12.97 17.52 17.51 17.11 17.41 0.00 0.00 0.00 0.00 0.01
S 0.01 0.00 0.01 0.00 0.00 0.02 0.01 0.00 66.67 66.17 16.60 16.02 0.09
K 4.71 4.79 4.73 4.65 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01
Ca 0.01 0.01 0.01 0.01 9.78 9.85 9.63 9.90 0.00 0.00 0.00 0.00 0.01
Fe 0.54 0.60 0.58 0.57 0.36 0.39 0.34 0.53 33.33 33.83 0.00 0.00 37.01
Ba 0.04 0.07 0.05 0.06 0.02 0.02 0.02 0.03 0.00 0.00 16.65 16.61 0.01
Total 100.01 100.00 100.02 100.01 100.01 100.01 100.00 100.00 100.00 100.00 100.00 99.99 100.00

Conclusion

From the results, it is evident that the quantitative compositions of mineral phases in a geological sample can be rapidly determined using the NORAN System 7 system with fully integrated UltraDry EDS and MagnaRay WDS detectors.

The dataset shown here are useful to understand the mineralogical and textural evolution of this calc-silicate rock during contact metamorphism.

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

For more information on this source, please visit Thermo Fisher Scientific – Materials & Structural Analysis.

Ask A Question

Do you have a question you'd like to ask regarding this article?

Leave your feedback
Submit