6. (6 pts) Students A and B receive plagioclase samples and are asked to determi

Question

6. (6 pts) Students A and B receive plagioclase samples and are asked to determine the chemical composition. Student A polishes the as-received sample to obtain a planar surface of approximately 6 cm2 and performs chemical analysis using XRF-WDS Student B strikes his sample with a hammer, chooses a representative grain, and performs EDS using an electron microprobe. They obtain the following: Table 1: chemical analysis of plagioclase by students A and B. Atomic percent [% (esd)] [% (esd)) Sodium 5.16 (3 3.38 (2) Calcim 5.94 (3 8.85(4) Aluminum 14.04 (7) 15.88 (8) Silicon 27.22 (14) 24.80 (12) Explain, in context of the experiment, the discrepencies between the results of students A and B

Explanation / Answer

(i) Here, student 'A' analyzed the sample after polishing the received sample to obtain a planar surface having area ~ 6 cm2.

But student 'B' analyzed the sample after striking the sample with a hammer and by choosing a representative grain among the obtained grains/pieces of the sample.

(ii) Student A performed the analysis by WDXRF (wavelength dispersive X-ray fluorescence) is one of two general types of X-ray Fluorescence instrumentations used for elemental analysis. In WDXRF spectrometers, all of the elements in the sample are excited simultaneously. The different energies of the characteristic radiation emitted from the sample are diffracted into different directions by an analyzing crystal or monochromator (similar to the action of a prism dispersing different colors of visible light into different directions).

But student B performed the analysis by EDS (energy dispersive spectroscopy) using an electron microprobe. Data collection and results of EPMA (electron probe microanalyzer) chemical analysis vary depending on the method. Semi-quantitative EDS analysis is generally limited to recognizing different elements that may be present in an unknown sample. If one just wants to know what elements are present, there is little instrument set-up required.

The above two descriptions are responsible for the discrepancies between the results of students A and B.

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