The Problem: A collection of ancient plaster figures has been uncovered. A small sample is taken: microscopic examination identifies calcite (CaCO3) and electron microprobe analysis reveals Ca, Si and Mg as the major heavier elements. A powder diffraction pattern is collected, which after initial data analysis, yields the parameters below (the third parameter "FWHM" is a measure of the peak width). Identify the two remaining phases in the sample. Explain, step by step, how you arrived at this solution. Comment on the solution.

ANSWERS TO THE PROBLEM SET 5

The following table identifies the various phases present in the plaster. The d-values indexed black are due to the calcite. The silicate (5-490) and magnesium carbonate (8-479) are indexed with blue and red respectively.

d (Å) I (%) FWHM(°)     d (Å) I (%) FWHM(°)     d (Å) I (%) FWHM(°)
4.27 28   0.27     2.237 4   0.23     1.700 5   0.35
3.86 12   0.30     2.128 8   0.23     1.672 6   0.23
3.343 82   0.25     2.102 6   0.35     1.659 2   0.24
3.035 100   0.28     2.095 18   0.27     1.626 4   0.29
2.845 3   0.28     1.980 4   0.23     1.604 8   0.28
2.742 12   0.35     1.939 2   0.37     1.587 2   0.29
2.503 2   0.36     1.927 5   0.29     1.541 12   0.23
2.495 15   0.28    1.913 17   0.28     1.525 5   0.28
2.458 10   0.22    1.875 17   0.28    1.518 4   0.30
2.284 29   0.43    1.817 15   0.22    1.510 4   0.42

The d-value indexed with the green is an overlap of calcite (113) and silica (102) peaks. This is also evident from thier high FWHM values. Similarly the d-value indextedThe d-value indexed with the violet is an overlap of (211) of magnesium carbonate and (217) of silica.

Thus the mixture contains three phases of decreasing % content

  1. CALCITE
  2. SILICA
  3. MAGNESIUM CARBONATE

The presence these three phases is further confirmed from electron microprobe analysis which revealed the presence of Ca, Si and Mg.