| d (Å) | I (%) | FWHM |
|---|---|---|
| 3.86 | 12 | 0.30 |
| 3.035 | 100 | 0.28 |
| 2.845 | 3 | 0.28 |
| 2.495 | 15 | 0.28 |
| 2.284 | 29 | 0.43 |
| 2.095 | 18 | 0.27 |
| 1.927 | 5 | 0.29 |
| 1.913 | 17 | 0.28 |
| 1.875 | 17 | 0.28 |
| 1.626 | 4 | 0.29 |
| 1.604 | 8 | 0.28 |
| 1.587 | 2 | 0.29 |
| 1.525 | 5 | 0.28 |
| 1.518 | 4 | 0.30 |
| 1.510 | 4 | 0.42 |
The intensity of the peak at 2.284 Å is 29 % but on the calcite 5-586 data card the intensity is only 18 and the FWHM is large. Therefore, a peak from another phase also present.
Comment: You have got the right idea, and well
done for noting the mismatch of intensity for the 2.284 line suggesting
another phase. A complementary list of the unaccounted (by calcite) lines
would have been useful at this stage.
4 Marks
2. The remaining peaks were listed and the peak with the largest intensity,
which was 3.34 Å, was decided to be the most appropriate
peak to search through the Hanawalt search manual from page 13 to 16 first. This
was because the first peak in the line in the Hanawalt manual is the most
intense. If there was a match then the other peaks were matched with other peaks
in the sample.
The matching peaks are shown in the table below:
| Hanawalt manual: | 3.34 | 4.26 | 1.82 | 1.54 | 2.46 | 2.28 | 1.38 | 2.13 |
| sample: | 3.343 | 4.27 | 1.817 | 1.541 | 2.458 | 2.284 | - | 2.128 |
The file no. for this phase is 5-490 and the formula is
SiO2.
This result also agrees with the electron microprobe
analysis because it indicated that Si was present.
Comment: This is the correct solution. Note that
the agreement should be in both positive and negative senses; i.e. you must
also explain that there are no lines missing in the sample that are present
in the data file - in fact there are 2 missing (see model answer) but this
can be explained on account of their weak intensities.
6 Marks
3. The peaks for SiO2 were also eliminated from the list and the
remaining peaks listed to find the third phase. Again the most intense peak was
chosen, 2.742 Å, and was searched for in pages 30-33 of the
Hanawalt manual. If a match was found then it was seen if there was match with
the other peaks.
The matching peaks are shown in the table below:
| Hanawalt manual: | 2.74 | 2.10 | 1.7 | 2.50 | 1.94 | 1.35 | 1.34 | 0.91 |
| sample: | 2.742 | 2.102 | 1.7 | 2.503 | 1.939 | - | - | - |
The file no. for this phase is 8-479 and the formula is
MgCO3.
This result also agrees with the electron microprobe
analysis because it indicated that Mg was present.
Comment: Again the correct solution, though the
missing line (in this case 2.318) requires comment (too weak to see etc.).
The table that follows in (4) is very good and makes it all much clearer;
for example, the attribution of 2.284 to calcite and quartz you have included.
A quibble is that similarly the 1.510 line should be attributed to both
magnesite and calcite, hence the broad peak (see model answer).
8 Marks
4. The results were checked by obtaining their full PDF patterns. Both matched the two phases in the sample and the table below accounts for all the peaks in the sample:
| d (Å) | I (%) | FWHM | Phase |
|---|---|---|---|
| 4.27 | 28 | 0.27 | SiO2 |
| 3.86 | 12 | 0.3 | CaCO3 |
| 3.343 | 82 | 0.25 | SiO2 |
| 3.035 | 100 | 0.28 | CaCO3 |
| 2.845 | 3 | 0.28 | CaCO3 |
| 2.742 | 12 | 0.35 | MgCO3 |
| 2.503 | 2 | 0.36 | MgCO3 |
| 2.495 | 15 | 0.28 | CaCO3 |
| 2.458 | 10 | 0.22 | SiO2 |
| 2.284 | 29 | 0.43 | CaCO3 & SiO2 |
| 2.237 | 4 | 0.23 | SiO2 |
| 2.128 | 8 | 0.23 | SiO2 |
| 2.102 | 6 | 0.35 | MgCO3 |
| 2.095 | 18 | 0.27 | CaCO3 |
| 1.980 | 4 | 0.23 | SiO2 |
| 1.939 | 2 | 0.37 | MgCO3 |
| 1.927 | 5 | 0.29 | CaCO3 |
| 1.913 | 17 | 0.28 | CaCO3 |
| 1.875 | 17 | 0.28 | CaCO3 |
| 1.817 | 15 | 0.22 | SiO2 |
| 1.700 | 5 | 0.35 | MgCO3 |
| 1.672 | 6 | 0.23 | SiO2 |
| 1.659 | 2 | 0.24 | SiO2 |
| 1.626 | 4 | 0.29 | CaCO3 |
| 1.604 | 8 | 0.28 | CaCO3 |
| 1.587 | 2 | 0.29 | CaCO3 |
| 1.541 | 12 | 0.23 | SiO2 |
| 1.525 | 5 | 0.28 | CaCO3 |
| 1.518 | 4 | 0.30 | CaCO3 |
| 1.510 | 4 | 0.42 | CaCO3 |
The phases present in the sample are calcite (5-586), silica (5-490) and magnesium carbonate (8-479).
The search of the two unknown phases was also assisted by the microprobe analysis results because the heavier elements present were known. The overlapping of peaks of the different phases was not major making the search easier. It is assumed that the measuremnt of the sample was taken accurately as possible and preferred orientation was minimised. The close agreement of the measured intensities with the peak intensities from the actual data cards indicates that the diffraction pattern had been taken carefully.
Comment: A good summing up which covered the main
points together with your earlier point that your assignments were consistent
with the microprobe analysis.
5 Marks
Comment: Overall a very good answer and correct
solution.