Mark Farnworth - Problem Set 5

Mark Farnworth - Pilkington European Technical Centre

Repeat the process of (2) for a potential third/second phase from the following range, again noting down the numerical address of your choice:

Four more pages of entries taken from the Hanawalt search manual:

Page:	30 (79 kBytes)
	31 (81 kBytes)
	32 (78 kBytes)
	33 (81 kBytes)

[10 Marks]

Of the remaining unidentified reflections, the most intense (A) is d= 2.742Å, the second most intense (B) is d= 2.102Å, the third most intense (C) is d= 1.700Å and the 'equal' forth most intense (D) reflections are d= 2.503Å and d= 1.939Å. At this stage, a strong assumption (because there were only a few reflections) was made that these five reflections were from the same phase and so this was borne in mind when searching the Hanawalt data pages 30 to 33.

I ran down the left hand column (most intense reflection) of each page, to find any entries that were 2.742+/-0.002. Having done this I then checked column 2 for correspondance with 2.102 +/- 0.002. If both column 1 and column 2 matched I then checked column 3 for correspondance with 1.700. ICDD Pattern 8-479 MgCO3, showed agreement with the three most intense unidentified reflections. Further confirmation was achieved when other entries of the of the 8-479 data were compared with the unidentified reflections.

Entry 4 (2.50) corresponded to reflection 2.503
Entry 5 (1.94) corresponded to reflection 1.939

Thus, matches had been obtained for all 5 of of the remaining unidentified reflections.

Comparison of the measured peak intensities for MgCO3 with the intensities given in the MgCO3 PDF card showed a very good correspondence. For example, the MgCO3 2.102Å reflection intensity should be 45% of the main 2.742Å MgCO3 reflection intensity according to the PDF card. 45% of 12% is 5.4%, which is very similar to the 6% measured value. Similar calculation showed similar agreement for the other MgCO3 reflections. It was noted that the elements in MgCO3 were consistent with the electron probe analysis (oxygen not detectable).

The magnesium carbonate ICDD Pattern Number 5-490 was typed into the dialogue box on the Problem Set 5 question sheet and the full pattern obtained. It was noted that magnesium carbonate has a reflection at 1.488 and is near to the calcite reflection at 1.510, this observation, therefore, explaining the unusually large FWHM for this reflection.

The reflections that can be attributed to magnesium carbonate are highlighted in green in the table below.

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

You should then comment on the solution.
[5 Marks]

A sample taken from ancient plaster figures has been found to contain three crystalline phases. These were:

Calcium carbonate (calcite), CaCO3 - ICDD Pattern 5-586
Quartz Silica, SiO2 - ICDD Pattern 5-490
Magnesium carbonate, MgCO3 - ICDD Pattern 8-479

All of the reflections present in the measured data can be explained by these three phases. The elements present in the three phases were also detected by electron microprobe analysis (excluding light elements C and O - as expected).

Addition of the intensities of all the reflections gives a total diffraction intensity of 441 units (counts).

The contribution of each phase to the total diffraction intensity is as shown below.

Calcite (243 units) = 55%
Silica (171 units) = 39%
Magnesium carbonate = 6%

The peak reflection intensities suggest that the main components are calcite and silica (together accounting for 94% of the total diffraction intensity).It should be noted that direct diffraction intensities are not a measure of concentration due to different detection sensitivities for each phase and any effects of preferred orientation. However, calculation has shown that the relative peak intensities of each of the three phases are similar to those shown on the respective PDF cards (excludind a calcite/silica reflection overlap at the 2.284Å position). Preferred orientation does not seem to be a big problem - assuming that the PDF data is free from preferred orientation!. The results are convincing enough to suggest that that the magnesium carbonate is a minor phase with calcite and silica present as major phases.