Tables for
Volume H
Powder diffraction
Edited by C. J. Gilmore, J. A. Kaduk and H. Schenk

International Tables for Crystallography (2018). Vol. H, ch. 3.7, pp. 309-310

Section Water-still deposit

J. A. Kaduka,b,c*

aDepartment of Chemistry, Illinois Institute of Technology, 3101 South Dearborn Street, Chicago, IL 60616, USA,bDepartment of Physics, North Central College, 131 South Loomis Street, Naperville, IL 60540, USA, and cPoly Crystallography Inc., 423 East Chicago Avenue, Naperville, IL 60540, USA
Correspondence e-mail: Water-still deposit

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A water still in my home eventually generates scale, much of which flakes off the walls, permitting easy analysis in the powder diffractometer. Any commercial search/match program will easily identify magnesian calcite (Fig. 3.7.2[link]; files kadu1389.gsas, kadu1389.raw and iitd26_0510.prm, available in the supporting information). There are, however, three additional weak peaks at d/I = 4.788/38, 3.3089/51 and 2.3697/56. In Naperville, Illinois, the tap water comes from Lake Michigan. The bedrock underlying the Chicago region is the Racine Dolomite. Given the identity of the major phase in the scale and the source of the water, it seems likely that any minor phases will be mineral-related and contain some combination of the elements Ca, Mg, C, O and H (to include the possibility of hydrates and hydroxides). Accordingly, a search of mineral-related entries containing `just' the elements Ca, Mg, C, O and H was performed and used as a filter in a Hanawalt search using these three peaks. This limits the search universe to 692 of the 328 660 entries in the PDF-4+ in 2012. The seven highest goodness-of-match entries in the hit list were brucite, Mg(OH)2. This phase was added to the Rietveld refinement. Analysis of the difference plot indicated an unaccounted-for peak at a d-spacing of 3.3089 Å. A search for mineral-related entries with the same chemistry and having one of their three strongest peaks in the range 3.309 (30) Å yielded the vaterite polymorph of CaCO3 as the hit with the highest goodness of match. This phase was added to the Rietveld refinement. The final quantitative phase analysis was: 94.7 (1) wt% Ca0.84Mg0.16(CO3), 5.2 (4) wt% Mg(OH)2 and 0.2 (1) wt% vaterite.

[Figure 3.7.2]

Figure 3.7.2 | top | pdf |

The result of applying a commercial search/match program (Jade 9.5; Materials Data, 2012[link]) to the powder pattern of a water-still scale. Weak peaks not accounted for by the major magnesian calcite phase are apparent and additional tools in the Powder Diffraction File were needed to identify the additional phases.

The composition of the major phase was refined, constraining the sum of the Ca and Mg site occupancies to equal 1.0. To understand how this fitted with previous magnesian calcites, a search for compounds containing only Ca, Mg, C and O, ambient conditions and space group No. 167 was carried out. Some manual editing of the hit list was required. Adjusting the preferences to include the display of composition in at.% and the unit-cell volume made it convenient to plot the variation in unit-cell volume as a function of Mg content x in Ca1−xMgx(CO3) (Fig. 3.7.3[link]). This magnesian calcite in the water-still scale has a higher Mg concentration than most, but falls close to the trend line. The flexibility and content of the Powder Diffraction File makes such data mining relatively straightforward.

[Figure 3.7.3]

Figure 3.7.3 | top | pdf |

Variation of the unit-cell volume with the magnesium content in magnesian calcites in the Powder Diffraction File.

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