Recently we have found weak extra reflections similar to
c-reflections in the ED patterns of calcite precipitated in a hot-spring (La Duke) near Yellow Stone National Park. However, X-ray microanalysis indicated that the amount of impurity cations such as Mg
2+ is very small and sulfur is the major impurity element. S/Ca atomic ratio is about 3%. The cell parameters of the La Duke sample determined from XRD, indicated that the
a-length (4.9803Å) is slightly decreased and the
c-length (17.076Å) slightly increased compared to the pure calcite (
a = 4.9857Å,
c = 17.0624Å). The Selected-area ED pattern along the [001] direction indicated that the extra reflections appear holding the three-fold symmetry of calcite. Combining these results and with a future experiment to determine the chemical species of sulfur by XPS, we are now constructing a model of the superstructure to form
c-reflections in the present specimen.
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