Natural and synthetic sulfate-containing calcite crystals were characterized using electron beam and X-ray techniques. Natural calcite was collected from LaDuke hot springs near Yellowstone National Park, U.S.A. Sulfur was found to be the predominant foreign element in the calcite, with a mean atomic S/Ca ratio of ca. 5%. X-ray microanalysis with a transmission electron microscope (TEM) indicated that sulfur was homogeneously incorporated in the crystals. The chemical species of sulfur was identified as sulfate ions () by X-ray photoelectron spectroscopy. Powder X-ray diffraction revealed the c-length of the calcite to be longer (ca. 0.25%) than that of geological pure calcite (Iceland spar), whereas the a-length was slightly shorter. Selected-area electron diffraction patterns from TEM showed extra reflections resembling those previously reported as c-reflections, corresponding to a super-cell with doubled ai-axes without breaking the three-fold symmetry. In order to investigate the origin of these crystallographic features in LaDuke calcite, synthetic calcite was precipitated from sulfate ion solutions with different concentrations. The sulfur content in the synthetic calcite crystals increased with the concentration of sulfate ions in the solutions and reached saturation at 4 at%. The synthetic calcite containing sulfate ions showed similar characteristics to those of natural LaDuke calcite, namely a longer c-length and the appearance of superstructure reflections, indicating that these characteristics are induced by the incorporation of ions in the calcite lattice.