Railsback's Some Fundamentals of Mineralogy and Geochemistry X-ray diffraction (XRD) of aragonite and calcite

Dickinson and McGrath (2001, The Analyst 126, 1118–1121) used the ratio of integrated peak areas (not heights) of the calcite 104 and aragonite 221 peaks to develop a quantitative relationship for ratio of aragonite to calcite. That approach yields good quantitative results, if with the somewhat unsettling proviso that a 50-50 mixture of the two ploymorphs has a 104/221 intensity ratio much greater than 1, because of the exceptional intensity of the calcite 104 peak. Another approach that yields a more intuitively acceptable result is to compare the calcite 113 and aragonite 221 peaks. In this case, an 50-50 equal mixture of calcite and aragonite generates peaks of about the same height, facilitating a quick-look approach. On a pattern generated with CoKα radiation, these peaks are at about 46 and 54 °2Θ, and thus conveniently symmetrically positioned around 50°. On a pattern generated with CuKα radiation, they're at about 40 and 46 °2Θ, not quite so conveniently symmetrically positioned around 43°, but not hard to find.