This dataset contains X-ray diffraction data of powder drilled from the shell of A. benedeni benedeni specimen SG-125. This measurement was carried out in order to determine whether the originally aragonitic shell of this specimen contained any calcite of diagenetic origin. The spectrum of this specimen was compared to that of a modern aragonitic shell measured in-house (XRD_Fresh_Aragonite_Cerastoderma_edule.csv), and a calcite mineral from an external database (RRUFF database, ID R040070; Lafuente et al. 2015). The bivalve shells were collected in 2013 from a shell bed at the top of the Oorderen Member of the Lillo Formation of the Pliocene in Belgium. The collection site was a construction-related temporary outcrop at the Deurganck Dock Lock (now Kieldrecht Lock) in the city of Antwerp, located at 51°16′44″N 4°14′52″E. X-ray diffraction analysis was carried out in spring 2021 at the department of Earth Sciences at Utrecht University, the Netherlands, on a Bruker D8 diffractometer that was calibrated with a corundum crystal. Measurement conditions were as follows: 40 kV voltage, 40 mA current, 1.5418 Å (CuKα) radiation, 0.165 mm divergence slit, 2.5° primary soller slit, no secondary soller slit, 15-70° 2theta measuring range, 0.02° 2theta step-size, 0.85 s counting time, 15 rpm sample rotation. Reported are the angle (in degrees 2theta) and the corresponding intensity (in counts per second).