Vera Rubin ridge (VRR), located in Gale crater, Mars, contains coarse-grained (>3-5 µm) gray hematite particles. VRR likely experienced multiple episodes of diagenesis in its past, some of which may have formed gray hematite from existing iron(III) minerals. The precursor mineralogy and diagenetic fluid conditions, however, are unknown. Experiments were performed on several iron(III) minerals (akaganeite, ferrihydrite, goethite, red hematite, jarosite, and schwertmannite) to simulate transformations in diagenetic fluid conditions. The solid transformation products were analyzed via powder X-ray diffraction, visible to near-infrared reflectance spectroscopy, and scanning electron microscopy. The compositions of the final fluids were determined via inductively-coupled plasma optical emission spectrometry. Final pH values of the fluid products were measured as well.