Coarsely crystalline gray hematite is an iron oxide (Fe2O3) initially discovered from orbit by the instrument TES (Thermal Emission Spectrometer). TES has detected gray hematite in this area of Capri Chasma, one of several large depressions that make up the Valles Marineris canyon system.
This HiRISE image shows light-toned units beneath darker mantles. At the Opportunity landing site in Meridiani Planum, the same gray hematite is found in millimeter size globules that have weathered out of the sulfate outcrop and become concentrated along upper soils.
It is likely that the same scenario is taking place here in Capri with the hematite grains forming in the light-toned sulfates and then eroding out and concentrating in the darker mantle soils.
Photo credit: NASA/JPL/University of Arizona