Life is composed of organic compounds, and characterizing preserved compounds provides insights into the presence of specific types of life, including early life on Earth. With growing evidence for a wet early Mars, excitement over the potential of an early Martian biosphere strongly motivates Mars exploration. However, the preservation potential of organic compounds in rocks on either Earth or Mars depends critically on mineralogy. Results from elemental, mineralogical, and morphological characterization of sedimentary rocks by the Opportunity rover team in the vicinity of the Challenger Memorial Station, Meridiani Planum, demonstrate an abundance of Fe(III) and sulfate minerals that formed from liquid water. The composition of these sedimentary rocks suggests that organic compounds are unlikely to be preserved within them, even if present when the rocks were deposited, based on comparisons with iron formation, acid lake deposits, and iron-containing concretions from Earth. No evidence consistent with the presence of organic compounds, such as the presence of Fe(II) minerals, has been reported from Meridiani Planum to date. Thus these Martian sedimentary rocks are not a good target to explore for organic compounds on Mars.