In situ investigations by the Mars Science Laboratory, Curiosity rover, have confirmed the presence of an ancient lake in Gale crater for up to 10 million years. The lake was filled with sediments that eventually converted to a compacted sandstone. However, it remains unclear whether the infilling of the lake was the result of background sedimentation processes or represents punctual flooding events in a largely isolated lake. Here we used the XRD data obtained with the Chemistry and Mineralogy instrument (CheMin), on board the Curiosity rover, to characterize the degree of disorder of clay minerals in the Murray formation (MF) at Gale crater. Our analysis shows that they are structurally and compositionally related to glauconitic clays, which are a sensitive proxy of quiescent conditions in liquid bodies for extended periods of time. Such results provide evidence of long periods of extremely low sedimentation in an ancient brackish lake on Mars, signature of an aqueous regime with slow evaporation at low temperatures. More in general, the identification of lacustrine glauconitic clays on Mars provides a key parameter in the characterization of aqueous Martian paleoenvironments that may once have harbored life.