X-Ray diffraction data from the Curiosity rover found evidence of glauconitic clays, which are sensitive to the duration of sedimentary conditions within bodies of water. This result is indicative of a low-temperature aqueous environment that remained in a steady state for extended periods of time, favouring its habitability. In situ investigations by the Mars Science Laboratory Curiosity rover have confirmed the presence of an ancient lake that existed 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 X-ray diffraction data obtained with the Chemistry and Mineralogy instrument onboard the Curiosity rover to characterize the degree of disorder of clay minerals in the Murray formation 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, the 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 palaeoenvironments that may once have harboured life.