In the most general use of this term, a sediment that consists of a mixture of calcium carbonate (CaCO₃) and any other constituents in varying proportions. Marls usually are fine-grained, so that most marls consist of CaCO₃ mixed with silt and clay. The dominant carbonate mineral in most marls is calcite (CaCO₃), but other carbonate minerals such as aragonite (another form of CaCO₃), dolomite (Ca, Mg (CO₃)₂), and siderite (FeCO₃) may be present also.