Earthy mixture of fine-grained minerals, which range widely in composition. Lime (calcium carbonate) is present as shell fragments of snails and bivalves, or as powder mixed with clay and silica-containing silt. Large deposits contain 80–90% calcium carbonate and less than 3% magnesium carbonate. With decreasing amounts of lime, calcium-containing marls are called clays and clayey limestones. Marls rich in potash (potassium carbonate), called greensand marls, are used as water softeners. Marls have also been used in the manufacture of insulating material and portland cement, as liming material, and in making bricks.