MARL

earthy mixture of fine-grained minerals. The term is applied to a great variety of sediments and rocks with a considerable range of composition. Calcareous marls grade into clays, by diminution in the amount of lime, and into clayey limestones. Greensand marls contain the green, potash-rich mica mineral glauconite; widely distributed along the Atlantic coast in the United States and Europe, they are used as water softeners. Both marine and freshwater marls most commonly are of a white, gray, or brownish colour; red and black marls also occur. In calcareous marine marls, some lime is present in the form of shells, whereas in others it is a fine impalpable powder mixed with clay and siliceous silt. Freshwater marl may be similar in composition to marine marl. Much of the calcium carbonate in lake deposits is precipitated by algae, but some lake marls contain numerous shell fragments from freshwater snails and bivalves. Large deposits of freshwater marl that contain from 80 to 90 percent calcium carbonate and less than 3 percent magnesium carbonate have been used as the calcareous material required in the manufacture of insulating material and portland cement. Marl also is used as a liming material and in making bricks.