A molecular mixture of thallium malonate and thallium formate. Used as a heavy solution for the separation of minerals. The solution has a maximum density of 4.25 g/cm₃at 20 degrees C. It is prepared by adding formic acid to one of two equal quantities of thallium carbonate, and adding malonic acid to the other until each is neutralized. The 621 two solutions are then mixed, filtered, and evaporated until almandite floats. Compare: Sonstadt solution; Klein solution; bromoform; methylene iodide.