Clerici solution is a solution of equal parts of thallium formate (Tl(CHO)) and thallium malonate (Tl(CHO)) in water. It is a freely flowing, odorless liquid which changes from yellowish to colorless upon decreasing the concentration of the thallium salts. At a density of 4.25 g/cm at 20 °C, the saturated Clerici solution is one of the heaviest aqueous solutions known. The solution was invented in 1907 by the Italian chemist Enrico Clerici (1862–1938) and introduced to mineralogy and gemology in 1930s as a valuable liquid, which allowed separating minerals by density with a traditional flotation method. Its advantages include transparency and variable and easily controllable density in the range 1–5 g/cm.
The density of Clerici solution is so high that minerals such as spinel, garnet, diamond and corundum can float in the solution. The density of the saturated solution (thallium formate plus thallium malonate in water) increases from 4.25 to 5.0 g/cm upon heating from 20 to . (Note that density of water decreases from 1.0 to 0.96 g/cm in the same temperature range.) It can be reduced by diluting the solution with water down to 1 g/cm. The refractive index shows significant, linear and well reproducible variation with the density; it changes from 1.44 for 2 g/cm to 1.70 for 4.28 g/cm. Thus the density can be easily measured by optical techniques.
The color of the Clerici solution changes significantly upon minor dilution. In particular, at room temperature the concentrated solution with the density of 4.25 g/cm is amber-yellow. However, a minor dilution with water to the density of 4.0 g/cm makes it as transparent as glass or water (absorption threshold 350 nm).
A common procedure for mineral density measurement using the Clerici solution is as follows: a grain of mineral is placed in a small tank filled with the concentrated solution so that the grain floats on the surface. Then water is added drop-wise until the grain becomes suspended in the liquid. Then the density of the liquid is evaluated either directly by weighing or indirectly through its refractive index, which is measured with the Abbe refractometer.
The drawback of the Clerici solution is its high toxicity and corrosiveness.