The formation of an insoluble material in a fluid. Precipitation can occur by a chemical reaction of two or more ions in solution or by changing the temperature of a saturated solution. There are many examples of this important phenomenon in drilling fluids. Precipitation occurs in the reaction between calcium cations and carbonate anions to form insoluble calcium carbonate: Ca⁺² + CO₃^-2 --> CaCO₃. When a saturated clear brine first crystallizes, the solid is a precipitate, and is often caused by changing temperature.