Barium oxide is made by heating barium carbonate. Barium hydroxide ("baryta") was known to alchemists, who produced it by heating barium carbonate. Reactions of barium hydroxide with ammonium salts are strongly endothermic. Other metal nitrates have been used, including barium and strontium nitrates. Industrially, barium hydroxide is used as the precursor to other barium compounds. Barium titanate crystals find use in nonlinear optics. It decomposes upon heating into barium carbonate. Early piezoelectric materials used in transducers such as barium titanate had poor temperature stability. Discolorations can result from mineral impurities such as pyrites or barium carbonates. It belongs to the ABO structure ferroelectrics like lithium tantalate and barium titanate. Liquid mixed metal stabiliser systems are primarily based on barium, zinc and calcium carboxylates. It has some properties similar to barium carbonate. White smoker vents emit lighter-hued minerals, such as those containing barium, calcium and silicon. The mineral is called bazirite, named after the chemical elements barium and zirconium. This application exploits the very low solubility of barium sulfate.