Potassium Chloride (KCl) is a salt composed of the ions potassium (K+) and chloride (Cl-). It occurs naturally as the mineral sylvite and is a common substance in the Earth's crust. In addition to its use as a fertilizer and in the production of potassium-containing chemicals, KCl is also used in some optical applications, such as in infrared spectroscopy, as a substrate for growing single crystals of other materials, and as a substrate for growing thin films of certain materials. Due to its transparency in the visible and near-infrared regions of the spectrum, KCl can be used as a window material in spectroscopic applications, and it has a high resistance to thermal and mechanical damage, making it suitable for use in high-power laser systems.