Feodosiyite is a very rare chloride mineral, just recently approved, with the formula Cu<sub>11</sub>Mg<sub>2</sub>Cl<sub>18</sub>(OH)<sub>8</sub>â¢16H<sub>2</sub>O. Its structure is unique. Feodosiyite comes from the Tolbachik volcano, famous for many rare fumarolic minerals. Chemically similar minerals, chlorides containing both copper and magnesium, include haydeeite, paratacamite-(Mg) and tondiite.