Abstract: The two most recent eruptions of Tolbachik Volcano, Kamchatka were used to demonstrate how joint volcanological and crystallochemical studies can make it possible to monitor active volcanoes, as well as to sample the products of their activity and perform joint investigations of these samples, ultimately leading to a better understanding of volcanic processes. In particular, X-ray crystallography and X-ray spectroscopy revealed that many minerals that were formed from volcanic gases contained uncommon cation oxy-centered tetrahedra [OA₄] (where A is a metal), e.g., [OCu₄]⁶⁺, unlike the well-known anion tetrahedra of the [SiO₄]^4– type. These studies have been conducted for 40 years, with the result that a new line of research has been systematized and generalized, viz., the crystal chemistry of inorganic compounds and minerals with assemblages of oxy-centered tetrahedra, or oxysalts. We set forth the essential principles of this science; in particular, the [OA₄] tetrahedra are treated as basic building units of minerals produced from volcanic exhalations, while the assemblages of such tetrahedra are considered to be a basic form of transfer of metals by volcanic gases in the high-temperature oxidizing environment of volcanism.