Incorporation of 3d elements in tourmalines: structural adjustments and stability

The crystal structure of 45 tourmalines containing trivalent cations (Me³⁺: Cr³⁺, V³⁺, Fe³⁺) and 43 tourmalines containing divalent cations (Me²⁺: Fe²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺) of 3d elements were analysed. We establish that the incorporation of Me³⁺ cations is controlled by the ratio between the sizes of YO₆ and ZO₆ octahedra and does not exceed eight atoms per formula unit (apfu), while the incorporation of Me²⁺ cations is controlled by the charge balance and does not exceed 3 apfu. We prove that there are no structural constraints controlling the incorporation of Co, Ni and Cu cations into the tourmaline structure at concentrations of up to 1 apfu. We show that, when the limiting content (less or equal to 1 apfu and 1.6 apfu for divalent and trivalent cations, respectively) is reached, these cations start to occupy not only YO₆, but also ZO₆ octahedra, to maintain stability of the crystal structure. As the content of 3d-element cations increases further, the degree of disorder grows and, at a content of ~7 apfu, their distribution is close to statistical. An increase in the content of 3d cations, accompanied by their disordering, causes a decrease in disparity between sizes of YO₆ and ZO₆ octahedra and their deformations.