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.

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