AbstractThe crystal structure of the rare skarn mineral trabzonite, Ca₄[Si₃O₉(OH)]OH, from the type locality near Ikizdere, Turkey and from the Upper Chegem caldera, Northern Caucasus, Kabardino-Balkaria, Russia has been solved and refined using single-crystal X-ray data. This shows that the chemical formula should be modified from Ca₄(Si₃O₁₀)˙2H₂O, reported in the original trabzonite description, to an OH-bearing composition. The crystal structure, which contains Si₃O₁₀ trimers embedded in a framework of CaO_6–8 polyhedra, has orthorhombic symmetry, space group Ama2, a = 20.6, b = 9.1, c = 10.3 Å. The orthorhombic A-centred cell is easily obtained by transformation from the original monoclinic cell of P2₁/m symmetry. The revised formula means that trabzonite and foshagite are polymorphs; foshagite has wollastonite-like silicate chains and in contrast to trabzonite it does not contain silanol groups. The structure and composition of killalaite from both localities was also studied. Single-crystal X-ray structure refinement of killalaite from the Northern Caucasus confirmed it to be non-stoichiometric with a composition between Ca₆[Si₂O₆(OH)]₂(OH)₂ and Ca₇[Si₂O₇]₂(OH)₂(Z = 2). Trabzonite, killalaite and dellaite form a series of modular structures which differ mainly in the degree of condensation of the SiO₄ units.