The 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 CaO6–8 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.