A new occurrence of katoite (Ca2.95Fe0.03A12.03(SiO4)1.12(OH)7.51; a = 12.286) found near Dunabogdány (Hungary), along with its structural refinement is reported here. The crystal-chemical data, the physical properties, and the structure refinement of this new member of the Ca3Al2(SiO4)3-Ca3Al2(O4H4)3 series indicate the presence of about 37% grossular. The positional disorder of the oxygen atom, a peculiar feature of the hydrogarnet structure, is described here in terms of both anisotropy along the d-O vector, and presence of two distinct oxygen sites (unsplit- and split-O models, respectively). The hydrogen atom was localised for both models in the difference Fourier map. The structural features of the katoite from Dunabogdány are compared with those of other hydrogrossulars and with the two end-members “anhydrous” grossular, and Si-free katoite. The results of the unsplit-O model refinement confirm the increase of the tetrahedron volume with the substitution of Si by 4H+. The geometry of the coordination polyhedra in the two configurations resulting from the split-O model are also discussed. This second model better describes from the crystallochemical point of view the substitution of Si4+ with 4H+ in the tetrahedra. However, the tetrahedra of the two dimensions are randomly distributed in the unit cell and hence domains of the two end-member configurations are not expected in katoite structure. This conclusion is also strengthened by the TEM study of katoite from Dunabogdany.**

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