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Single crystals of katoite hydrothermally synthesized were examined by single-crystal X-ray diffraction, EPMA, and Raman spectroscopic techniques. Chemical formulas of the katoite fall inside the miscibility gap proposed by Kyritsis et al. (2009). The systematic absences observed with the singlecrystal X-ray diffraction were completely consistent with the cubic space group Ia-3d. In two katoites with chemical formulas, Ca3Al2(SiO4)0.57(H4O4)2.43 and Ca3Al2(SiO4)0.69(H4O4)2.31, O atom position was split into two independent crystallographic sites, O1 and O2; the O1 is coordinated to Si, whereas the O2 forms a tetrahedral interstice. The a lattice parameter monotonically decreased as increasing Si content. The coordination volume of T site decreased with Si incorporation into the T site. The variation lay on the straight lines between grossular and Si-free katoite solid solution. The coordination volume of CaO8 dodecahedra also decreased with the Si incorporation into the T site because edges of the CaO8 dodecahedron are shared with the adjacent TO4 tetrahedra. These contractions lead to the monotonous decrease of the a lattice parameter. Volumes of AlO6 octahedra, on the other hand, increased with the Si incorporation. There are no clear structural constraints resulting in the miscibility gap in the solid solution. The Raman band corresponding to the OH stretching vibration was observed at 3650 cm, but with the substitution of Si for H a new Raman peak appeared at 3580 cm (Fig. 1). The two Raman band positions remained unchanged with Si content. These results strongly suggest that there are two kinds of OH stretching vibration in the siliceous katoite. We therefore conclude that with Si substitution for H the O position is split into two inequivalent sites that correspond to SiO4 and H4O4 tetrahedra (Fig. 2).

Crystal structure change in grossular–Si–free katoite solid solution: Oxygen position splitting in katoite