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

Abstract

Single crystals of katoite hydrothermally synthesized were examined by single–crystal X–ray diffraction, EPMA, and Raman spectroscopic techniques. The chemical formulas of the katoite fell inside the miscibility gap proposed by Kyritsis et al. (2009). The systematic absences observed through single–crystal X–ray diffraction were completely consistent with the cubic space group Ia3d. In two kinds of katoite with chemical formulas Ca₃Al₂(SiO₄)_0.57(H₄O₄)_2.43 and Ca₃Al₂(SiO₄)_0.69(H₄O₄)_2.31, the O atom position was split into two independent crystallographic sites, O1 and O2; the O1 is coordinated with Si, whereas the O2 forms a tetrahedral interstice. The a lattice parameter monotonically decreased as increasing Si content. The variation lay along a straight line between grossular and Si–free katoite solid solution. The coordination volume of the T site decreased with Si incorporation into the T site. The coordination volume of CaO₈ dodecahedra also decreased with the Si incorporation into the T site because the edges of the CaO₈ dodecahedron are shared with the adjacent TO₄ tetrahedra. These contractions lead to a monotonous decrease of the a lattice parameter. The volume of the AlO₆ octahedra, on the other hand, increased with the Si incorporation. There were no clear structural constraints resulting in a miscibility gap in the solid solution. A Raman band corresponding to the OH stretching vibration was observed at 3650 cm⁻¹, but with the substitution of Si for H, a new Raman peak appeared at 3580 cm⁻¹. The two Raman band positions remained unchanged with increasing Si content. These results strongly suggest that there are two types of OH stretching vibration in siliceous katoite. We therefore conclude that with Si substitution for H the O position is split into two inequivalent sites that correspond to the SiO₄ and H₄O₄ tetrahedra. The oxygen position splitting in katoite results in the emergence of two Raman bands at 3580 and 3650 cm⁻¹.