Détermination et étude de la structure cristalline de l'henritermiérite, hydrogrenat de symétrie quadratique

Henritermierite, Ca₃ (Mn₁,₅ , Al₀,₅) [(SiO₄)₂/(OH)₄] is tetragonal with a = 12.39 Å, c = 11.91 Å and eight formula units in the cell. The space group is I4₁/acd-D²⁰[subscript 4h]. The crystal structure has been determined with the use of three-dimensional F₀ and (F₀-Fc) Fourier synthesis and refined by least-squares methods. Each oxygen of a SiO₄ tetraedron is shared with an MnO₆ octaedron. Four of the six oxygens of MnO₆ are shared with SiO₄ tetraedra, the two last are OH. Calcium atoms lie in distorted cubic cavities ; the study of electroneutrality shows that the structure is ionic. Henritermierite belongs to the hydrogarnet family with regular replacement of SiO₄ by (OH)₄ tetraedra.