Orthorhombic Jahn-Teller distortion and Si-OH in mozartite, CaMn³⁺O[SiO₃OH]: A single-crystal X-ray, FTIR, and structure modeling study

The structure of mozartite, CaMn³⁺O[SiO₃OH], was refined in space group P2₁2₁2₁ from X-ray single-crystal data collected at 100 K (R = 2.45%, R_w = 2.62%), 300 K (R = 2.60%, R_w = 2.68%), and 500 K (R = 2.79%, R_w = 2.81%). The Mn³⁺O₆ octahedron shows approximately orthorhombic geometry, which is explained by a combination of a tetragonally compressed Jahn-Teller effect with lattice-induced stress. Comparison with isostructural vuagnatite, CaAl(OH)SiO₄, which shows no distortions due to electronic effects, indicates that the distorted octahedral geometry in mozartite causes shifts in valence sums of the O atoms that are hydrogen bonded. As a result, the OH group in mozartite is located at the isolated SiO₄ apex and is not linked to the octahedron as reported for isostructural minerals. Isostructural minerals of the adelite group with tetrahedral As⁵⁺ and V⁵⁺ exhibit a different Jahn-Teller distortion with tetragonally elongated geometry for octahedral Cu₂₊O₆. The O-H ⋯ O distance of <2.5 Å in mozartite is one of the shortest hydrogen bonded O ⋯ O distances in minerals and leads to a diffuse FTIR absorption peak (stretching mode) polarized parallel to b between 1300 and 1700 cm^-1.