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 P2(1)2(1)2(1) 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 Angstrom 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).