The structure of mozartite, CaMn (super 3+) O[SiO 3 OH], was refined in space group P2 1 2 1 2 1 from X-ray single-crystal data collected at 100 K (R = 2.45%, Rw = 2.62%), 300 K (R = 2.60%, Rw = 2.68%), and 500 K (R = 2.79%, Rw = 2.81%). The Mn (super 3+) O 6 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)SiO 4 , 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 SiO 4 apex and is not linked to the octahedron as reported for isostructural minerals. Isostructural minerals of the adelite group with tetrahedral As (super 5+) and V (super 5+) exhibit a different Jahn-Teller distortion with tetragonally elongated geometry for octahedral Cu (super 2+) O 6 . The O-H...O distance of <2.5 Aa 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 (super -1) .

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