Summary: Three-dimensional counter-diffractometer data corrected for absorption and a full matrix least-squares method have been used to refine the crystal structure of an oxy-kaersutite in the space group C2/m. The chemical composition of the amphibole is Na0·543K0·434Ca1·916Mg3·188Fe1·0648+Ti0·515Mn0·009Al0·370Si5·878Al2·122O22(O2−)2 with cell parameters a 9·8920(11), b 18·064(2), c 5·3116(7)Å and β 105·388(5)°. Cation site occupancies were refined using bulk chemical constraints and the A-site positionally disordered. The final conventional R-factor for 1041 observed reflections is 4·0%.

The rotation of the tetrahedral chains in calcic clino-amphiboles is shown to be a function of the Al^iv content of the tetrahedral sites. Comparison of this oxidized amphibole with an hydroxyamphibole of similar chemical composition indicates that on oxidation the maximum amount of structural relaxation takes place in the vicinity of the M(I) octahedra, which distort to a high-energy configuration.