Three-dimensional counter-diffractometer data and a full-matrix least-squares method have been used to refine the crystal structure of a ferrotschermakite in the space group C2/m. The chemical composition of the amphibole is Na0·23K0·14Ca1·86Mg1·22Fe2·102+Mn0·02Ti0·10Fe0·303+Al1·30Al2·00Si6·00O22(OH)2 with cell parameters a 9·8179(7), b 18·1060(14), c 5·3314(5) Å, and β 105·00(I). Cation site-occupancies were initially assigned by ionic-radius criteria and refined with the constraint that the sum total of the site-occupancies be equal to the chemical analysis.

The A-site shows positional and substitutional disorder and can be better represented by two sites, one on the mirror plane and the other on the 2-fold axis.

Unit weights were used throughout the refinement and the final R-factor for 1207 observed nonequivalent reflections was 4·5 %.