Abstract: The structure refinement of a hastingsite (Cl 0.91 p.f.u.) from West Ongul Island, Lützow-Holm Bay, East Antarctica, has been carried out in order to characterise the effect of Cl on the crystal structure. The composition of the Cl-rich hastingsite is K_0.69Na_0.26Ca_1.99Mg_1.09Fe²⁺_2.71Fe³⁺_0.92Mn_0.04Ti_0.10-Al_2.31Si_5.83O₂₂OH_0.99Cl_0.91F_0.10 with unit cell parameters a 9.962(3), b 18.283(7), c 5.372(1) Å and β 104.87(3)°.

In the Cl-rich hastingsite, Al, Fe³⁺ and Ti occupy only the M(2) site, and Fe²⁺ strongly prefers the M(1) and M(3) sites to the M(2) site with respect to Mg. The Mg-Fe²⁺ distribution suggests that the Mg-Cl avoidance is realised in the intracrystalline exchange reaction among the octahedral sites in the Cl-rich hastingsite. The individual position of Cl and OH was refined and the effective ionic radius of Cl in fourfold coordination is determined as 1.79 Å. Substitution of Cl for OH expands the sizes of the M(1) and M(3) sites and extends the double chains in comparison with hydroxy calcic amphiboles. Cl especially, is close to the O(6) and O(7) oxygens beyond the minimum distance calculated from effective ionic radii. Therefore, the Cl-rich hastingsite is considered to be deformed locally by Cl.