Effect of chlorine on the crystal structure of a chlorine-rich hastingsite

The structure refinement of a hastingsite (CI 0.91 p.f.u.) from West Ongul Island, Liitzow-Holm Bay, East Antarctica, has been carried out in order to characterise the effect of CIon the crystal structure. The composition of the CI-rich hastingsite is KO.69Nao.26Ca1.99Mg1.09Fe2+ 2.71Fe3+O.92MnO.\l4 Tio.lOAI2.31Sis.R30220Ho.99Clo.91Fo.10with unit cell parameters a 9.962(3), b 18.283(7), c 5.372(1) A and ~ 104.87(3r. In the CI-rich hastingsite, AI, Fe3+ and Ti occupy only the M(2) site, and Fe2+ strongly prefers the M(I) and M(3) sites to the M(2) site with respect to Mg. The Mg-Fe2+ distribution suggests that the Mg-CI avoidance is realised in the intracrystalline exchange reaction among the octahedral sites in the C\-rich hastingsite. The individual position of CI and OH was refined and the effective ionic radius of CI in fourfold coordination is determined as 1.79 A. Substitution of CI for OH expands the sizes of the M(I) and M(3) sites and extends the double chains in comparison with hydroxy calcic amphiboles. CI especially, is close to the 0(6) and 0(7) oxygens beyond the minimum distance calculated from effective ionic radii. Therefore, the CI-rich hastingsite is considered to be deformed locally by CI. KEYWORos: CI-rich hastingsite, structure refinement, chlorine effect, amphibole. Introduction CALCIC amphibole with CI replacing OH occurs in metamorphic, plutonic and volcanic rocks (Suwa et ai., 1987). The highest content of CI recorded so far is 7.24 wt. % in a calcic amphibole from a skarn in Dashkesan (Krutov, 1936). High concentrations (over 3.0 wt.%) of CI in calcic amphiboles have been reported by Jacobson (1975), Dick and Robinson (1979), Sharma (1981), Kamineni et ai. (1982), Yanko (1986), and Suwa et al. (1987). These CI-rich calcic amphiboles are high in Fe, and are mostly of hastingsite defined by Leake (1978). Suwa et ai. (1987) pointed out that the CI-rich hastingsites are high in AllY and (Na + K) contents and K/(Na + K) value. The chemical behaviour of Cl-rich amphiboles has been interpreted from a structural point of view by Ito and Anderson (1983), Volfinger et ai. (1985), Suwa et ai. (1987), and Yamaguchi (1989). No structure refinement of CI-bearing amphiboles, however, has been made with special Mineralogical Magazine, December ]993, Vol. 57, pp. 677-685 @ Copyright the Mineralogical Society attention given to the role of CI in the structure. In this paper, the crystal structure of a CI-rich hastingsite is described fully and the effect of chlorine on the structure is shown clearly.