Structural studies of boracites. The cubic phase of chromium chlorine boracite, Cr₃B₇O₁₃Cl

See also ibid., vol.7, no.21, 3840 (1974). Results are presented for the first full three-dimensional structural study of a single growth sector of a pure synthetic boracite in its cubic phase. The sample chosen was a (100) growth sector of chromium chlorine boracite, Cr₃B₇O₁₃Cl. The data were collected at room temperature with Zr-filtered MoK alpha radiation on a four-circle diffractometer out to sin Theta / lambda =0.95 AA^-1. The techniques of constrained refinement and statistical testing have been used to test carefully the significance of (i) various features of the positional parameters and (ii) the inclusion of parameters to describe third- and fourth-order (anharmonic) terms in the thermal motion of ions displaying unusually high vibrational amplitudes. It is considered that the positional parameters describe a structure essentially the same as that found for cubic Mg₃B₇O₁₃Cl by Ito, Morimoto and Sadanaga (1951).