The crystal structure of braunite II, Ca(Mn,Fe)143+SiO24, has been determined on a crystal from the Wessels Mine, near Hotazel, South Africa. It is tetragonal, space group I41/acd, with cell dimensions a = 9.431(2), c = 37.774(4)Å. The structure was refined by use of anisotropic thermal parameters and an extinction correction to a conventional R = 0.058. The structure consists of an (A′ABA)4 stacking sequence of three non-equivalent edge- and corner-linked polyhedral sheets. The A- and A′-sheets consist of edge- and comer-linked (Mn,Fe) octahedra, whereas the В-sheets consist of Ca in cubic coordination, Si in tetrahedral coordination, and (Mn,Fe) in octahedral coordination. The A- and A′-sheets are present in bixbyite with stacking sequence (A A′)2, whereas the stacking sequence of braunite is (AB)4. The A-, A′-, and В-sheets are virtually identical in the three structures. The structure determination confirms the validity of the braunite II structure as proposed previously. Comparative calculated X-ray diffraction data are presented so that distinctions can be made among these three minerals.

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