In an effort to provide additional information on the coordination of Cr3+ in chlorites, complete structural refinements were undertaken of two chromian chlorites from Day Book Body, North Carolina, and Siskiyou Co., California, both of which have been reported to contain predominantly tetrahedral Cr3+. In addition, eight one-dimensional electron density projections were constructed from X-ray intensity data taken from eight different chromian chlorites, three of which were reported to have tetrahedral Cr3+. The one-dimensional projections indicate the Cr3+ and other heavy atoms to be concentrated in the interlayer octahedral sites. There is good agreement between the number of electrons per formula unit derived from integration of cation electron density peaks and the number calculated from the microprobe analyses under the assumption that all heavy atoms are concentrated in the interlayer. Both three-dimensional studies were done on IIb-4 polytypes in triclinic space group Cl. In both structures the Cr3+ was preferentially concentrated in the M(4) octahedron on the inversion center within the interlayer. Si and Al were found to be disordered over the two independent tetrahedra. We propose that a combination of cation repulsion and crystal field effects concentrates the Cr3+ in the M(4) octahedron, and that a disordered Si,Al distribution in tetrahedral sites provides the most favorable balance of charge around M(4). The cell angle α was found to be triclinic, and it is postulated that repulsion between the tetrahedral and M(4) cations causes this slewing of the structure. As no evidence of Cr3+ in tetrahedral coordination was found, we propose that the names kotschubeite and kämmererite be discarded.

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