The structure of triclinic chloritoid and chloritoid polymorphism

The crystal structure of triclinic chloritoid from Chibougamau, Quebec, [a = 9.46(1), b = 5.50(1), c = 9.15(1)Å, α = 97.05(2), β = 101.56(2), and γ = 90.10(2)°; Z = 4] was solved using symbolic addition methods, and refined in space group Cl to R = 0.080 (R_w = 0.060). The triclinic structure is topologically very similar to that of its monoclinic polymorph. Substitution of Fe³⁺ for Al and Mg for Fe²⁺ occurs in the same two sites in the trioctahedral layer, M(1A) and M(1B) respectively, as in the monoclinic polymorph, and the observed average M–O bond distances for these sites (1.948 and 2.155Å) are in good agreement with those calculated using the refined occupancies of these sites. Other average bond distances in the structure are: M(2A)−0 = 1.907, M(2B1)−O = 1.906, M(2B2)–O = 1.906 (octahedral sites occupied by aluminum), and T–O = 1.638Å (tetrahedral site occupied by silicon). Because the structures are so similar, interlayer hydrogen bonding is assumed to occur in the same way as it does in the monoclinic structure, although no hydrogen positions were determined for triclinic chloritoid. The monoclinic and triclinic structures cannot be strictly considered polytypes because there are significant differences between the observed monoclinic atomic coordinates and those calculated from the triclinic atomic coordinates.