The crystal structure of type manganhumite from the Brattfors mine, Nordmark, Sweden [(Mn0.68Mg0.30Fe0.01Ca0.01)7(SiO4)3(OH)2, Pbnm, a = 4.815(1), b = 10.580(2), с = 21.448(5)A] has been refined by conventional least-squares methods to R = 0.051 (Rw = 0.081), using 1562 structure factors with Fobs > 2σ(Fobs). Mean Si–О distances for the isolated tetrahedra are 〈Si(l)–O〉 = 1.628A and 〈Si(2)–0〉 = 1.632A. The apical Si–О distances of 1.600 and 1.604A are the shortest yet observed in humite-group minerals. Site refinement yielded Mg/(Mg+Mn) ratios of 0.30 in the M(1)O6 octahedron (mean M–О distance = 2.170A), 0.0 in the M(2)O6 and M(2)O5OH octahedra (〈M–O〉 = 2.222 and 2.206A), and 0.75 in the M(3)O4(OH)2 octahedron (〈M–О〉) = 2.117A). The correlation coefficient of Mg/(Mg+Mn) occupancy and mean octahedral bond length is r = 0.986, and as expected, octahedral sizes correspond almost exactly to the weighted radii of the larger Mn and smaller Mg cations (r = 0.989).

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