The crystal structure of norrishite, KLiMn23+Si4O12: An oxygen-rich mica

The crystal structure of norrishite-1M was refined by least squares using 649 single crystal X-ray intensities to an R value of 0.078 (R_w = 0.098). The structure is monoclinic (C2/m) with cell parameters: a = 5.289(3) Å, b = 8.914(3) Å, c = 10.062(7) Å, β = 98.22(5)°. The O atom [O(4)] usually associated with H⁺ in an ideal mica is highly undersaturated (about 1.7 e.v.u.) based on weighted Pauling bond-strength calculations. Hence, the octahedrally coordinated Mn³⁺ cation is displaced from the polyhedral center toward the O(4)-O(4) shared edge [Mn-O(4) = 1.855 Å vs. Mn-O(3) = 2.023 Å]. The asymmetrical displacement of M(2) toward O(4) produces a charge deficiency on the apical O atom [O(3)], which is balanced by a short Si-O(3) distance of 1.571 Å.

Jahn-Teller distortions associated with Mn³⁺ are consistent with the distortions from electrostatic effects; octahedra are lengthened approximately parallel to [100] and shortened with respect to [010] and [001]. Relative lengths of shared octahedral edges are similar to those observed in dioctahedral micas and are the result of the size and charge differences between M(1) and M(2) (Li⁺ vs. Mn³⁺). These distortions, in addition to the relatively large ionic radii of Mn and Li, extend the lateral dimensions of the octahedral sheet and minimize tetrahedral rotation (α = 0.6°), thereby forming a hexagonal interlayer cation site. A narrow interlayer region (3.275 Å) results, in part, from coulombic interactions between O(4) and K. Displacement of Si toward O(3) may be due, in part, to Si-Si repulsion across the interlayer.