Abstract

The crystal structure of norbergite, Mg2SiO4·MgF1.8(OH)0.2 from Franklin, New Jersey (a = 4.7104(l); b = 10.2718(3); c = 8.7476(4) Å; Pbnm,ρ(obs) = 3.177 g/cc) was refined by least-squares techniques to R=0.052 using 867 intensities weighted by the range-estimate method (weighted 7? = 0.024). The structure is based on a slightly distorted hexagonal close-packed array of anions with one-half the octahedral sites occupied by Mg and one-twelfth the tetrahedral sites occupied by Si. F is ordered in the array and bonded to three Mg; its temperature factor is twice that of four-coordinated O. Comparable bond lengths involving F and O indicate that the radius of F is ~0.11 Å smaller than that of O.

As in forsterite, the dominant structural unit in norbergite is a zigzag chain of edgesharing octahedra lying parallel to z. A detaiied study of the structure in terms of bondangle strains shows that the distortions from an ideal hexagonal close-packed model can be explained qualitatively in terms of ca-tion-cation repulsion across shared polyhedral edges. The Si-O bond lengths (1.635, 1.638 Å) opposite shared edges are significantly longer than that (1.612 Å) opposite the unshared edges of the tetrahedron.

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