The crystal structure of humite [Mg6.6Fe0.4(SiO4)3F(OH); a 4.7408(1); b 10.2580(2); c 20.8526(4) Å; Pbmn] has been refined to a residual of R = 0.042. The steric details of the structure are similar to those of norbergite and chondrodite and can be understood in terms of geometrical and electrostatic interactions. In humite there are four distinct octahedra: M(1)O6 and M(2)Oe like those in olivine and M(2)O5(F,OH)1 and M(3)O4(F,OH)2 like those in chondrodite.
Ferrous iron is ordered in equal amounts (∼0.1 Fd2+) into the more distorted octahedra with six oxygen ligands but avoids the less distorted octahedra with one or two (F,OH ligands. However, in Mg/Fe- containing amphiboles, Fe2+ prefers the more distorted M(3)O4(OH,F)2 and M(1)O4(OH,F)2 octahedra over the less distorted M(2)O6 octahedron, despite the ligancy. This is rationalized on the basis that the M (2) cation is bonded ro two highly electrostatically undersaturated oxygens, which may be less polarizable than OH- F-.
Size criteria fail to predict the Mg/Fe distributions observed in chondrodite and humite. In these close-packed orthosilicates all anions are charge-balanced and Fe2+ prefers the more distorted siles and those with the more polarizable ligands. In Mg/Fe olivines the M(1)O6 octahedron is significantly smal]er but slightly more distorted than, M(2)O6. This may explain the highly disordered Mg/Fe configurations observed in olivines (Brown, 1970) and the slight ordering of Fe2+ into M(1)O6 reported by Finger (1970) for terrestrial and lunar olivines.
Considerations of M-(F,OH) bond lengths in tremolite, protoamphibole, and the humite minerals and Fe⇄OH-related volume changes in topaz give radii of 1.325Å and 1.34 Å, respectively, for two- and three-coordinated OH ions, based on Shannonand Prewitt’s (1969) fluorine radii.