CRYSTALLOGRAPHIC CONSEQUENCES OF MN SUBSTITUTION IN WILLEMITE

Willemite, Zn₂SiO₄, is a widely distributed but comparatively uncommon silicate occurring primarily as an ore or secondary mineral in zinc-rich deposits. Although it has an olivine-like formula and the Zn ionic radius lies between those of Mg and Fe, willemite crystallizes with the trigonal phenakite structure, having all cations in tetrahedral coordination (i.e., a "tectozincosilicate"). Significant to minor substituents reported for Zn are Mn, Mg, Fe, and Cu, with Mn substitution producing marked green fluorescence. We have performed electron microprobe analyses and single-crystal structure refinements on a specimen with ideal composition from Kabwe Province, Zambia, and on 6 Fe-free specimens containing up to 0.26 apfu Mn from Franklin, New Jersey, USA.  

Zn occupies two symmetrically distinct tetrahedra, Zn1 and Zn2. Zn2 is intrinsically larger than Zn1, as determined both by previous structure refinements of synthetic Zn₂SiO₄ and by the current refinement of our specimen from Zambia, suggesting Zn2 as the preferred site of Mn substitution. Population refinements of our Mn-rich specimens confirm a moderate preference: On average, about 25% more Mn enters Zn2 than enters Zn1. Substitution of Mn causes isotropic expansion of the unit cell. Both <Zn1-O> and <Zn2-O> show strong positive correlations with unit-cell volume, whereas <Si-O> is nearly constant among our specimens.  

Analysis of bond-length, bond-angle, and edge-length distortions reveals the following: (1) Distortion indices of SiO₄ tetrahedra are nearly constant across all specimens, as are bond-angle distortions for ZnO₄ tetrahedra. (2) Edge-length distortion parameters for ZnO₄ tetrahedra cluster in two groups that display little internal spread. (3) A weak negative correlation exists between refined electron population and bond-length distortion indices for Zn1 and Zn2. Because Si is bonded to all four unique oxygen atoms, these observations are consistent with a rather rigid oxygen framework in which Si-O bonds maintain essentially fixed equilibrium lengths, and in which the mean Zn1 and Zn2 positions shift in response to average chemical occupancy. Zn and Mn may have different equilibrium positions in willemite, similar to the observed behavior of these two cations in Zn-rich rhodonite, reported previously.