THE PYROPHANITE-GEIKIELITE SOLID-SOLUTION SERIES: CRYSTAL STRUCTURES OF THE MN1-XMGXTIO3 SERIES (0 < X < 0.7)

Abstract

Members of the pyrophanite-geikielite solid-solution series, Mn1-xMgxTiO3 (0 < x < 0.7 apfu Mg), were obtained by solid-state synthesis at 1000°C at ambient pressure in air. In common with ilmenite, ternary Mn-Mg titanates adopt ordered R3 structures. The maximum solubility of Mg in MnTiO3 under the given conditions is considered to be ~0.6 apfu Mg, as compounds with greater Mg content could not be synthesized. The structures of these titanates were refined by the Rietveld method from powder X-ray-diffraction data. Within the solid-solution series, unit-cell parameters and unit-cell volumes decrease with increase in Mg content. All compounds consist of distorted TiO6 and AO6 (A = Mn,Mg) octahedra, and in common with geikielite, pyrophanite, and ilmenite (sensu lato), the TiO6 octahedra are distorted to a greater degree than (Mn,Mg)O6. The extent of displacement of (Mn,Mg) and Ti from the centers of their coordination polyhedra varies irregularly with increasing Mg content, reaching a maximum for x = 0.1 and 0.2 apfu Mg. Entry of Mg2+ into the VIA site results in "puckering" of layers consisting of TiO6 octahedra (less distorted) above and below planes parallel to (001), and decreased "puckering" of the AO6 octahedra (more distorted). The interlayer distance across the vacant octahedral site in the TiO6 layer decreases regularly with entry of the smaller Mg2+ cation into the VIA site. The absence of natural solid-solutions between geikielite and pyrophanite seems to be due to the contrasting geochemistry of Mn and Mg rather than to crystallochemical reasons.