MG-FE SUBSTITUTION IN SYNTHETIC TALC

The incorporation of ferrous iron into talc has been experimentally investigated to provide further insights into (1) the extent to which this substitution can occur and (2) the changes that this substitution has on the cell parameters of the talc structure.  The present study extends the range of bulk compositions examined in the earlier study of Forbes (1969, Am. Mineral.).

            Material was treated in 2.0 mm diameter Pd₅₀-Ag₅₀ tubing.  Six different starting bulk compositions were made along the join Mg₃Si₄O₁₀(OH)₂ - Fe₃Si₄O₁₀(OH)₂ in intervals of 0.5 atoms per formula unit (apfu) of Fe.  Starting mixtures were made using reagent MgO, SiO₂, and finely powdered metallic iron.  All experiments were done using cold seal vessels made from Renè 41 with water as the pressure medium and using the vessel to control the oxygen fugacity, which is assumed to be very close to the Ni-NiO buffer.  All experiments were done at 500^o C, 2.0 kbars, and with 15 wt% water added to the solid starting mixture.  Synthesis products were analyzed using X-ray powder diffraction and the unit-cell parameters were derived from Reitveld structural refinements of the powder patterns.  Talc with minor quartz was observed over the range of Fe contents of 0.0 to about 1.0 Fe apfu.  Above 1.0 Fe apfu, the phase assemblage included quartz, fayalite, and magnetite along with the talc solid solution.  It was observed that the unit-cell volume of the talc phase increased with increasing Fe content in the starting bulk compositions.  Where the talc formed from starting bulk composition of 0.5 Fe apfu had a volume of 453.3 ± 0.7 ų, and the talc formed from starting bulk composition of 2.5 Fe apfu had a volume of 457.4 ± 1.3 ų.  Based on this information Fe saturation in synthetic talc occurs at 1.8 ± 0.1 apfu, which correlates with a volume of 456.9 ± ~1.0 ų.  These results agree with the limit of Fe found in natural talc, where the Fe content rarely exceeds 2.0 Fe apfu.  This saturation point in the amount of iron content is probably related to the crystal-structural limit of Fe incorporation in talc.