The experimental incorporation of Fe into talc: a study using X-ray diffraction, Fourier transform infrared spectroscopy, and Mössbauer spectroscopy

Talc is a common Mg-rich trioctahedral layer silicate that occurs both as a primary and as a secondary mineral in a wide range of rock types. Substitution of Fe2+ for Mg is fairly extensive in certain rock types, particularly banded iron formations, yet there is relatively limited fundamental crystal-chemical information on this substitution. This study is an experimental investigation of Fe2+ substitution for Mg using X-ray diffraction, infrared spectroscopy, and Mossbauer spectroscopy. Talc was synthesized in 0.5 Fe cation [0.17 X Fe, X Fe = Fe/(Fe + Mg)] increments along the join Mg3Si4O10(OH)2–Fe3Si4O10(OH)2 over the range of 350–700 °C, oxygen fugacities (fO2) from ~Ni–NiO to 3.3 log(fO2) units below Ni–NiO, and at a pressure of 0.2 GPa. High yields of talc without any coexisting Fe-bearing phases were obtained up to 0.33 X Fe, beyond which talc coexisted with fayalitic olivine, magnetite, or both, indicating saturation in Fe for syntheses along the talc join. Infrared spectroscopy was used to determine independently the X Fe of talc, showing a deviation from the observed and expected composition starting at X Fe of 0.37 ± 0.03. Minor additional solid solution occurred beyond this to a maximum X Fe solubility of 0.50. Mossbauer spectroscopy indicated the dominance of octahedral Fe2+ in talc with octahedral Fe3+ ranging from 2.9 to 21.5 at.%, depending on the ambient fO2. X-ray diffraction analysis did not confirm the strong dependence of the interplanar spacing d 003 on the oxygen fugacity as reported earlier in the literature. This study provides the first experimentally constrained unit-cell volume of 474.4 ± 2.2 A3 (142.6 ± 0.7 cm3/mol) for the end-member Fe3Si4O10(OH)2. The observed upper limit of iron solubility in talc of about 0.5 X Fe agrees with the majority of analyses reported for talc, and that values above this are attributed to intergrowths of talc with the structurally distinct minnesotaite.