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

Abstract

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 XFe, XFe = 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 XFe, 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 XFe of talc, showing a deviation from the observed and expected composition starting at XFe of 0.37 ± 0.03. Minor additional solid solution occurred beyond this to a maximum XFe 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 d003 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 XFe 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.