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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 Fe^sup 2+^ 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 Fe^sup 2+^ substitution for Mg using X-ray diffraction, infrared spectroscopy, and Mössbauer spectroscopy. Talc was synthesized in 0.5 Fe cation [0.17 X ^sub Fe^, X ^sub Fe^ = Fe/(Fe + Mg)] increments along the join Mg^sub 3^Si^sub 4^O^sub 10^(OH)^sub 2^-Fe^sub 3^Si^sub 4^O^sub 10^(OH)^sub 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 ^sub 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 ^sub Fe^ of talc, showing a deviation from the observed and expected composition starting at X ^sub Fe^ of 0.37 ± 0.03. Minor additional solid solution occurred beyond this to a maximum X ^sub Fe^ solubility of 0.50. Mössbauer spectroscopy indicated the dominance of octahedral Fe^sup 2+^ in talc with octahedral Fe^sup 3+^ 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 ^sub 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 Å^sup 3^ (142.6 ± 0.7 cm^sup 3^/mol) for the end-member Fe^sub 3^Si^sub 4^O^sub 10^(OH)^sub 2^. The observed upper limit of iron solubility in talc of about 0.5 X ^sub 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.