rautasubstituution

Rautasubstituution is a term used in mineralogy and solid-state chemistry to describe the substitution of iron ions into crystal lattices, forming solid solutions with other cations. In practice, iron (Fe2+ and Fe3+) can replace magnesium, manganese, calcium, aluminum and other cations in minerals such as olivine, pyroxene, amphibole, garnet, and magnetite, or vice versa. Substitution occurs when the substituting ion has a similar charge and roughly comparable ionic radius, allowing the lattice to accommodate the different species with limited distortion. The extent of substitution defines a solid-solution series and is influenced by temperature, pressure, and oxygen fugacity.

In olivine, the (Mg,Fe)2SiO4 series shows a continuous range from forsterite to fayalite as Fe substitutes for Mg. Pyroxenes and amphiboles also display Fe-Mg solid solutions such as enstatite-ferrosilite and augite-hornblende. In oxide systems, magnetite (Fe3O4) and hematite (Fe2O3) form a subsystem with varying Fe3+/Fe2+ proportions, affecting magnetic properties and color. In feldspar and other framework silicates, iron substitution commonly occurs with coupled substitutions to maintain charge balance.

Rautasubstituution is important for interpreting petrology and geochemistry. The solid-solution behavior records magmatic differentiation, temperature, and redox conditions, and it underpins techniques such as geothermobarometry and ore genesis models. Analytical approaches include X-ray diffraction, electron microprobe analysis, and Mössbauer spectroscopy. See also solid solution, isostructural substitution, and ferric/ferrous mineral systems for related concepts.