Thermal behaviour of iron-containing micas in oxidising, reducing, and neutral atmospheres

Abstract

The reactions which occur on heating iron-containing micas depend on the ambient atmosphere. In vacuo, the principal reaction is dehydroxylation, accompanied by evolution of hydrogen according to the equation, 4Fe²⁺+ 4OH-→ 4Fe³⁺+ 4O^2–+ 2H₂. Between 500 and 600° in air or oxygen, dehydrogenation occurs before dehydroxylation. This involves simultaneous reaction of iron(II) and hydroxy-ions according to the equation, 4Fe²⁺+ 4OH^–+ O₂→ 4Fe³⁺+ 4O^2–+ 2H₂O, and proceeds until either the iron(II) or the hydroxy-content of the mica has been consumed. Between 500 and 600° in hydrogen, reduction of iron(III) occurs according to the equations, 4Fe³⁺+ 4O^2–+ 2H₂→ 4Fe²⁺+ 4OH^– and 4Fe³⁺+ 2O^2–+ 2H₂→ 4Fe²⁺+ 2H₂O. Similar reactions in deuterium provide a convenient method for introducing OD^– ions into iron-containing silicates.