Chemical changes during the alteration of micas

Abstract

Di- and trioctahedral micas were altered to vermiculite-like minerals by extracting the K with sodium tetraphenylboron. Chemical analysis of original and altered micas shows that loss of K is accompanied by an increased loss on ignition, oxidation of some Fe²⁺ to Fe³⁺, loss of divalent octahedral cations, mainly Mg²⁺, loss of OH⁻ (or sorption of H⁺) and decrease in net negative charge.The following reactions are suggested to explain these changes: (1) replacement of K by Na at interlayer sites; (2) release of structural OH⁻ ions exposed by replacement of K, which decreases the negative charge and allows the structure to expand and more K to be replaced; (3) oxidation of Fe²⁺ ions by the reaction4Fe²⁺ + 4 structural (OH⁻) + O₂ → 4Fe³⁺ + 4 structural (O²⁻) + 2H₂O;and (4) release of divalent octahedral ions, possibly through some of the holes left when structural OH⁻ is lost.The wide implications of these proposals are discussed.