Chemical changes during the alteration of micas

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 Fe 2+ to Fe 3+ , loss of divalent octahedral cations, mainly Mg 2+ , 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 Fe 2+ ions by the reaction 4Fe 2+ + 4 structural (OH − ) + O 2 → 4Fe 3+ + 4 structural (O 2− ) + 2H 2 O; 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.