Summary: The net cationic charge of the octahedral and tetrahedral layers was calculated from the analyses of 100 biotites and 93 muscovites and compared to the observed hydroxyl contents (OH+F+Cl). The correlation coefficient between these parameters of −0·96 for biotites and −0·82 for muscovites demonstrates that the hydroxyl content in these micas varies in response to cationic substitutions. Thus substitutions in micas involve cation-hydroxyl replacement couples in addition to other modes previously recognized. Tetrasilicic phlogopite, K₂Mg₆Si₈O₂₂(OH)₂, which is related to phlogopite by the substitution Al³⁺ + H⁺ = Si⁴⁺ was synthesized but is not a stable phase under the conditions investigated. This mica crystallizes as a 1M polymorph with a = 5·289(2) Å, b = 9·151(3) Å, c = 10·247(3) Å, β = 100°4(2).

Exchange reactions, such as Al³⁺ + H⁺ ⇌ Si⁴⁺, between phlogopite and pyroxenes at depth may provide a mechanism for release of H⁺ within the mantle.