Published online by Cambridge University Press: 05 July 2018
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.