Chlorite-Biotite Equilibrium Reactions in a Carbonate-Free System

M. J. MCNAMARA
1966 Journal of Petrology  
In a system open to both potassium and water, the conditions for two different chloritebiotite equilibrium reactions may be depicted on an isobaric temperature-[K + ]/[H + ] diagram. At moderate values of the [K + ]/[H+] ratio in the fluid phase pervading the system, muscovite is involved in the equilibrium reaction: chlorite+quartz+KOH = biotite+muscovite+H,O. At high values of [K + ]/[H + ], K-feldspar is involved in the equilibrium reaction: chlorite + quartz+KOH = biotite+K-feldspar+H,O.
more » ... +K-feldspar+H,O. When the same system is closed to potassium, assemblages forming at lower values of [K+]/[H + ] resemble those developed on progressive metamorphism, whereas higher [K+]/ [H+] assemblages in the closed system resemble certain retrogressive mineral assemblages. The assemblage muscovite+biotite is characteristic of higher temperatures and lower values of ^K+/^H+ than the assemblage K-feldspar+chlorite in carbonate-free rocks where both potassium and water were perfectly mobile components and SiO, was present in excess.
doi:10.1093/petrology/7.3.404 fatcat:rgi77yrodvbw7kx3v3kpus2yeu