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The new biotite activity model and standard-state thermodynamic properties of Ann, Phl, and Eas presented in part-I were used to make pseudosections of bulk compositions representing experimental Fe–Mg exchange equilibria and (model) pelitic bulk rock compositions in the system K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O (KFMASH), using mainly the software  Perple_X . These pseudosection calculations (termed ‘ our calculation ( s )’ in the following) were compared to analogous ones performed with the solution model of biotite and thermodynamic data cited in White et al. (J Metamorph Geol 32:261–286, 2014, 10.1111/jmg.12071), termed ‘ W14 calculation ’.  Our calculations  with the experimental bulk composition used by Zhou (Ti–Mg–Fe biotites: formation, substitution, and thermodynamic properties at 650 to 900 °C and 1.1 Kb with fO 2  defined by the CH 4 –graphite buffer. PhD thesis, State University of New York, 1994) in his experimental study of the Fe–Mg exchange between biotite (Bt) and olivine (Ol) confirm that biotite had no or only minimal octahedral Al (Al VI ) in these experiments. The experimental data of Ferry and Spear (—FS78, Contrib Mineral Petrol 66:113–117, 1978, 10.1007/BF00372150) on the Fe–Mg distribution between biotite and garnet (Grt) are well reproduced by  our calculations . The computed composition of biotite ( X  Fe ) in equilibrium with garnet of Alm 90 Py 10  composition and the resulting ln K  D  values as a function of temperature are in good agreement with the experimental brackets. An analogous  W14 calculation  on the same Fe-rich bulk composition predicts too high  X  Fe  Bt  in order of 0.1 mol fraction. The Al VI  contents of biotite of about 0.3–0.45 apfu, as measured by Gessman et al. (Am Mineral 82:1225–1240, 1997, 10.2138/am-1997-11-1218) in similar biotite–garnet exchange experiments performed with Alm 80 Py 20  and Alm 70 Py 30  garnets, are well reproduced by  our , as well as by  W14 calculations . The extent of Tschermak substitution in biotite in the FS78 experiments, which had Fe-richer bulk compositions, has not been measured. Comparing the FS78 biotites with the ones from Gessman et al. (Am Mineral 82:1225–1240, 1997, 10.2138/am-1997-11-1218), it is very likely that the biotites reported in FS78 contained Al VI  in the same order of ca. 0.3–0.4 apfu. A  T – X  Fe  (= molar FeO/(FeO + MgO) pseudosection demonstrates the bulk composition dependence of ln K  D  of the Mg/Fe Grt/Bt  exchange reaction in high-variance fields. Further comparisons, demonstrating the application of the new biotite solution model in the KFMASH system, are presented in pseudosections constructed for an average model pelite, as well as for a natural high- T /low- P  and a natural high- P  metapelite. The pseudosections show that biotite according to  our  biotite model breaks down at lower temperatures and pressures than predicted from the  W14  biotite model in the KFMASH system. This means that KFMASH biotite can break down before the wet solidus is reached, which can explain the existence of dry high- T /low- P  metapelites. At higher pressures, biotite according to  our calculations  breaks down at lower pressures than computed with the  W14  biotite model. Before biotite breaks down, however, its Al VI  content based on  our calculations  could potentially be used for pseudosection barometry, similarly as the Si-in-phengite barometer. These trends need to be confirmed by a future extension of our model which incorporates Ti, Fe 3+  and a di–tri-octahedral substitution.

A new activity model for Fe–Mg–Al biotites: II—Applications in the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system