Actually I found two distinct generations of biotites in genetically related syenites and pegmatites. I did EMPA and trace element analyses both generations showing good changes in their chemistries.I plot the classification diagrams the first biotite generation is biotite and second generation is phlogopite. Now I am confused can I used this phlogopite for geothermometer, geothermobarometry, and oxygen fugacity calculations or not. Further can I used their chemistries to evaluate magma evolution tend from syenitic to pegmatitic stage.
Biotite is a solid solution series with Fe-enriched members (annite) and Mg-enriched ones (phlogopite). In your case biotite seems to be representative of the reducing part of the system when bivalent Fe was accommodated into the lattice of the s.s.s. while phlogopite attests to more oxidizing conditions as trivalent Fe occurred in the system and was prevented from entering the mica structure. I know from Přibyslavice near Čáslav annite to be present in a pegmatoidic schlieren and phlogopite from the Věžná I pegmatite phlogopite to be present besides biotite in a serpentinized lherzolite forming the host rock.
Dear Amjad,
As it is not common to get the pure-end members of minerals so it is obvious some of your analyzed points (or even entire grain) will show biotite or phlogopite. Check petrographic or SEM images of the grains so you can see the relationship or can be able to differentiate if both annite and phlogopite exist in the same grain or they occur in petrographically different domain. As answered above by Prof. Harald G. Dill the oxidizing and reducing conditions cause these minerals to form,
I think you can use both these cases for geothermobarometry. The results may indicate the magma evolution. If the obtained results are in accord with the petrographic evidence, the results are meaningful.
Phlogopite can be used for geothermobarometry.
T(K) = 838/(1.0337 – Ti/Fe2+) (after Luhr et al. 1984)
P(kb) = 3.03·TAl – 6.53(±0.33) (after Uchida et al. 2007)
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