Experimental investigation in the Mn-rich portion of the system MnO–MgO–FeO–SiO
2–CO
2 has been carried out to study the effect of X
Fe on the phase relation in the ternary system MnSiO
3–MgSiO
3–FeSiO
3. The experiments were conducted with oxygen and/or graphite buffers in the system C–O–H at pressure-temperature conditions corresponding to normal crustal metamorphism to understand the evolution of mineral assemblages from Ca-poor Mn–Mg–Fe carbonates admixed with silica.
Stability field of pyroxmangite solid solution containing Fe can appear at lower temperature than the MnSiO
3 end member. The appearance of single phase pyroxmangite
ss is insensitive to variations in X
Mn of the bulk in the range ∼0.8 and above. With increasing X
Mg and X
Fe in the bulk composition, carbonate
ss+quartz react to produce pyroxmangite
ss+Mn-clinopyroxene
ss and Mn-clinopyroxene
ss successively. All the reaction boundaries are insensitive to pressure, at least up to 8 kb, and have gentle slope in temperature. These reactions are also independent of the X
CO2 of the fluid phase in the region X
CO2=0.21−0.53 above moderate temperatures. The slope of the lnX
CO2−10
−3/T K relationship of these reactions is similar to that of the pure pyroxmangite forming reaction. Mn-clinopyroxene
ss can be stabilized at a low temperature than pyroxmangite
ss from favourable bulk composition [X
(Mg+Fe)=0.4]. With increasing Fe content, stability field of Mn-clinopyroxene
ss extends toward higher X
Mn in region as compared to that in Fe-free system.
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