Liquidus isotherms and phase equilibria have been determined experimentally for a pseudo-ternary section of the form “MnO”-(CaO + MgO)-(SiO +Al O) with a fixed Al O /SiO weight ratio of 0.17 and MgO/CaO weight ratio of 0.25 for temperatures in the range 1 393-1 673 K. The primary phase fields found in the investigated section include manganosite (Mn, Mg, Ca)O; dicalcium silicate α-2(Ca, Mg, Mn)O·SiO; merwinite 3CaO (Mg, Mn)O 2SiO; melilite [2CaO·(MgO, MnO, AlO)·2(SiO· , Al O )]; wollastonite [(Ca, Mg, Mn)O·SiO ]; diopside [(CaO, MgO, MnO, Al O )·SiO ]; tridymite (SiO ); rhodonite [(Mn, Mg, Ca)O·SiO ]; anorthite (CaO·Al O 2SiO ) and tephroite [2(Mn, Mg, Ca)O·SiO ]. The liquidus temperatures and primary phase fields are significantly different to those in the ternary system “MnO”-CaO-SiO , but are close to those previously reported pseudo-ternary section “MnO”-(CaO + MgO)-(SiO +Al O ) for Al O /SiO weight ratio of 0.17 and MgO/CaO weight ratio of 0.17. The partitioning of CaO, MgO and MnO between liquid and solid phases was measured using EPMA, and the extents of solid solutions for a range of bulk compositions and temperatures were characterised.