The phase relations in the join acmite-CaAl
2SiO
6 at 1 atmospheric pressure were studied and the phase diagram of the join was constructed. The solubility limit of CaAl
2SiO
6 in acmite is between 5 and 7wt. %. When the CaAl
2SiO
6 content increases, the phase assemblages below the solidus temperature in this join are changed as follows: pyroxene
ss (ss: solid solution), pyroxene
ss+nepheline
ss+plagioclase
ss, (pyroxene
ss+nepheline
ss+ plagioclase
ss+hematite), pyroxene
ss+nepheline
ss+plagioclase
ss+melilite
ss, nepheline
ss+plagioclase
ss+melilite
ss+(Ca0.6Al
2O
3)
ss, and plagioclase
ss+melilite
ss+(CaO.6Al
2O
3)
ss. At high temperatures pyroxene
ss melts incongruently to hematite and liquid. The compositions of pyroxeness in this join change from NaFeSi
2O
6-CaAl
2SiO
6 to NaFeSi
2O
6-CaAl
2SiO
6-CaFeAlSiO
6 with the decrease of sodium content in the bulk composition. The bearing of this join on natural acmitic pyroxene is discussed.
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