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The a-phase of the AI-Mn-Si ternary system has been precipitated from four liquid solutions (2.8wt% Mn-3.7wt% Si, 4.5wt% Mn-7wt% Si, 7.7wt% Mn-10wt% Si and 8.7wt% Mn-12.6 wt% Si) and equilibrated slightly over the melting point of the (secondary) solid solution. Quenching into water enabled the precipitates to be separated from the solid solution and the co-nodal tie-lines for the a-phase and liquid to be determined. The silicon content varied from 9.8 to 11.8wt%, while the manganese content, 29.6 ± 0.6wt%, did not vary significantly. The lattice parameter of the cubic unit cell decreased from 1.267 6(2) nm to 1.265 10(4) nm with increasing silicon. Neutron powder diffraction showed silicon to constitute approximately 50% of atoms on the icosahedral aluminium sublattices and 70% of a six-fold aluminium site in the interstices between the almost spherical, 54-atom clusters. This decrease in unit cell volume is interpreted as evidence for substitution of the silicon atoms on the six-fold sublattice.

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