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Manganese is added to many magnesium alloys to control impurity iron, but its effects on phase transformations and microstructure formation remain incompletely understood. Here we show that an Al-Mn-based decagonal quasicrystal (d-QC) forms in the late stages of solidification in AZ31 and AZ91 magnesium alloys at relatively slow cooling rates, here down to 0.1 K/s. The d-QC has a periodicity of ∼ 12 Å, grew as decagonal rods and commonly shared interfaces with Al 8 Mn 5 and eutectic Mg 17 Al 12. A reproducible orientation relationship (OR) was measured with Al 8 Mn 5 only, indicating that the d-QC nucleated on Al 8 Mn 5. The OR is consistent with the structural relationship between quasicrystals and gamma brasses and gives the d-QC nucleation advantages over LT-Al 11 Mn 4 during solidification. A subsequent heat treatment at 410 °C caused both Al 8 Mn 5 and the d-QC to transform into LT-Al 11 Mn 4. [Display omitted] [ABSTRACT FROM AUTHOR]