Grain boundaries coarse particles (GBCP) are observed in an Al-Mg-Si alloy in the as-received extruded state and after various heat treatments in the temperature range (540-580 °C) for holding times up to 4 h. These particles are also observed as agglomerates in the face-centred cubic (FCC) aluminium matrix. In this study, the chemical composition, the structural and crystallographic characterization (Crystal system, Bravais lattice, Point and Space Groups) of these particles are carried out using optical microscopy, X-rays diffraction, transmission electron microscopy using Convergent beam électron diffraction (CBED) in microdiffraction mode and energy-dispersive spectroscopy (EDS) techniques. GBCP crystallizes in the body-centred cubic (BCC) Bravais lattice and belongs to the I 2 m 3 space group with a lattice parameter a = 1.2615nm. The EDS analysis indicates a quaternary phase − Al 14 (Fe, Mn)Si. This phase would develop by a diffusion mechanism. The spatial orientation relationship of this intermetallic phase with the surrounding aluminium matrix is found to be: (2OO) ∕∕ 220 Al , (O2O) ∕∕ 111 Al and [001] ∕∕[112] Al. The symmetry analysis shows that the rational orientation relationship developed between the α-phase and the aluminium matrix is consistent with the intersection point group H 4 = 2 m , belonging to the monoclinic system, leading to 12 variants per grain of aluminium matrix. The derived morphology is a {(010) or (h0l)} − pinacoid , which is a crystal form consisting of two parallel and opposite faces.