This work relates the crystal chemistry and the magnetic behaviour of a trioctahedral mica (chemical formula: K0.90Na0.01Ca0.01Ba0.01&0.07)(Al0.05Fe2+ 1.20Mg1.38Ti0.32Mn0.01&0.04)(Al1.12Si2.88)O10 (F0.27OH1.27O0.46); unit cell parameters: a = 5.345(2), b = 9.261(4), c = 20.189(8) A; b =95.075(8)º) from Minto Block (Ungava peninsula, northern Quebec, Canada), a region characterized by high magnetic anomalies. Crystallographic and X-ray absorption spectroscopy data suggest a prevalentdivalent oxidation state for Fe and a disordered Fe2+ distribution in the two octahedralsites M1 and M2. The realpart of magnetic susceptibility shows two peaks at ~5.2 K and 120 K. However, asdemonstrated by AC magnetic susceptibility measurements, the origin of the two effects is different: the peak position of the first one (i.e. the effect revealed at 5.2 K) is frequency-dependent, thus suggesting a spin-glass like behaviour. The effect at 120 K can instead be attributed to the occurrence of diluted phases in mica matrix, such as Fe oxides.