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Magnetic behaviour of trioctahedral mica-2M₁ occurring in a magnetic anomaly zone

¹ Dipartimento di Scienze della Terra, Università di Modena e Reggio Emilia, Via S. Eufemia, 19, I-41100 Modena, Italy
² Dipartimento di Fisica, Università di Modena e Reggio Emilia, Via G. Campi 213/a, I-41100 Modena, Italy

^* E-mail: mariafranca.brigatti{at}unimore.it

This work relates the crystal chemistry and the magnetic behaviour of a trioctahedral mica (chemical formula: (K_0.90Na_0.01Ca_0.01Ba_0.010.07)(Al_0.05Mg_1.38Ti_0.32Mn_0.010.04)(Al_1.12Si_2.88)O₁₀ (F_0.27OH_1.27O_0.46); unit cell parameters: a = 5.345(2) Å, b = 9.261(4) Å, c = 20.189(8) Å; β = 95.075(8)°) from Minto Block (Ungava peninsula, northern Quebec, Canada), a region characterized by high magnetic anomalies. Crystallographic and X-ray absorption spectroscopy data suggest a prevalent divalent oxidation state for Fe and a disordered Fe²⁺ distribution in the two octahedral sites M1 and M2. The real part of magnetic susceptibility shows two peaks at 5.2 K and 120 K. However, as demonstrated 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.

KEYWORDS: magnetic behaviour, crystal chemistry, trioctahedral mica, Ungava, Canada