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Ferro-ferri-nybøite, NaNa2(Fe2+3Fe3+2)(Si7Al)O22(OH)2, is a new mineral of the amphibole group from Poudrette quarry, Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie (formerly Rouville County), Québec, Canada. It occurs in an igneous microbreccia associated with a eudialyte-group mineral, an astrophyllite-group mineral, albite, and nepheline. Crystals are prismatic parallel to [001] with {100} and {110} forms and cleavage surfaces, and the prism direction is terminated by irregular fractures. Grains are up to 3 cm long, and occur as blocky aggregates. Crystals are black with a greyish-green to black streak. Ferro-ferri-nybøite is brittle, has a Mohs hardness of 6 and a splintery fracture; it is non-fluorescent with perfect {110} cleavage, no observable parting, and has a calculated density of 3.424 g/cm3. Crystals show extreme optical absorption due to intervalence charge transfer, which inhibited measurement of optical properties.<br/><br/>Ferro-ferri-nybøite is monoclinic, space group C2/m, a 9.9190(5), b 18.0885(8), c 5.3440(3) Å, β 103.813(1)°, V 931.09(13) Å3, Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d in Å(I)(hkl)]: 8.520(100)(110), 3.162(55)(310), 2.834(24)(330), 1.671(19)(461), 2.732(10)(151), 2.552(10)(Embedded Image02), 2.344(9)(Embedded Image51), 3.298(7)(240), 2.606(6)(061), 1.446(6)(Embedded Image61,4.10.0). Analysis by a combination of electron microprobe and Mössbauer spectroscopy gives SiO2 45.80, Al2O3 3.11, TiO2 0.50, Fe2O3 11.18, FeO 23.45, MnO 2.28, ZnO 0.12, MgO 0.23, CaO 0.99, Na2O 8.01, K2O 1.30, F 0.81, H2Ocalc 1.47, O ≡ F−0.34 sum 98.91 wt.%. The formula unit, calculated on the basis of 24 (O + OH + F) with (OH + F) = 2 apfu is (Na0.66K0.27)Σ0.93(Na1.83Ca0.17)Σ2.00(Mg0.06Fe2+3.15Mn0.31Zn0.01Fe3+1.38Ti0.06Al0.03)Σ5.00(Si7.35Al0.65)Σ8.00O22(OH1.58F0.42)Σ2.00. Ferro-ferri-nybøite, ideally NaNa2(Fe2+3Fe3+2)(Si7Al)O22(OH)2, is related to endmember nybøite, NaNa2(Mg3Al2)(Si7Al)O22(OH)2 by the substitutions Fe2+ → Mg and Fe3+ → Al.