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Nolzeite, Na(Mn,□)2[Si3(B,Si)O9(OH)2]·2H2O, is a new mineral found in altered sodalite syenite at the Poudrette quarry, La Vallée-du-Richelieu, Montérégie (formerly Rouville County), Québec, Canada. Crystals are colourless to pale green and are acicular with average dimensions of 5 μm × 8 μm × 55 μm. They occur as radiating to loose, randomly oriented groupings within vugs associated with aegirine, nepheline, sodalite, eudialyte-group minerals, analcime, natron, pyrrhotite, catapleiite, steedeite and the unidentified mineral, UK80. Nolzeite is non-pleochroic, biaxial, with nmin = 1.616(2) and nmax = 1.636(2) and has a positive elongation. The average of six chemical analyses gave the empirical formula: Na1.04(Mn1.69□0.24Fe0.05Ca0.02)Σ=2.00(Si2.96S0.04)Σ=3.00(B0.70Si0.30)Σ=1.00O9(OH)2·2H2O based on 13 anions. The Raman spectrum shows six distinct bands occurring at ∼3600–3300 cm–1 and 1600–1500 cm–1 (O–H and H–O–H bending), 1300–1200 cm–1 (B–OH bending), 1030–800 cm–1 (Si–O–Si stretching) as well as 700–500 cm–1 and 400–50 cm–1 (Mn–O and Na–O bonding, respectively). The FTIR spectrum for nolzeite shows bands at ∼2800–3600 cm–1 (O–H) stretching, a moderately sharp band at 1631 cm–1 (H–O–H) bending, strong, sharp bands at ∼650–700 cm–1, ∼800–840 cm–1, and ∼900–1100 cm–1 (Si–O and B–O) bonds. Nolzeite is triclinic, crystallizing in space group P with a = 6.894(1), b = 7.632(2), c = 11.017(2) Å, α = 108.39(3), β = 99.03, γ = 103.05(3)°, V = 519.27 Å3, and Z = 2. The crystal structure was refined to R = 12.37% and wR2 = 31.07% for 1361 reflections (Fo > 4σFo). It is based on chains of tetrahedra with a periodicity of three (i.e. a dreier chain) consisting of three symmetrically independent SiO4 tetrahedra forming C-shaped clusters closed by BO2(OH)2 tetrahedra, producing single loop-branched dreier borosilicate chains. The chains are linked through shared corners to double chains of edge-sharing MnO5(OH) octahedra. Nolzeite is a chain silicate closely related to steedeite and members of the sérandite–pectolite series. Paragenetically, nolzeite is late-stage, probably forming under alkaline conditions and over a narrow range of low pressures and temperatures.