Steedeite, ideally NaMn2Si3BO9(OH)2, is a new mineral discovered in altered sodalite syenite at the Poudrette quarry, La Vallée-du-Richelieu, Montérégie (formerly Rouville County), Québec, Canada. Crystals of steedeite are colorless to pale pink, and acicular with average dimensions of 0.006 × 0.011 × 0.51 mm. They occur as radiating to loose, randomly oriented groupings within vugs associated with aegirine, nepheline, sodalite, eudialyte-group minerals, analcime, natron, pyrrhotite, catapleiite, and two other unidentified minerals temporarily designated UK78 and UK80. Steedeite is transparent to translucent with a vitreous luster and has a weak pale green to pale yellow fluorescence under medium-wave radiation. No partings or cleavages were observed, although crystals exhibit an uneven fracture. The calculated density is 3.106 g/cm3. Steedeite is nonpleochroic, biaxial with nmin= 1.636(2) and nmax = 1.656(2) and a positive elongation. Chemical analyses (n = 14) from seven crystals gave an average (range, standard deviation) of: Na2O 7.51 (6.78–8.32, 0.44), CaO 0.17 (0.08–0.22, 0.03), MnO 31.02 (29.91–32.83, 0.93), FeO 0.86 (0.76–1.01, 0.07), SiO2 46.34 (40.39–49.29, 2.56), S 0.39 (*b.d.– 2.36, 0.71) (*b.d. = below detection), B2O3 (calc.) 8.73, and H2O (calc.) 4.52, total 99.53 wt.%. The empirical formula is: Na0.97(Mn1.75Fe0.05Ca0.01)∑1.83(Si3.07S0.02)∑3.09 BO9(OH)2, or ideally NaMn2Si3BO9(OH)2. The presence of both B and OH in steedeite were inferred from the refinement of the crystal structure. The Raman spectrum for steedeite show bands at 3250–3500 cm–1 attributed to O–H stretching, strong sharp bands at 575–750 cm−1 and 825–1075 cm−1 attributed to Si–O bonds and possibly B–O bonding, as well as weak to strong bands at 50–500 cm−1 attributed to Na–O/Mn–O bonds. Steedeite crystallizes in space group P(#2) with a 6.837(1), b 7.575(2), c 8.841(2) Å, α 99.91(3), β 102.19, γ 102.78(3)°, V 424.81 Å3, and Z = 2. The strongest six lines on the X-ray powder-diffraction pattern [d in Å (I) (hkl)] are: 8.454 (100) (00 ), 7.234 (39) (00 ), 3.331 (83) (1 1, 0 , 20 , 1 2), 3.081 (38) (0 ), 2.859 (52) (0 3), and 2.823 (80) (21 ). The crystal structure of steedeite was refined to R = 1.68% and wR2 = 4.96% for 2409 reflections (Fo > 4σFo). It is based on silicate chains with a periodicity of three (i.e., dreier chain) consisting of four-membered borosilicate rings of composition [BSi3O9(OH)2]5−. The borosilicate chains are classed as single loop-branched dreier chains that are linked together through shared corners to bands of edge-sharing MnO5(OH) octahedra. Steedeite is a chain silicate mineral closely related to the sérandite-pectolite series of the pyroxenoid group and is the first mineral found to contain single loop-branched dreier silicate chains.