AbstractThe new pyroxenoid barrydawsonite-(Y) occurs at the Merlot Claim, North Red Wine Pluton, Labrador, Canada (62°32'38.54'' W; 54°08'1.37'' N). The host rock is a metamorphosed eudialyte syenite consisting of eudialyte, potassic arfvedsonite, jadeitic aegirine, nepheline, albite and potassium feldspar with accessory Y-bearing pectolite, britholite and steenstrupine. Barrydawsonite-(Y) crystals commonly have discrete thin rims of Y-bearing pectolite. The average empirical formula (based on nine anions p.f.u.) is Na1.54Ca0.74Mn0.15Fe0.07Y0.38Nd0.01Sm0.01Gd0.02Tb0.01Dy0.04Ho0.01Er0.02Yb0.01Si3.00O9H. The simplified formula is Na1.5Y0.5CaSi3O9H. Barrydawsonite-(Y) is related to pectolite by the substitution ½[NaM3+Ca–2] (M3+= Y,REE), and is exceptional in being the only member of the pectolite group that has the structure of the monoclinicM2abcpolytype. The crystal structure has been determined in monoclinic space groupP21/a: a= 15.5026(2),b= 7.0233(1),c= 6.9769(1) Å, β = 95.149(1)°,V= 756.58(2) Å3(Z= 4). Final agreement indices areR1= 0.038,wR2= 0.068, Goof = 1.136. The asymmetric unit of barrydawsonite-(Y) has three metal sites:M(1) = Ca,M(2) = Na0.5(Y,REE)0.5,M(3) = Na.M(1) andM(2) are octahedrally-coordinated sites, whereasM(3) is [8]-coordinated as in pectolite and serandite. The structural formula for the empirical composition isM(3)Na1.00M(2)(Na0.50Y0.38REE0.13)Σ=1.01M(1)(Na0.04Ca0.74Mn0.152+Fe0.072+)Σ=1.00Si3O9H. There is excellent agreement between the refined site-scattering values and those calculated based upon the structural formula.Barrydawsonite-(Y) is biaxial (+) with α = 1.612(1), β = 1.617(1), γ = 1.630(1) (white light) and 2V = 63(1)°. The five strongest peaks in the X-ray powder diffraction pattern are [dobs(Å),Iobs%, (hkl)]: [2.905, 100, (023)], [3.094, 30, (210,211,121,202)], [1.7613, 29, (127,323,040)], [3.272, 27, (202,104)], [1.7016, 27, (140,227,325)].