Barrydawsonite-(Y), Na1.5CaY0.5Si3O9H: a new pyroxenoid of the pectolite–serandite group

The 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 Na 1.54 Ca 0.74 Mn 0.15 Fe 0.07 Y 0.38 Nd 0.01 Sm 0.01 Gd 0.02 Tb 0.01 Dy 0.04 Ho 0.01 Er 0.02 Yb 0.01 Si 3.00 O 9 H. The simplified formula is Na 1.5 Y 0.5 CaSi 3 O 9 H. Barrydawsonite-(Y) is related to pectolite by the substitution ½[Na M 3+ Ca −2 ] ( M 3+ = Y, REE ), and is exceptional in being the only member of the pectolite group that has the structure of the monoclinic M2abc polytype. The crystal structure has been determined in monoclinic space group P 2 1 / a : a = 15.5026(2), b = 7.0233(1), c = 6.9769(1) A, β = 95.149(1)°, V = 756.58(2) A 3 ( Z = 4). Final agreement indices are R 1 = 0.038, wR 2 = 0.068, Goof = 1.136. The asymmetric unit of barrydawsonite-(Y) has three metal sites: M (1) = Ca, M (2) = Na 0.5 (Y, REE ) 0.5 , M (3) = Na. M (1) and M (2) are octahedrally-coordinated sites, whereas M (3) is [8]-coordinated as in pectolite and serandite. The structural formula for the empirical composition is M (3) Na 1.00 M (2) (Na 0.50 Y 0.38 REE 0.13 ) Σ=1.01 M (1) (Na 0.04 Ca 0.74 Mn 0.15 2 + Fe 0.07 2 + ) Σ=1.00 Si 3 O 9 H. 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 [ d obs ( A), I obs %, ( hkl )]: [2.905, 100, (023)], [3.094, 30, (210,2I11,1I21,202)], [1.7613, 29, (127,323,040)], [3.272, 27, (2I02,104)], [1.7016, 27, (140,2I27,3I25)].