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Peatite-(Y), Li4Na12(Y,Na,Ca,HREE)12(PO4)12(CO3)4(F,OH)8, and ramikite-(Y), Li4Na12(Y,Ca,HREE)6Zr6(PO4)12(CO3)4 O4(OH,F)4, are two new minerals discovered in the core of the Poudrette pegmatite at Mont Saint-Hilaire, Quebec. Epitactic-like, euhedral crystals (pseudocubes) of both minerals range from 0.1 to 1 mm in size (average: 0.2 mm), with ramikite-(Y) forming yellowish-white cores (dominant) and peatite-(Y) occurring as thin (< 50 um) pale pink rims. Crystals of peatite-(Y) exhibit the dominant forms pinacoid {100}, {010}, and {001} and the minor forms rhombic prism {110}, {101}, and {011}, with crystals of ramikite-(Y) showing the possible forms pedion {100}, {00 Formula }, {010}, {0 Formula 0}, {001}, and {00 Formula }. The most common associated minerals include albite, rhodochrosite, siderite, chabazite-Na, synchysite-(Ce), and sabinaite. Peatite-(Y) displays a brittle fracture with very good {100}, {010}, and {001} cleavages; ramikite-(Y) has a splintery fracture with possible weak to poor {100}, {010}, and {001} cleavages. Peatite-(Y) has a vitreous luster and ramikite-(Y) has a vitreous to dull luster. Both minerals have a white streak and neither shows any discernible fluorescence under long-, medium-, or short-wave ultraviolet radiation. Both minerals have an approximate Mohs hardness of 3. Peatite-(Y) has a calculated density of 3.62(1) g/cm3 and ramikite-(Y) of 3.60(1) g/cm3. Both minerals have a very low birefringence (~100), exhibit parallel extinction, and give poor interference figures; the optic sign and measured 2V of both are unknown. Only one refractive index for each could be measured: peatite-(Y), β = 1.601(1) and for ramikite-(Y), β = 1.636 (1). Four analyses of peatite-(Y) gave an average (range) of (wt. %): Li2O 1.96 (calc.), Na2O 12.95 (12.50–13.30), CaO 1.15 (0.98–1.51), Y2O3 37.32 (37.01–37.52), Gd2O3 0.61 (0.54–0.74), Dy2O3 3.08 (2.91–3.44), Ho2O3 0.67 (b.d.−1.02), Er2O3 2.88 (2.59–3.15), Tm2O3 0.28 (b.d.−0.40), Yb2O3 1.78 (1.67–1.92), ZrO2 0.67 (0.63–0.70), ThO2 0.37 (b.d.−0.56), P2O5 27.29 (27.09–27.64), F 4.35 (4.03–4.62), CO2 5.79 (calc.), H2O 0.31 (calc.), O = F −1.83, total 99.75, corresponding to Li4Na12(Y10.06Na0.72Ca0.62Dy0.50Er0.46Yb0.28Zr0.17Ho0.11Gd0.10Tm0.04Th0.04Tb0.02)Σ13.12 (PO4)11.70(CO3)4[F6.97(OH)1.03]Σ8 and the simplified formula, Li4Na12(Y,Na,Ca,HREE)12 (PO4)12(CO3)4(F,OH)8. For ramikite-(Y), 22 analyses gave an average (range) of (wt. %): Li2O 2.01 (calc.), Na2O 11.25 (10.32–13.34), CaO 4.15 (4.01–4.27), Y2O3 16.48 (14.88–18.25), La2O3 0.11 (b.d.−0.48), Ce2O3 0.10 (b.d.−0.40), Nd2O3 0.08 (b.d.−0.31), Dy2O3 1.11 (0.96–1.23), Er2O3 1.18 (1.01–1.36), Yb2O3 0.57 (0.46–0.68), ZrO2 23.40 (22.66–24.70), ThO2 0.49 (b.d.−0.70), HfO2 0.69 (0.48–0.92), Al2O3 0.14 (0.09–0.22), P2O5 28.10 (27.47–28.58), F 0.62 (0.24–0.90), CO2 5.92 (calc.), H2O 0.92 (calc.), O = F −0.26, total 97.06, corresponding to Li4(Na10.79Ca1.21)Σ12(Y4.34Ca0.99Dy0.18Er0.18Yb0.09La0.02Ce0.02Nd0.01)Σ5.83(Zr5.65Hf0.10Th0.06)Σ5.81 [(P0.98Al0.01)Σ0.99 O4]12 (CO3)4O4[(OH)3.03F0.97]Σ4.00 and the simplified formula, Li4(Na,Ca)12 (Y,Ca,HREE)6 Zr6(PO4)12(CO3)4O4(OH,F)4. In both peatite-(Y) and ramikite-(Y), the presence of Li2O was confirmed via crystal-structure and LAM-ICP-MS analyses and both H2O and CO2 via results of crystal-structure, infrared, and Raman analyses. Peatite-(Y) crystallizes in space group P222 with a 11.167(2), b 11.164(2), c 11.162(2) Å, V 1391.7(1) Å3, and Z = 1, and ramikite-(Y) in space group P1 with a 10.9977(6), b 10.9985(6), c 10.9966(6) Å, α 90.075(4), β 89.984(4), γ 89.969(4)°, V 1330.1(1) Å3, and Z = 1. The strongest six lines on the X-ray powder-diffraction pattern [d in Å (I) (hkl)] for peatite-(Y) are: 4.56(57)(211,121,112), 3.95(57)(220,202,022), 3.54(46) (310,301,130), 2.99(83)(321,312,231), 2.63(100)(330,303,033), 2.149(42)(333) and for ramikite-(Y): 11.04(76)(0 Formula 0,100,00 Formula ), 7.80(79)(0 Formula 1,110,101), 6.36(75)(11 Formula ,1 Formula 1,111,1 Formula Formula ), 3.89(100)(0 Formula 2,220,202), 2.94(98)(13 Formula ,12 Formula ,23 Formula ), 2.59(98)(0 Formula 3,330,303). The crystal structure of peatite-(Y) was refined to R = 3.37 % and wR2 = 9.36 % for 3816 reflections and that of ramikite-(Y) to R = 5.13% and wR2 = 13.06% for 8272 reflections. While not strictly isostructural, both minerals have similar crystal structures dominated by Mφ8 polyhedra (M = Y,Zr; φ = unspecified ligand). These are linked into six-membered, edge- or corner-sharing clusters, which in turn are joined together by PO4 tetrahedra. Both LiO6 octahedra and CO3 groups are positioned within the corner-sharing clusters. Linkages among all these polyhedra produce an open, equidimensional framework structure, with Na occupying the resulting cavities. Although possessing complex crystal structures, both minerals may be considered more simply as homeotypes of body-centered cubic Fe (or CsCl) or, alternatively, as complex derivatives of cation-deficient perovskite-related structures. Both minerals are late-stage products, possibly related to the in situ alteration of the pre-existing mineral assemblage (dawsonite, burbankite-group minerals, sabinaite, muscovite-polylithionite, etc.) present in the core of the Poudrette pegmatite.<br/>