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Tundrite-(Ce), ideally Na2Ce2Ti02(Si04)(C03)2, has been found recently in the Mont Saint-Hilaire quarry, Quebec. It occurs as yellow-green, bladed, singly terminated crystals up to 9 mm in length. Minerals associated with tundrite-(Ce) are aegirine, analcime, an astrophyllite-group mineral, catapleiite, a eudialyte-group mineral, gonnardite, mangan-neptunite, monazite-(Ce), natrolite, rhodochrosite, sphalerite and vinogradovite. Tundrite-(Ce) is triclinic, space group PI, with the following unit-cell parameters, refined from powder-diffraction data: a 7.5702(6), b 13.949(2), c 5.0235(5) A, Cl 100.19(1), 1370.88(1), 'Y101.26(1)°, V 487.91(6) A3 and Z = 2. Crystals are tabular on {OlO} and elongate on (001), with pinacoid forms: {100}, {OlO}, {20l} and {lOl}. Electron-microprobe analyses give the average and ranges: Na20 9.44 (9.07-9.66), CaO 0.34 (0.16-0.42), La203 16.12 (15.26-17.95), Ce203 27.01 (25.41-28.56), Pr203 2.13 (1.72-2.38), Nd203 6.16 (5.61--6.58), Sm203(0.38 (0.18-0.73), Ti02l1.17 (10.73-11.93), Nb20s 2.30 (1.14-3.11), Ta20S 0.07 (0.00--0.15), Si02 9.32 (9.10--9.54), CO2 13.91 (TGA), H20 0.36 (TGA), for a total of 98.80 (97.43-100.17) wt. %. The empirical formula based on the crystal-structure analysis, ideally showing 24 oxygen atoms, is 2((Na1906Ca0039)1194S (Cel030La0621Ndo229Pr0081Sm0014)1197S (Tio87sNbo109)lo98402Ho2so (Si097104)(C098903)2). The structure has been refined to an R index of 2.7% for 2827 unique reflections. The structure has three chemically distinct layers parallel to (010): (1) a Na- Ti-Si-O slab, (2) a Ce-C-O slab and (3) a Na-O slab. Slabs are cross-linked by independent (C03f-and (Si04)4- groups. Minor amounts of OH are present to ensure charge balance as a result of the substitutions Nb ~ Ti and Ca ~ REE.

TUNDRITE-(Ce) FROM MONT SAINT-HILAIRE, QUEBEC: CRYSTAL-STRUCTURE ANALYSIS AND SPECIES CHARACTERIZATION