LUKECHANGITE-(CE), A NEW RARE-EARTH-FLUOROCARBONATE MINERAL FROM MONTSAINT-HILAIRE, QUEBEC

Lukechangite-(Ce), ideally Na3Ce2(CO3)(4)F is a new mineral from Mont Saint-Hilaire, Quebec. It occurs as tabular, short prismatic, and barr el-shaped crystals up to 1 mm. It is colorless to pale beige with a wh ite streak and a vitreous, or somewhat pearly on {0001}, luster. Assoc iated minerals include microcline, analcime, sodalite, aegirine, seran dite, eudialyte, catapleiite, fluorite, petersenite-(Ce), siderite, as trophyllite, and albite. Lukechangite-(Ce) is soft, Mohs hardness appr oximate to 4 1/2, brittle, with an uneven to conchoidal fracture, and with perfect {0001} cleavage. Lukechangite-(Ce) is uniaxial negative, omega = 1.728(3) and epsilon = 1.542(1). It is hexagonal, space group P6(3)/mmc, a = 5.068(1), c = 22.87(5) Angstrom, V = 509(1) Angstrom(3) , and Z = 2. The strongest X-ray powder diffraction Lines [d (Angstrom ), I, hkl] are: 5.71(50)(004), 4.31(100)(101), 3.804(50)(006), 3.169(7 0)(105), 2.877(60)(106), 2.534(70)(110), 2.192(90B)(109,200,201), 1.97 8(70)(205), and 1.658(50)(209,210,211). An average of the electron-mic roprobe analyses gave Na2O 14.94, CaO 0.10, SrO 0.12, La2O3 16.36, Ce2 O3 29.48, Pr2O3 1.95, Nd2O3 5.88, F 3.58, CO2 (28.40), and O equivalen t to F - 1.51, total 99.30 wt%. CO2 was calculated by stoichiometry fr om the results of the crystal-structure analysis. D-calc is 4.02 g/cm( 3). The atomic arrangement of lukechangite-(Ce) has been refined to R = 3.4%. The structure is layered parallel to (001), with CO3 groups or iented parallel to the layering forming thick slabs incorporating eith er Ce or Na cations and a separate Na-F layer. The structure of lukech angite-(Ce) resembles that of huanghoite-(Ce)and baiyuneboite-(Ce) and is isostructural with synthetic Na3La2(CO3)(4)F.