Abstract

Lukechangite-(Ce), ideally Na 3 Ce 2 (CO 3 ) 4 F, is a new mineral from Mont Saint-Hilaire, Quebec. It occurs as tabular, short prismatic, and barrel-shaped crystals up to 1 mm. It is colorless to pale beige with a white streak and a vitreous, or somewhat pearly on {0001}, luster. Associated minerals include microcline, analcime, sodalite, aegirine, serandite, eudialyte, catapleiite, fluorite, petersenite-(Ce), siderite, astrophyllite, and albite. Lukechangite-(Ce) is soft, Mohs hardness nearly equal 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) Aa, V = 509(1) Aa 3 , and Z = 2. The strongest X-ray powder diffraction lines [d (Aa), I, hkl] are: 5.71(50)(004), 4.31(100)(101), 3.804(50)(006), 3.169(70)(105), 2.877(60)(106), 2.534(70)(110), 2.192(90B)(109,200,201), 1.978(70)(205), and 1.658(50)(209,210,211). An average of the electron-microprobe analyses gave Na 2 O 14.94, CaO 0.10, SrO 0.12, La 2 O 3 16.36, Ce 2 O 3 29.48, Pr 2 O 3 1.95, Nd 2 O 3 5.88, F 3.58, CO 2 (28.40), and O identical with F--1.51, total 99.30 wt%. CO 2 was calculated by stoichiometry from 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 CO 3 groups oriented parallel to the layering forming thick slabs incorporating either Ce or Na cations and a separate Na-F layer. The structure of lukechangite-(Ce) resembles that of huanghoite-(Ce) and baiyuneboite-(Ce) and is isostructural with synthetic Na 3 La 2 (CO 3 ) 4 F.

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