Abstract

Abenakiite-(Ce) is a new mineral species discovered at the Poudrette Quarry, Mont Saint-Hilaire, Quebec. It occurs as a single 2X1 mm pale brown ellipsoidal grain in association with aegirine, eudialyte, mangan-neptunite, polylithionite, serandite, and steenstrupine-(Ce). The streak is white, and the luster, vitreous. The mineral is brittle, with a conchoidal fracture and a poor {0001} cleavage. It displays no discernible fluorescence under long- or short-wave ultraviolet radiation, has a Mohs hardness greater than 4, and exhibits a very weak effervescence in 1:1 HCl. D(meas.) is 3.21, D(calc.) 3.27 g/cm 3 for the empirical formula. Abenakiite-(Ce) is uniaxial negative, omega 1.589(1) and epsilon 1.586(1). The average of four electron-microprobe analyses is Na 2 O 25.32, Ce 2 O 3 15.31, La 2 O 3 5.94, Nd 2 O 3 7.78, Pr 2 O 3 2.02, Sm 2 O 3 0.74, ThO 2 1.42, SrO 0.12, SiO 2 13.30, P 2 O 5 14.11, SO 2 2.28, CO 2 8.90 (calc.), total 97.24 wt. %, corresponding to Na (sub 24.23) (Ce (sub 2.77) Nd (sub 1.37) La (sub 1.08) Pr (sub 0.36) Th (sub 0.16) Sm (sub 0.13) Sr (sub 0.03) ) (sub Sigma 5.90) Si (sub 6.56) P (sub 5.90) C 6 S (sub 1.06) O 63 on the basis of O = 63 or, ideally, Na 26 REE 6 (SiO 3 ) 6 (PO 4 ) 6 (CO 3 ) 6 (S (super 4+) O 2 )O with Z = 3. It is rhombohedral, R3, with a 16.018(2), c 19.761(4) Aa, V 4390.9(1) Aa 3 . The strongest eight reflections in the X-ray powder pattern [d in Aa(I)(hkl)] are 11.414(75)(101), 8.036(85)(012), 6.554(85)(021), 4.646(75)(104), 3.773(90)(131), 3.591(80)(312), 3.150(70)(125), 2.674(100)(036). The name honors the Abenaki Indian Tribe, which inhabited the area around Mont Saint-Hilaire. The crystal structure has been refined to R = 3.1%, wR = 3.0% for 2116 observed [F>6sigma (F)] reflections. The structure, when viewed along c, may be best described in terms of a {6 3 } hexagonal net, with chains of NaO 7 polyhedra joined to PO 4 groups at the nodes of the net and columns of six-membered SiO 4 , NaO 7 , and NaO 7 -REEO 8 polyhedral rings in the center. Within the columns are NaO 6 octahedra, CO 3 groups, and disordered SO 2 groups.

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