Home > Mineral Abstracts > CanMin Vol. 32, pp. 843-854 (1994)




Vol. 32, pp. 843-854 (1994)



Abenakiite-(Ce), a new silicophoshate carbonate mineral from Mont Saint-Hilaire, Quebec: Description and structure determination


Andrew M. McDonald* and G. Y. Chao
Ottawa-Carleton Geoscience Centre, Department of Earth Sciences,
Carleton University, Ottawa, Ontario K1S 5B6



Joel D. Grice
Mineral Sciences Section, Earth Sciences Division,
Canadian Museum of Nature, Ottawa, Ontario K1P 6P4


ABSTRACT

     Abenakiite-(Ce) is a new mineral species discovered at the Poudrette Quarry, Mont Saint-Hilaire, Quebec. It occurs as a single 2 x 1 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- 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/cm3 for the empirical formula. Abenakiite-(Ce) is uniaxial negative, w 1.589(1) and e 1.586(1). The average of four electron-microprobe analyses is:


Na2O - 25.32
Ce2O3 - 15.31
La2O3 - 5.94
Nd2O3 - 7.78

Pr2O3 - 2.02
Sm2O3 - 0.74
ThO2 - 1.42
SrO - 0.12

SiO2 13.30
P2O5 - 14.11
SO2 - 2.28
CO2 8.90 (calc.)



     Total 97.24 wt.%, corresponding to:


Na24.23(Ce2.77Nd1.37La1.08Pr0.36Th0.16Sm0.13Sr0.03)S5.90


Si6.56P5.90C6S1.06O63 on the basis of O = 63 or, ideally,


Na26REE6(SiO3)6(PO4)6(CO3)6(S4+O2)O with Z = 3.


     It is rhombohedral, R3, with a 16.018(2), c 19.761(4) Å, V 4390.9(1) Ä3. The strongest eight reflections in the X-ray powder pattern (d in Å(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 > 6s(F)] reflections. The structure, when viewed c, may be best described in terms of a {63} hexagonal net, with chains of NaO7 polyhedra joined to PO4 groups at the nodes of the net and columns of six-membered SiO4, NaO7, and NaO7-REEO8 polyhedral rings in the center. Within the columns are NaO6 octahedra, CO3 groups, and disordered SO2 groups.

Keywords:  abenakiite-(Ce), new mineral species, silicophosphate carbonate, X-ray data, chemical composition, crystal structure,
Mont Saint-Hilaire, Quebec.


© 1994  The Canadian Mineralogist