Vol. 16, pp. 335-340 (1978)


Donnayite, NaCaSr3Y(CO3)6 · 3H2O, a new mineral
from Mont Saint-Hilaire, Québec



George Y. Chao, Paul R. Mainwaring and Judith Baker
Department of Geology, Carleton University, Ottawa, Ontario K1S 5B6


ABSTRACT

     Donnayite occurs in pegmatite dykes, miarolitic cavities and interstices in nepheline syenite at Mont St-Hilaire, Québec.  Usually associated with microcline, analcime, calcite, natrolite, chlorite, aegirine, arfvedsonite and other minerals, donnayite, commonly occurs in syntactic intergrowths with ewaldite, a Sr analogue of ewaldite, mckelveyite, and rarely with synchysite.  Crystals, 0.05 to 2mm, vary in habit from platy, tabular, columnar, saucer-shaped, barrel-shaped to irregularly granular; they are usually pale yellow to reddish brown, but also colorless, white, grey, brown to reddish brown, with a white streak and vitreous lustre.  Hardness - 3.
Dobs = 3.30 and Dcalc = 3.266 g/cm3.  Basal cleavage indistinct to fair.  Dissolves in HCl with strong effervescence.  Optically, biaxial negative, with a 1.551-1.561, b 1.646, g 1.652, 2V 0-20º, X  = c*.  Donnayite is isomorphous with weloganite (as shown by the following unconventional setting):  triclinic P1, a 9.000, b 8.999, c 6,793 Å, a 102.77, b 116.28, g 59.99º, Z = 1.  The nine strongest powder diffraction lines are: 

6.103 (4)(001) 4.368 (7)(120, 211, 110) 3.209 (3)(121, 111, 212)
2.839 (10)(211, 112, 122) 2.598 (4)(331, 030, 301) 2.038 (3)(310, 113, 212,
    003)
2.018 (3)(242, 222, 420) 1.978 (3)(333, 303, 032,
    301, 331)
1.694 (3)(542, 512, 140,
    150, 411, 451, 214, 113)

     Twinning by rotation about [103]120º and by reflection across (010, (301) or (331) is extremely common.   Electron microprobe analysis gives: 

Na2O 3.37 CaO 5.75 BaO 0.85 SrO 35.8 Y2O3 13.1
Nd2O3 1.83 La2O3 0.45 CO2 (30.98) H2O (6.34)

sum 98.47%.  CO2 and H2O are assumed to be stoichiometric.  The analysis corresponds to Na0.93(Ca0.87Nd0.09La0.03)(Sr2.95Ba0.05)(Y0.99(CO3)6 · 3H2O, or ideally NaCaSr3Y(CO3)6 · 3H2O.  New data show that mckelveyite is also triclinic, isomorphous with weloganite and donnayite; its original analysis may be recalculated to (Na1.29K0.02)(Ca0.73U0.17)(Ba2.72Sr0.17)(Y0.70RE0.32)(CO3)6 · 3.48 H2O, close to NaCaBa3Y(CO3)6 · 3H2O.



© 1999  The Canadian Mineralogist

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