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.
D_obs = 3.30 and D_calc = 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: 

     Twinning by rotation about [103]_120º and by reflection across (010, (301) or (331) is extremely common.   Electron microprobe analysis gives: 

sum 98.47%.  CO₂ and H₂O are assumed to be stoichiometric.  The analysis corresponds to Na_0.93(Ca_0.87Nd_0.09La_0.03)(Sr_2.95Ba_0.05)(Y_0.99(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 (Na_1.29K_0.02)(Ca_0.73U_0.17)(Ba_2.72Sr_0.17)(Y_0.70RE_0.32)(CO₃)₆ · 3.48 H₂O, close to NaCaBa₃Y(CO₃)₆ · 3H₂O.