Museum Mont Saint-Hilaire is the type locality for gaidonnayite. In the summer of 1972 more than a hundred small crystals were found on a natrolite specimen and much of the previous work was repeated with the new material. It is the orthorhombic dimorph of catapleiite. Some crystals are found with a very fine coating of a chlorite-like mineral, causing a grayish or greenish color.
Color ranges from pale yellow, very pale brown, pale gray and gray-green to colorless.
Luster is adamantine to vitreous. Diaphaneity is usually transparent, occasionally translucent to opaque. Fracture is conchoidal. Streak is white. Associated Minerals include aegirine, albite, amphibole group, ancylite, burbankite, calcite, catapleiite, chlorite group, elpidite, epididymite, eudialyte, fluorite, galena, goethite, hilairite, microcline, natrolite, pyrite, pyrochlore, rhodochrosite, s randite, siderite, sphalerite and zircon.