"mixed" carbonate layers. In this "mixed" layer, the CO3 triangles are tilted within the layer, which allows the H2O groups and Na octahedral to share the slab with the CO2 polyhedra. The (Na, Y)CO3·H2O layer of donnayite is comparable to this. However, the Sr atoms of donnayite have 9-fold coordination with an extra layer of flat-lying carbonate groups. This is shown in the figure below. In the structure of the REE carbonate, donnayite, the effects of H-bonding are negligible. Physical properties Typical donnayite crystals are very small, ranging from 0.05 to 1.0 mm and rarely reaching 2.0 mm. Donnayite is commonly pale yellow to yellow, but can also be colorless, white, gray, and very rarely, reddish brown, due to hematite inclusions. The streak of this mineral is white, and it has a vitreous luster. Donnayite ranks at a hardness of 3 on Mohs scale and has fair to imperfect cleavage on the c axis {001}. Donnayite crystals usually display trigonal or hexagonal symmetry and belong to the point group 1 or 3m and the space group P1. Optically, donnayite is biaxial negative with a 2v (measured) ranging from 0° to 30°. Geologic occurrence Donnayite occurs in minute quantities in the pegmatite dykes, miarolitic cavities, and interstices in the nepheline syenites at Mont St-Hilaire, Quebec. It is usually found in rocks from the alkaline complexes and in carbonatites. During crystallization of the nepheline syenite, the alkalinity continues to increase until the very last stage, when the alkalinity suddenly drops, resulting in decreasing temperature and an increase in H activity. This is exemplified by increasing amounts of Na carbonates and REE minerals. In addition, during the last stages of crystallization in more acidic conditions, minerals form with increasing amounts of H2O, and REE minerals rich in Y. In 1973, donnayite was first discovered at Mont St-Hilaire, but was incorrectly identified as "brockite". Finally, in 1978 it became possible to gather sufficient data to characterize this sample as the new species, donnayite. In addition to this geologic occurrence, donnayite can be found in Russia at Mt. Kukisvumchorr, Khibiny massif, Kola Peninsula, and at the Vishnevogorsk complex, Vishnevy-Ilmen Mountains, Southern Ural Mountains. Recently, donnayite has been discovered along with ewaldite, epitaxially intergrown, in crystals from the Narsaarsuk pegmatite in South Greenland. Related minerals include ewaldite, mckelveyite, synchysite, calcite, sphalerite, microcline, and analcime. Special characteristics As of today, donnayite has no real historical or political