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Donnayite-(Y)
Donnayite-(Y) is a rare earth carbonate mineral containing the rare earth metal yttrium. It was first discovered in 1978 at Mont St-Hilaire, Quebec. Donnayite was subsequently identified and named after Joseph D.H. Donnay and his wife, Gabrielle Donnay. Both were prominent mineralogists and crystallographers, and J.D.H. Donnay was awarded the Roebling Award by the Mineralogical Society of America in 1971 for his emphasis on the importance of optical mineralogy and crystal morphology. Donnayite tends to occur in small quantities in the pegmatite dykes and miarolitic cavities of mountainous regions. It crystallizes in this environment with increasing alkalinity values until the alkalinity suddenly drops during the last stage of crystallization. This results in increasing amounts of Na carbonates and REE minerals. First discovered at Mont St-Hilaire, donnayite has since been found in the Southern Ural Mountains of Russia and the Narssarssuk pegmatite of South Greenland. Donnayite crystals tend to be small and the color is commonly pale yellow to yellow with a white streak and a vitreous luster. Donnayite crystals usually display trigonal or hexagonal symmetry and have a hardness of 3. Twinning is extremely common in this mineral. Minerals closely related to donnayite include synchysite, calcite, sphalerite, microcline, and analcime. Donnayite is isomorphous with weloganite and mckelveyite.
==Composition==
The chemical formula of donnayite is: NaCaSr3Y(CO3)6∙3H2O. Donnayite is composed of the cations sodium, calcium, strontium, and yttrium. Of these cations, strontium and yttrium are the most prominent. In addition to this, donnayite contains a significant amount of water, making it a hydrated mineral (more specifically, a hydrated carbonate). Generally speaking, mineral hydration occurs when water is added to the structure of the mineral, which occurs in retrograde metamorphism (when temperatures during crystallization suddenly drop, and H activity increases). Donnayite was first analyzed using a Cambridge MK5 electron microprobe. Electron microprobe analysis of donnayite in 1978 yielded the following weight percent oxides (Baker et al., 1978):
The CO2 and H2O values given were calculated on the basis of 6(CO3)-2 and 3(H2O) per formula by comparison with weloganite.
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