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S0RENSEN, HENNING; MAKOVICKY, MILOTA and ROSE-HANSEN, JOHN, 1985: Reconnaissance studies on the synthesis and stability of steenstrupine. Contribution to the mineralogy of Ilimaussaq no. 75. Bull. Geol. Soc. Finland 57, Part 1—2, 103—112. Steenstrupine, a complex silicophosphate of sodium, the lanthanides, calcium, manganese, iron, zirconium and thorium, is the principal uranium-bearing mineral (about 0.1 "Its U) in the Kvanefjeld uranium deposit in the northern part of the Ilimaussaq alkaline complex, South Greenland. The steenstrupine occurs in a number of varieties in lujavrites, in fenitized country rocks and in pegmatites and hydrothermal veins, thus spanning a range of conditions of formation. Its geological occurrence indicates that steenstrupine is formed only in sodium-rich rocks containing albite, analcime and other Na-rich minerals. The stability conditions for steenstrupine have been investigated in a series of hydrothermal runs at 1000 atm and 400°—700°C using »synthetic» steenstrupine compositions produced from pure chemicals and gel charges obtained by decomposition of steenstrupine as starting materials. The hydrothermal solutions were aqueous solutions of NaOH and NaCl of various compositions. It was found that steenstrupine at 1000 atm may form in the whole temperature range investigated, 400°—700°C, its formation being determined by the chemical composition of the initial solid phases and of the solutions. It formed only when the starting material was sodium-rich and in the case of »synthetic» compositions only when the starting material was rich in Zr. Other minerals produced in the experiments were: monazite, lessingite and britholite. The results of the experiments are in accordance with the field observations and petrological evidence for the occurrence of steenstrupine and monazite. Britholite has been observed rarely in the rocks of Ilimaussaq, lessingite has not been indentified to date in this locality.