The phoshate mineral associations of the Tsaobismund pegmatite,Namibia |
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| Authors: | André -Mathieu Fransolet Paul Keller François Fontan |
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| Institution: | (1) Institut de Minéralogie, Université de Liège, Place du Vingt-Août, 9, B-4000 Liège, Belgium;(2) Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Pfaffenwaldring 55, D-7000 Stuttgart 80, Vaihingen, Federal Republic of Germany;(3) Laboratoire de Minéralogie, Université Paul-Sabatier de Toulouse, Allées Jules-Guesde 39, F-31400 Toulouse, France;(4) Present address: Institut de Minéralogie, Université de Liège, Bât. B18, Sart Tilman, B-4000 Liège 1 |
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| Abstract: | A detailed mineralogical investigation using the classical methods of identification by X-ray diffraction and by optical properties in thin sections, has revealed thirty one phosphate minerals occurring in the Tsaobismund pegmatite. This investigation is complemented by wet chemical and, mainly, electron microprobe analyses performed on the phosphates known to be typomorphic or considered to be relevant to the hydrothermal alteration. Additionally, microprobe analyses are also given for garnet, gahnite, and ferrocolumbite associated with the phosphates. On the basis of their chemical composition, particularly in terms of their Fe, Mn, and Mg contents, three types of triphylites are distinguished. Triphylite 1 only occurs as a primary phase, triphylite 2 shows exsolution lamellae of sarcopside, and triphylite 3 is partly replaced by a fluorophosphate of the triplite-zwieselite series. These minerals constitute three generations of the parent phases, which were progressively transformed by metasomatic processes, hydrothermal alteration, and by weathering, to give finally three types of complex associations. The Li(Fe,Mn)PO4 minerals appear to be more sensitive to such transformations than those of the (Fe,Mn)2PO4F series. Four main stages of hydrothermal alteration processes have been recognized in the Tsaobismund pegmatite: (i) the Mason-Quensel sequence results from a progressive oxidation of Fe and Mn, and a concomitant Li-leaching of triphylite yielding ferrisicklerite and heterosite, successively; (ii) the metasomatic exchange of Na for Li produces alluaudite; in the present case, the formation of hagendorfite from triphylite 2 is considered to be earlier than the generation of alluaudite-Na occurring in the three associations; (iii) the hydration phase mainly transforms the parent Li(Fe,Mn)PO4 phase into grey hureaulite, associated with barbosalite and tavorite; (iv) the formation of fluorapatite, not particularly widespread, replaces alluaudite-Na, as well as zwieselite s.l. The following crystallization sequence of the initially formed phosphate minerals is proposed: triphylite 1  triphylite 2 + sarcopside (associated with garnet) triphylite 3 + zwieselite s.l. The most prominent feature of this succession is the increase in the Mg and Zn contents in the composition of the phosphates, as well as the decrease in their Li contents. The variations of the Fe/Mn ratios in this sequence are discussed. The succession triphylite-zwieselite within weakly differentiated and Li-poor pegmatites is of general significance. |
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