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PETROGRAPHIC EVIDENCE FOR PRIMARY HAGENDORFITE IN AN UNUSUAL ASSEMBLAGE OF PHOSPHATE MINERALS, KIBINGO GRANITIC PEGMATITE, RWANDA

¹ Laboratoire de Minéralogie, Département de Géologie, Université de Liège, Bâtiment B18, Sart Tilman, B-4000 Liège, Belgique
² Laboratoire de Minéralogie, Université Paul-Sabatier de Toulouse, 39, Allées Jules-Guesde, F-31000 Toulouse, France

E-mail address: amfransolet{at}ulg.ac.be

A sample from the Kibingo granitic pegmatite, Rwanda, shows a striking assemblage of an alluaudite-group mineral with heterosite. The core of alluaudite s.l. is strongly pleochroic in blue green to pinkish grey hues, and is rimmed by an irregular border, weakly pleochroic in yellow brown to greenish yellow tints. The pleochroic core is very rich in Na (up to 9.35% Na₂O), whereas the yellowish green border is depleted in Na (3.75% Na₂O) and more oxidized. The decrease of the Na contents is not accompanied by significant variations of the ratio Fe_tot/(Fe_tot + Mn). This mechanism represents the progressive transition of hagendorfite to alluaudite sensu stricto, and results from an oxidation, Na⁺ + Fe²⁺ + Fe³⁺. Heterosite exhibits cracks that are parallel to one of the cleavage directions, and that are favorable to the development of a product of alteration, identified as cyrilovite. Heterosite shows a chemical composition close to the idealized formula (Fe_0.85 ³⁺Mn_0.15 ³⁺)PO₄, with a Mg content constant and quite low, i.e., 0.023 Mg apfu. The Kibingo heterosite seems to be the Fe-richest member known so far. The heterosite corresponds to an oxidized Fe-rich member of the triphylite – lithiophilite series, a member that was initially in equilibrium with the primary hagendorfite. The ratio Fe_tot/(Fe_tot + Mn) of the two minerals of this assemblage is different: the value is about 0.70 in hagendorfite, and 0.85 in the former triphylite. The temperature of formation of hagendorfite is estimated at about 600°C. Contrary to the information in the literature, the Kibingo pegmatitic body differs from the amblygonite pegmatite of Mwaka and could be a poorly evolved neighboring pegmatitic lens.

Keywords: hagendorfite, alluaudite, heterosite, phosphate mineral assemblage, Kibingo granitic pegmatite, Rwanda.

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