THE STABILITY OF ALLUAUDITE IN GRANITIC PEGMATITES

Alluaudite, a Na-Mn-Fe-bearing phosphate mineral, is known to occur in Li-rich granitic pegmatites. During the past decade, several compounds with the alluaudite structure were synthesized, but their stability was scarcely envisaged. Because alluaudite can occur as a primary phosphate in granitic pegmatites, we decided to investigate its stability. The hydrothermal synthesis were performed between 400 and 800°C at 0.1 GPa, using horizontally arranged Tuttle-type cold-seal bombs. The oxygen fugacity was controlled by the Ni-NiO buffer.

Starting from chemical compositions corresponding to those of unoxidized alluaudites, Na₂(Mn_1-xFe²⁺_x)₂Fe³⁺(PO₄)₃, we obtained pure alluaudite at 400 and 500°C, whereas the association alluaudite + maricite appears between 500 and 700°C. The upper stability limit of alluaudite is defined by the crystallization of maricite, because this mineral has never been observed in granitic pegmatites. This limit corresponds to 550-600°C for Na₂Fe²⁺₂Fe³⁺(PO₄)₃, and to 450-500°C for Na₂MnFe²⁺Fe³⁺(PO₄)₃. At 800°C, alluaudite and maricite are associated with an undetermined phase ("X-phase"), whereas fillowite crystallizes above 600°C, in the Mn-rich part of the diagram.

Electron microprobe analyses of the synthesized phosphates were also performed, in order to establish phase relations in the central part of the Na-Mn-Fe (+PO₄) ternary diagram. When the temperature increases from 400 to 800°C, the composition of alluaudite shifts towards the Na-poor part of the diagram. This behavior is related to the crystallization of maricite, a phase richer in Na than alluaudite.

Supplementary experiments were also performed in the Na-Li-Mn-Fe²⁺-Fe³⁺-P-O system, in order to better understand the stability limits of the alluaudite + triphylite paragenesis, which was observed in the Li-rich granitic pegmatites of Buranga, Kibingo (Rwanda), and Hagendorf-Süd (Germany). Below 600°C, alluaudite is associated with triphylite, whereas maricite appears between 600 and 700°C. At 800°C, "X-phase" crystallizes together with triphylite. The upper stability limit of the alluaudite + triphylite paragenesis, without maricite, corresponds to 600°C, a temperature which is in good agreement with the conditions generally considered to occur in granitic pegmatites.