Crystal chemistry of lithium in pegmatite phosphates: a SIMS investigation of natural and synthetic samples

Lithium plays an essential geochemical role in granitic pegmatites, and this element can be used to monitor the degree of pegmatite differentiation (Ginzbourg 1960). Among primary phosphates, Li mainly occurs in minerals of the triphylite-lithiophilite [LiFe(PO4)-LiMn(PO4)] and amblygonitemontebrazite [LiAl(PO4)F-LiAl(PO4) OH] series. Starting from the triphylitelithiophilite series, a mechanism of oxidation coupled with Li-leaching is frequently observed, leading to the formation of ferrisicklerite-sicklerite [Li1- x(Fe3+,Mn2+)PO4-Li1-x(Mn2+,Fe3+)PO4] and of heterosite-purpurite [Fe3+(PO4)- Mn3+(PO4)] (Quensel 1957, Mason 1941). In order to constrain the conditions of temperature and oxygen fugacity which occurred in pegmatites, we decided to reproduce experimentally several associations of pegmatite phosphates. Primary alluaudite + triphylite assemblages were reported in the Hagendorf-Süd (Germany), Buranga, and Kibingo (Rwanda) pegmatites, and the hydrothermal experiments (P = 1 kbar, T = 400-800°C) lead to the crystallisation of alluaudite + triphylite at 400 and 500°C, and of alluaudite + triphylite + maricite at 600 and 700°C (Hatert et al. 2005). Triphylite and sarcopside, which show lamellar exsolution textures in several pegmatites, were reproduced between 400 and 700°C (P = 1 kbar), starting from the compositions LiFe2.5(PO4)2, LiFe2Mn0.5(PO4)2, and LiFe1.5Mn(PO4)2 (Hatert et al. 2007). The aim of this study is to measure the Li-content of several natural and synthetic phosphates by SIMS, in order to shed some light on the crystal chemistry of lithium in pegmatite phosphates.