The distribution of F between amblygonite (Amb, LiAlPO4F)–montebrasite (Mbr, LiAlPO4OH) solid solutions and metaluminous haplogranitic melt has been calibrated at 585 °C and 200 MPa H2O. The partition coefficient for F between the crystalline phase and melt, DFMbr/melt, is linear between 0 to ~10 wt% F in amblygonite, which contains 13 wt% F at the end-member:
Values of DFAmb/melt decrease sharply above 10 wt% F in amblygonite as the amblygonite reaches saturation in F at 200 MPa H2O. In natural occurrences, however, the vast majority of primary amblygonite-montebrasite solid solutions contain ~4–7 wt% F, well within the linear range of the calibrated exchange reaction, and the montebrasite-bearing assemblages are among the last to crystallize. If the F contents of the montebrasite are magmatic, then these most-fractionated residual melts of the LCT (Li-Cs-Ta, and mostly peraluminous S-types) rare-element class generally contained up to ~1.0–1.8 wt% F near the end of their crystallization. The modest F contents of pegmatites are consistent with the common association of Li aluminosilicates and with the general paucity of topaz in these occurrences. In topaz-bearing granites of Western Europe, however, high-F amblygonite (~10–11 wt% F) reflects >3 wt% F in melt during crystallization of these magmatic phases.