Li-bearing amphiboles: synthesis, stability and composition of clinoholmquistites

metasomatic plutonic environments, and usually coexist with mineral phases rich in economically important elements, such as Rb, Ba, U, Th, etc. Therefore a better knowledge of the stability of this group of Li-bearing amphiboles is of major importance in ore-deposits prospecting. Synthetic analogues of clinoholmquistite were obtained by hydrothermal synthesis in the system Li2O-Na2O-FeO-MgO-Al2O3-Fe2O3-Si2O-H2O (LNFMAFSH), at T ranging between 450 °C and 900 °C and P between 0.1 and 1.5 GPa; fO2 was varied between NNO-0.5 and NNO+3. In the Al, Mg-bearing, LMASH system, which is by far the most common in nature, synthetic amphiboles were never obtained in laboratory. This instability is reflected by the paucity of clinoholmquistite specimens in Nature (cf. Cámara and Oberti, this meeting). The introduction of ferric iron in the system significantly stabilises the monoclinic Li-amphiboles. At lower T (500-600 °C) and P (0.1 GPa), only very limited amounts of Al, around 10%, are incorporated into the structure. On the contrary, Mg and Fe2+ show complete miscibility at the M1 and M3 sites of ferriclinoholmquistite. Mg-rich terms show a much higher thermal stability than Fe2+-rich terms. Mg end-members are stable up to 800 °C and are replaced by lithian clinopyroxene between 800 and 900 °C; ferrous end-members are stable up to 500-600 °C. The fO2 conditions imposed during the syntheses strongly affect the Fe3+/Fe2+ ratio of the amphibole. The results obtained on our synthetic analogues of ferriclinoholmquistites are in good agreement with the stability data inferred for similar natural systems (Oberti et al., 2003). The run products have been characterised by EMP, FTIR and Mössbauer analysis and by XRD powder diffraction. In a very few cases, crystal size allowed single-crystal structure refinement. The combination of these techniques allowed understanding of the chemical shifts induced by the presence of both BLi and CLi, as well as a quantitative evaluation of the various short-range configurations for the A-, Band C-cations. This knowledge will be useful for the interpretation of cation order in other Li-bearing amphiboles, as well as for the understanding of the relative stability of clinoholmquistites and holmquistites.Oberti et al. (2003) Eur. J. Mineral., 15, 309.