We describe here the synthesis of amphiboles along the nominal Na(NaMg)Mg5Si8O22(OH)2−Na(LiMg)Mg5Si8O22(OH)2 join, at 800 °C, 0.4 GPa. High amphibole yields (>90%) plus minor quartz and enstatite have been obtained at all compositions; amphibole crystals are acicular and their size rarely exceeds 20–30 × 0.5–3 μm. TEM analysis shows the presence of h+k odd reflections in all samples, indicative of a P-lattice. By similarity with closely related amphiboles from the literature (e.g., Oberti et al. 2000; Cámara et al. 2003) a P21/m space group was assigned to the amphiboles synthesized here. Refined cell-parameters from X-ray powder-patterns show a linear decrease as a function of increasing Li at M4, a and β being the most affected parameters. The four infrared OH-stretching spectra all show two main bands at 3741–3748 and 3712–3716 cm−1, respectively. They are assigned to two independent O-H groups in the P21/m structure, interacting with a strongly delocalized ANa. The spectra show in addition two minor absorptions at about 3688 and 3667 cm−1, respectively; these bands are assigned to vacant A-sites in the structure and indicate slight departure of the nominal composition toward cummingtonite. The present work shows that one apfu of BNa can also be completely replaced by one apfu of BLi (M+), in synthetic Na(M+Mg)Mg5Si8O22(OH)2, and that all compositions have P21/m symmetry at ambient conditions.

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