Synthesis and crystal-chemistry of Na(NaMg)[Mg.sub.5] [Si.sub.8][O.sub.22][(OH).sub.2], a P[2.sub.1]/m amphibole

In the present work, we characterize the amphibole Na(NaMg)[Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2] synthesized at 0.4 GPa and 750, 800, and 850 [degrees]C, and 0.5 GPa, 900 [degrees]C. Experiments at 800 and 900 [degrees]C yielded crystals suitable for single-crystal data collection. Structure refinement shows that synthetic Na(NaMg)[Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2] has P[2.sub.1]/m symmetry at room T. The two non-equivalent tetrahedral double-chains differ in their degree of stretching and kinking. The infrared spectrum of synthetic Na(NaMg)[Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2] has two well-defined absorption bands at 3742 and 3715 [cm.sup.-1] which can be assigned to O-H bands associated with the two independent anion sites (O3A and O3B) in the structure. The higher frequency band is assigned to the shorter O3B-H2 bond, and the lower tYequency band is assigned to the longer O3A-H1 bond. The broader shape of the 3743 [cm.sup.-1] band is consistent with a stronger interaction of the H2 atom with [sup.A]Na, which is confirmed by structure refinement. Increasing Tof synthesis causes a progressive departure from the ideal stoichiometry via the A[[cube root of].sub.1][sup.B][Mg.sub.1][sup.A][Na.sub.-1][sup.B][Na.sub.-1] substitution, as confirmed by EMPA, structure refinement, and FFIR spectroscopy.