Synthesis and crystal-chemistry of NaNaMgMg5Si8O22(OH)2, a P21/m amphibole

In the present work, we characterize the amphibole Na(NaMg)Mg5Si8O22(OH)(2) synthesized at 0.4 GPa and 750, 800, and 850 degreesC, and 0.5 GPa, 900 degreesC. Experiments at 800 and 900 'C yielded crystals suitable for single-crystal data collection. Structure refinement shows that synthetic Na(NaMg)Mg5Si8O22(OH)(2) has P2(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)Mg5Si8O22(OH)(2) has two well-defined absorption bands at 3742 and 3715 cm(-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 frequency band is assigned to the longer O3A-H1 bond. The broader shape of the 3743 cm(-1) band is consistent with a stronger interaction of the H2 atom with Na-A, which is confirmed by structure refinement. Increasing T of synthesis causes a progressive departure from the ideal stoichiometry via the (A)square(1)(B)Mg(1)(A)Na(-1)(B)Na(-1) substitution, as confirmed by EMPA, structure refinement, and FTIR spectroscopy.