The structure determination of synthetic LiAlGe2O6 [Z = 4, space group P21/n, a = 9.8892(5), b = 8.3929(5), c = 5.3995(3) Å, b = 110.646(3)°], is a new pyroxene structure-type, and represents the first structural example of P21/n pyroxene symmetry. The crystal structure of the Ge-analog phase of spodumene was solved from single-crystal X-ray diffraction data by classical Patterson methods with a subsequent structure refinement converging to R1 = 0.0169. The new P21/n pyroxene polymorph was found to consist of a single S-type rotated tetrahedral chain type, which is-similar to ordered P21/n omphacite-composed of alternating Ge1O4 and Ge2O4 tetrahedra located at two distinct sites within a single chain. This (Ge1Ge2O6)n chain is S-rotated, strongly bent (O4-O2-O4 = 154.8°) compared to that of the C2/c spodumene structure, and assumes angles comparable to those of the two chains of spodumene at 3.3 GPa within the P21/c symmetry. As a consequence of the interplay between the M1 and M2 sites, the new polymorph reveals a larger angular distortion for the AlO6 octahedra, and the Li coordination is reduced from sixfold to fivefold coordination. This establishes LiAl(Ge1 Ge2O6) as the corresponding crystallochemical formula for the new P21/n representative within the monoclinic (clino)pyroxene family.
© 2015 by Walter de Gruyter Berlin/Boston