Abstract

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, β = 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 P2/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 Li[5]Al[6](Ge1[4]Ge2[4]O6) as the corresponding crystallochemical formula for the new P21/n representative within the monoclinic (clino)pyroxene family.

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