Examination of length-fast chalcedony by transmission electron microscopy reveals significant structural differences between microcrystalline fibrous quartz and ideal α qvarlz. Individual crystals that make up chalcedony fibers are elongate along [110] and are twisted about the fiber axis. Streaks in electron diffraction pattems of single fibers indicate pervasive disorder among the {101} and possibly the {011} planes, and in dark-field images the texture of this structural disorder appears dendritic. Because the streak systems associated with {101} and with {011} intersect at the expected locations of primary diffractions associated with the mineral “moganite”, demonstrations of the presence of “moganite” require complementary characteization techniques, such as X-ray powder diffraction. In addition, genuine superperiodicities of three, four, and five times the primary translations in quartz are discerned through electron diffraction of chalcedony crystals, and violations of systematic absences associated with the 31 screw symmetry are apparent in X-ray diffraction experiments.

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