STRUCTURAL MODULATION IN SARTORITE - AN ELECTRON-MICROSCOPE STUDY

Sartorite, PbAs2S4, has been examined by transmission electron microsc opy. The mineral has a monoclinic (but metrically orthorhombic) subcel l with a = 19.62, b = 7.89, c = 4.19 angstrom, beta = 90-degrees, and space group P2(1)/n. In addition to the strong substructure reflection s, G, corresponding to this monoclinic subcell, it also exhibits satel lite reflections (in general incommensurate with respect to the subcel l reflections) at G +/- mq, where m is an integer and q, the primary m odulation wave vector approximately 6/13 (101). Some variation in the direction of the primary modulation wave vector was observed with the direction of the modulation being rotated up to 3-degrees away from ( 101) toward (001)*. Crystals with twin-related super-lattices were al so observed. Superlattice formation in sartorite is believed to result primarily from modulations in the lengths of arsenic sulfide chains w ithin the structure.