A transmission electron microscope observation of 138A period in Pb-As-S sulfosalts

摘要

A small crystal fragment whose diffraction pattern shows a long period of around138A was found under the electron microscope, and based upon its high-resolution TEM image the essential crystal structural features were discussed. It belongs to the rathite group of lead sul-farsenites. The crystal structures of the members of this group can be interpreted as various combinations of two different structural units, namely, sartorite unit "S" with about 9.8A in width and rathite unit "R" with about 12.6A in width. The present material has been proved to have an ordered arrangement of RSRSR • RSRSRSR. This result is consistent with the conclusion derived by Takeuchi( 1970) that the crystal structure of each member of the rathite group is constructued by a regular arrangement of S and R units. Although the 138A period is reported for rathite-IV, the characteristics of the diffraction pattern of the present phase appear to be different from those of rathite-IV. The occurrence of the present material is discussed.