ON TYPE III PLESSITE IN CHONDRITES

Questions are raised concerning the possible sources of heat necessary for converting martensite to coarse Type III plessite in ordinary chondrites. It is suggested that the unusual Type III plessite in the Kingfisher, Oklahoma black chondrite was formed by partial homogenization of preexisting Type III plessite as a result of shock reheating of the metal into the γ field of the Fe-Ni phase diagram, rather than by decomposition of shock reheated prior martensite in the α + γ field, as originally proposed by Taylor and Heymann. Because martensite is sporadically distributed within Kingfisher plessite it is suggested that microstructures of this kind be called Type II-III plessite.