Abstract
Determination of the phase relations in the system MgS-MnS-CaS-FeS shows that niningerite (MgS) and alabandite (MnS) have extensive and strongly temperature dependent solid solutions towards FeS. The CaS-contents of niningerites and alabandites, and the MgS- and MnS-contents of oldhamites (CaS) in the assemblage oldhamite plus troilite plus niningerite or alabandite respectively, are also strongly temperature dependent. Interpretation of sulfide compositions in enstatite chondrites after Keil (1968) shows that type I, intermediate, and type II enstatite chondrites all have minimum temperatures of formation in the range 600°C to 800°C. Type II's cannot have been derived from type I's by metamorphism.
Quenching rate studies of niningerites and alabandites coexisting with troilite at 800°C show that type I and intermediate type enstatite chondrites must have cooled at rates at least as fast as 0.1°C/minute, and are presumably debris resulting from one or more impact events. Type II enstatite chondrites cooled much more slowly and in support of Keil's (1968) contention, must have a completely separate origin from type I's.
