Santa Catharina and the origin of cloudy taenite in meteorites.
CLOUDY taenite appears as a dark-etching rim that occurs at the high nickel border of taenite fields in iron meteorites and mesosiderites. Transmission electron microscopy (TEM) shows it to consist of a foam-like association of interpenetrating single crystals of taenite and kamacite when well developed(1) with an orientation relationship between those of the Kurdjumow-Sachs and the Nishiyama(2). It occurs between compositional limits that have been reported as approximately 25-40% Ni(1) and 30-40% Ni(3). Observed taenite particle sizes range from 0.4 microm in the Estherville mesosiderite(1) to considerably less than 0.1 microm for many iron meteorites (unpublished data). The mechanism of cloudy taenite formation has not been previously explained, although it is believed(2) to be a phenomenon related to the Invar effect, which occurs in Fe-35% Ni (Invar) and several other alloy systems. Santa Catharina is a wholly taenitic anomalous iron meteorite that contains 35% Ni and 0.6% Co (ref. 4). This composition, which is very similar to that of Invar, places it well inside the range where cloudy taenite is known to form. The optical microstructure of Santa Catharina suggested that the matrix phase of the entire meteorite may consist of cloudy taenite, so I have now examined it by TEM and the results are reported here.
Jago
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