A transmission electron microscope study of iron‐nickel carbides in the matrix of the Semarkona unequilibrated ordinary chondrite

The fine‐grained matrix of the Semarkona unequilibrated ordinary chondrite contains an assemblage of Fe carbides that are associated with oxides, sulfides, carbonates, and hydrated silicates. The carbides consist of fine‐scale epitactic intergrowths of cohenite (Fe3C) and Hägg carbide (Fe5C2) that formed as by‐products of gas‐solid reactions on the Semarkona parent body. The carbide intergrowths resulted from prograde reactions at moderate temperatures in the presence of a highly reducing carbon‐bearing gas. The carburization occurred prior to the aqueous alteration episode that produced the Fe‐rich phyllosilicates throughout the interchondrule matrix and the thin magnetite rims surrounding the carbide grains. The occurrence of the carbide intergrowths places an upper limit of <500 °C on the maximum postaccretional thermal processing that occurred on the Semarkona parent body.