CLASSIFICATION OF SECONDARY REDUCTION TEXTURES IN UREILITES

Introduction: Ureilites are primitive achondrites dominated by olivine and pigeonite, with minor augite, orthopyroxene, graphite, sulfide and metal [1]. After classifying several dozen ureilites, we observed a pattern of increased reduction of olivine and to a lesser extent, pyroxenes, with concomitant loss of graphite and increased degree of hardness. From these observations, we have developed a new classifying element useful in distinguishing various types of ureilites. Methodology: We recognize four stages of reduction based upon the volume ratio of graphite to reduction rim metal, and the thickness of the reduction rims on olivine. Those ureilites that have unreacted graphite and very lightly reduced silicates are assigned lowest reduction grade, R1. Ureilites where no graphite remains and olivine rims are heavily reduced with over 50 vol. % of grain mass affected, are assigned highest grade, R4.