Petrography and classification of refractory inclusions in the Allende and Mokoia CV3 chondrites

More than 600 refractory inclusions in 22 thin sections of Allende and Mokoia were studied by optical and scanning-electron microscopy. Olivinerich inclusions and Ca, Al-rich inclusions (CAI's) are aggregates of various combinations of three fundamental petrographic constituents : rimmed concentric objects, Ca, Si-rich chaotic material, and mafic inclusion matrix. A new classification system for refractory inclusions is developed that is based on the size and abundance of these three fundamental constituents. Olivine-rich inclusions consist primarily of inclusion matrix and are divided into two textural varieties : unrimmed olivine aggregates (Type 1A inclusions) and rimmed olivine aggregates (Type 1B inclusions). Many Type 1B inclusions contain Ca, Al-rich nodules that are aggregates of concentric objects. CAI's consist primarily of concentric objects and are divided into three varieties : unrimmed complex CAI's (Type 2 inclusions), rimmed complex CAI's (Type 3 inclusions), and simple CAI's (Type 4 inclusions). Simple and complex CAI's are divided into melilite-rich and melilite-poor varieties. Several varieties of refractory inclusions grade into each other chemically, mineralogically, and texturally. Our new classification system avoids several problems that are inherent to other classification systems, which use the term "coarse-grained" too restrictively for many simple CAI's and inaccurately for most melilite-rich complex CAI's.