Abstract:
The mineralogy of fine-grained rims in ALH 81002, a relatively primitive CM carbonaceous chondrite, has been determined by using transmission electron microscopy. The most abundant phase is Mg-rich serpentine, which occurs in small (=<20 nm) crystals with cylindrical or fibrous morphologies. Cronstedtite, an Fe-rich phyllosilicate, occurs as relatively large (100 to 2500 nm), platy crystals. Some cronstedtite is coherently intergrown with tochilinite. In many cases, cronstedtite has been partially altered to serpentine. The compositions and textures of these two phases provide a mineralogical and morphologic alteration sequence that parallels the known compositional trend for CM chondrite matrix. Accessory minerals embedded within the phyllosilicates include chlorite, pentlandite, gypsum, olivine, kamacite, taenite, and chromite. Regions containing only anhydrous minerals also occur. The hydrated and anhydrous regions are in direct contact with each other, suggesting that the rims accreted material from multiple reservoirs.