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The Karoonda CK4 chondrite exhibits pronounced silicate darkening of matrix and chondrule mesostases. Our scanning electron microscope study showed that the Karoonda matrix contains unusual olivine, with a high density of vesicles and inclusions of mainly magnetite and pentlandite, all between 0.1 and 3 μm in diameter. The vesicular and nonvesicular olivines occur as an intermixture, and vesicularity is particularly abundant in regions that exhibit a high degree of silicate darkening. Our transmission electron microscope study revealed that the vesicular olivine is entirely crystalline and contains a high density of much smaller (< 10 to 100 nm) vesicles and inclusions of mostly pentlandite. In contrast, nonvesicular olivine contains no such inclusions. The observations indicate that the silicate darkening is caused by the dispersion of not only opaque mineral inclusions but also vesicles, and the microinclusion-rich vesicular olivine is the principal cause of the silicate darkening. The texture and mineralogy of the vesicular olivine suggest that it was formed by crystallization of melt that was produced from nonvesicular olivine by shock at a high temperature.

Microinclusion-rich vesicular olivine in the Karoonda CK4 chondrite: transmission electron microscopy