Amoeboid Olivine Aggregates

Amoeboid olivine aggregates are common millimeter size inclusions in carbonaceous chondrite meteorites. Some are amoeba shaped, some are more chondrule-like. Their textures and compositions are variable, too – some appear to be transitional between calcium-aluminum-rich inclusions and some transitional between chondrules. The olivine is the magnesium-rich end member, forsterite. It was formed not from a melt of precursor dust but as a condensate from vapor. Though formation was in an environment of near-solar elemental composition, according to researchers, AOA tend to be low in metal. This might be due to aerodynamic sorting in the early solar nebula. Still, some AOA are outlined with small blebs of nickel iron.

Typical amoeboid olivine aggregate inclusion in Moss CO3.6. Cross-polarized light. Field of view is 3 millimeters wide.

AOA in Murchison CM2. XPL. FOV=3mm

Calcium-aluminum-rich inclusion and AOA in Allende CV3. XPL. FOV=3mm

Close-up of the same Allende AOA. XPL. FOV=0.3mm

AOA in DaG 1040 CV3. Partially cross-polarized light for clarity. FOV=3mm

Close-up of the same AOA in DaG 1040 CV3. Partially XPL. FOV=0.3mm

AOA in NWA 3118 CV3 with melt inclusion (dark) containing long crystals. XPL. FOV=3mm

Another AOA in NWA 3118 CV3 with a melt inclusion (left center edge of AOA). Partially XPL. FOV=3mm

Close-up of that melt inclusion. Partially XPL. FOV=0.5mm

A third AOA in NWA 3118 CV3 with a melt inclusion. Partially crossed-polarized light to see through the glassy melt. FOV=3mm

Top portion of that melt inclusion with mineral crystals. The glass contains vesicles. Partially XPL. FOV=0.5mm

Closer view of that glassy melt inclusion with skeletal crystals implying rapid crystallization. Partially XPL. FOV=0.25mm

Another close view of the glassy melt inclusion with a crystal appearing ribbon-like and frayed. Partially XPL. FOV=0.25mm

Oval fine-grained AOA (center) in DaG 978 C-ung. XPL. FOV=3mm

Close-up of the same fine-grained AOA. The AOA is about 0.7mm long. XPL

Another close-up of that fine-grained AOA, now in incident light to highlight metal blebs.

About the Author

John is a natural history enthusiast living in Oregon.

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