Amoeboid olivine aggregate
An amoeboid olivine aggregate is an irregular, fine-grained clump of olivine with small calcium-aluminium-rich nodules that formed by condensation in the early solar nebula.
Amoeboid olivine aggregates (AOAs) take their name from their lobed, irregular outlines. They consist mainly of fine grains of almost pure forsterite (magnesium-rich olivine), often with minor metal, enclosing small refractory nodules of minerals such as spinel, anorthite and aluminium-rich diopside. Their fine grain size and porous texture show that most were never completely melted, unlike chondrules.
AOAs are a link between CAIs and chondrules. Their forsterite is thought to have condensed from nebular gas after the most refractory minerals, so they record a slightly lower-temperature stage of condensation. Like many CAIs, AOAs are typically rich in oxygen-16, suggesting they formed in a similar setting near the young Sun. Their mineralogy is easily altered, so pristine examples are best studied in the least metamorphosed chondrites, where the olivine has not exchanged iron with its surroundings.
AOAs occur in most carbonaceous chondrite groups, including CV chondrites such as Allende and in CO and CR chondrites. In hand specimen they may appear as small, irregular pale or greenish patches. Confident identification requires a polished section examined with back-scattered electron imaging and electron microprobe analysis.
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