Mineralogy of chondrite components
Our description of meteorite mineralogy starts with the minerals characteristic of the calcium–aluminum-rich inclusions (CAIs). The mineralogy of CAIs varies systematically with their composition. The most Al-rich CAIs contain spinel, hibonite, and/or grossite. More rarely, corundum or calcium mono-aluminate is present. As the bulk composition becomes more Si-rich, the melilite solid solution becomes important. With additional Mg and Si in the bulk composition, fassaite and anorthite are present.
Inclusions that are predominantly melilite with minor spinel, perovskite, and hibonite are referred to as Type A. Most Type-A CAIs have a porous structure and are called fluffy Type-A CAIs. Some Type-A CAIs have a compact form and generally rounded shapes. These are referred to as “compact” Type-A CAIs. Type-B1 CAIs are characterized by coarse-grained, melilite-rich mantles surrounding cores composed of melilite, spinel, fassaite, and anorthite. Type-B2 inclusions have the same mineralogy, but lack the melilite-rich mantle. Type-B3 inclusions contain significant amounts of forsterite in addition tomelilite. Type-C inclusions are similar to Type B2s, but anorthite is more abundant than melilite. All Type-B and Type-C inclusions have compact morphologies.
Aluminum-rich chondrules are a broad class of objects with compositions intermediate between those of CAIs and the more common ferromagnesian chondrules. Their bulk compositions are generally Mg-, Si-rich and Ca-, Al-poor relative to most CAIs.