Chondrule Classification – Textural Types
Channel: Dominik Hezel
Date: 2023-04-12

"Chondrules are classified into various textural types. Most abundant (typically more than 90%) are chondrules with large grains of olivine and/or pyroxene, and interstitial either fine-crystalline or glassy Ca,Al-rich, feldspathic like material, in cases an intergrowth of feldspar, pyroxene and maybe silica. This interstitial material is called mesostasis. Such chondrules are called porphyritic chondrules, because of their large crystals. Porphyritic chondrules are often mineralogically zoned, with olivine in the core and low-Ca pyroxene at the rim.
These zoned chondrules are the dominant porphyritic type in almost all chondrites, and called mineralogically zoned porphyritic – or MZP – chondrules. Porphyritic chondrules that are unzoned are consequently called mineralogically unzoned porphyritic – or MUP – chondrules. Porphyritic chondrules likely formed from a melt with sufficient nuclei for the olivine and pyroxene grains to crystallise from. The next two most abundant types are barred olivine (BO) and radial pyroxene (RP) chondrules (ca. 5-10%). BO chondrules have about parallel olivine bars with interstitial mesostasis. The parallel olivine bars are a skeletal structure resulting from quick undercooling of a rather nuclei-free melt. BO chondrules further often have a magmatic rim surrounding the chondrule. RP chondrules likely also formed from an almost nuclei-free, more silica-rich melt. A seed crystal at the rim of the chondrule – that is in cases observable – triggered crystallisation, and fine pyroxene laths instantly crystallised radially from the seed crystal. There are many additional chondrule textural types that are either rare or restricted to certain chondrite groups. For example, CH and CB chondrites contain cryptocrystalline and skeletal chondrules, that are largely absent in other chondrite types. Further rare or sub-textural types are e.g., micro-porphyritic, granular olivine, or silica-rich chondrules."