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Howardites

Synonyms: orthopyroxene-pigeonite-plagioclase achondrites

 

General: Howardites are named for Edward Howard, a renowned British chemist of the 18th century, and one of the pioneers of meteoritics. In 1802, Howard published the results of his research on the historic Wold Cottage fall, convincing a growing number of contemporary scientists that meteorites actually represent extraterrestrial matter.

Description: Like the closely related eucrites, howardites are Ca-rich achondrites, and they also develop a glossy black fusion crust during their passage through the atmosphere. Their interiors are always heavily brecciated, displaying a wealth of different clasts cemented in a pulverized or impact-melted eucritic matrix.

Mineralogy: Consisting primarily of eucritic and diogentic clasts and fragments, howardites are always polymict breccias. Some howardites also contain accessory clasts of carbonaceous chondritic matter, xenolithic inclusions, and impact melt clasts, indicating a regolith origin for the members of this group.

Formation history: Obviously, the howardites represent the surface of 4 Vesta, a regolith breccia, consisting of eucritic and diogenitic debris that was excavated by the large impact that created the enormous crater near Vesta's south pole. These fragments have been mixed with parts of the chondritic impactor, and this mixture has been subsequently pulverized and metamorphosed by smaller impacts and the solar wind to form an entirely new type of rock called regolith. Similar regoliths cover the surface of the Moon, and as with the howardites, these regolith breccias display high values for noble gases that have been implanted into the rock by the solar wind.

Origin: Asteroidal. As the eucrites and diogenites, the howardites are thought to have their origin on the main belt asteroid Vesta - have a look at our introductory text about the HED group. Most howardites show similar formation and CRE ages, with only two exceptions: the howardites Luotolax, and Lohawat, both witnessed falls, appear to be older than all the other group members, and it is not clear up to this date if they just sample an older impact on Vesta, or if they actually represent a so far unsampled parent body, the regolith of another large differentiated asteroid, similar to Vesta.

Members: Howardites are pretty rare, and there are only about 65 members to this group if we exclude probable pairings. Renowned members of the howardite group are the witnessed falls of Bialystok from Poland, Kapoeta from Sudan, and Pavlovka from Russia. During the last few years, several new howardite finds have been reported from the hot deserts of Africa and Asia.

> Meteorite Classification


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