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Howardite - Eucrite - Diogenite |
| The Howardites, Eucrites, Diogenites, also named HED achondrites, make up a suite of magmatic rocks formed at high temperature. The parent body, most probably a large asteroid, shortly after formation, melted to form a metallic core and basaltic magmas. |
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Subsurface formation : Metamorphism and mechanical brecciation : |
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| The lastest studies, including dynamic impact simulations, infrared reflectance spectra as well as Hubble Space Telescope observations show a connection between asteroid 4 Vesta and these basaltic achondrites. Vesta, with a diameter of 540 km is the third largest asteroid so far discovered. Nevertheless, HED meteorites, which have a relative abundance of 5% of all meteorite falls, suggest that there is a possible larger parent family. This parent family has been aptly named the Vestoids. |
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- Diogenites are igneous rocks almost monomineralic: orthopyroxenite (95%) and a few percent olivine. With the addition of calcium, orthopyroxenes are inverted into clinopyroxenes and pigeonite. It is the beginning of Eucrite classifications. |
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Dar al Gani 609
monomict Eucrite |
Pyroxenes in Dar al Gani 609 are indicative of Eucrites. Numerous analytical tests confirm the absence of olivine while revealing a substantial metamorphic effect in the chemical equilibrium of Mg, Fe, and Ca in pyroxenes. DaG 609 is an iron-rich monomict eucrite with chromite grains. |
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- Eucrites are the oldest known basalts in the solar system (4.54 Ga). Minerals in Eucrites are pyroxenes (60%) and plagioclase (30%) with minor amounts of ilmenite, chromite, apatite and quartz. Complex processes of partial melting, fractional crystallisation, thermal metamorphism, brecciation and mechanical mixing are evident in the HED meteorites. These complex processes resulted in different lithologies which coexist in the form of polymict Eucrites. |
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Dar al Gani 863
polymict Eucrite |
DAG 863 is a polymict eucrite showing the result of the mixing of two different lithologies at some time in the past. |
| Most of the meteorites are representative samples of "recent" asteroid collisions. After ejection from its parent body, the average orbital lifetime for a meteorite is 20 Ma, with maximum ages of 80 Ma. HED meteorites show two peaks at 22 Ma and 39 Ma which could coincide with two major impact events on their parent body. |
| HED show a black fusion crust, they are little magnetic and break easily. |
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Their light interior makes them hard to find when they are broken. |
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- Howardites are shown as the result of such impact, they are mechanical mixtures of clasts of eucritic and diogenitic material, with a minor carbonaceous chondritic component. There is a continuous series between Howardites and polymict Eucrites. Some Howardites contain small amounts of diogenitic orthopyroxene, on the other hand some polymict Eucrites contain minor diogenitic components. The dividing line generally used for classification is 10 vol% of orthopyroxenes. |
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Dar al Gani 844
polymict breccia Eucrite/Howardite |
Typical polymict breccia showing many different inclusions which bear testimony to past impact events. Dar al Gani paired finds, DaG 844, DaG 671... have two classifications for the same fall, because small research samples can cross only the eucritic zone inside the Howardite. |
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HED meteorites are igneous rocks formed as a basaltic flow shortly after the formation of their parent body. HED meteorites show both a complex thermal history as well as the formation of impact brecciation. HED meteorites represent material from the very early history of our solar system. Of this material, 600 kg have been recovered and most of them are currently preserved in museums and institutions. Many well representative samples are available for collectors at reasonable prices. |
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