An achondrite is a type of stony meteoriteMeteorite composed of silicate minerals, but that may have up to 25% Ni-Fe metal by weight. Stony meteorites are extremely heterogeneous as a group, ranging from samples of primordial matter that have remained more or less unchanged for the last 4.56 Ga (chondrites) to highly evolved younger rocks from differentiated whose precursor was of chondritic origin and experienced metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. reached a sufficient size that through heating due to radioactive decayProcess in which an isotope's nucleus changes ('decays') to produce another isotope. The original atom is called the 'parent' and the resulting atom, the 'daughter'. There are three modes of radioactive decay: • Emission of a particle (He nucleus) that decreases the atomic number (Z) by 2 and the atomic of 26Al (aluminum isotopeOne of two or more atoms with the same atomic number (Z), but different mass (A). For example, hydrogen has three isotopes: 1H, 2H (deuterium), and 3H (tritium). Different isotopes of a given element have different numbers of neutrons in the nucleus.) and gravitational pressure allowed the parent body to differentiate into a coreIn the context of planetary formation, the core is the central region of a large differentiated asteroid, planet or moon and made up of denser materials than the surrounding mantle and crust. For example, the cores of the Earth, the terrestrial planets and differentiated asteroids are rich in metallic iron-nickel., mantleMain silicate-rich zone within a planet between the crust and metallic core. The mantle accounts for 82% of Earth's volume and is composed of silicate minerals rich in Mg. The temperature of the mantle can be as high as 3,700 °C. Heat generated in the core causes convection currents in and crustOutermost layer of a differentiated planet, asteroid or moon, usually consisting of silicate rock and extending no more than 10s of km from the surface. The term is also applied to icy bodies, in which case it is composed of ices, frozen gases, and accumulated meteoritic material. On Earth, the that may also have experienced periods of volcanism. Some achondrites formed or were further processed though hypervelocity impacts on their parent body that could have been chondritic or achondritic in origin. As the name implies, the resulting rock lacks chondrulesRoughly spherical aggregate of coarse crystals formed from the rapid cooling and solidification of a melt at ~1400 ° C. Large numbers of chondrules are found in all chondrites except for the CI group of carbonaceous chondrites. Chondrules are typically 0.5-2 mm in diameter and are usually composed of olivine and, at a minimum, has lost its chondritic texture. Further processing may cause the resulting achondrite to also lose its chondritic composition. ... Expand
Hypervelocity impacts result in the total melting of the target rock and the complete loss of chondrules (chondritic texture and mineralogy). However, achondrites that formed through metamorphic and igneous processes may exhibit rare chondritic inclusions, occasional chondrules and relict chondrules.
After their formation, achondrites are typically excavated at varying depths from the parent body crust, or in rare cases from the deeper crust-mantle boundary by an impact event large enough that fragments can escape its parent body’s gravityAttractive force between all matter - one of the four fundamental forces.. The impact event is not necessarily a hyper-velocity impact. Achondrites represent less than 10% of all meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and falls with the majority of achondrites originating from the crust of asteroid 4 VestaThird largest and fourth brightest asteroid; it was discovered in 1807 by Heinrich Olbers and named for the ancient Roman goddess of the hearth. 4 Vesta has a basaltic surface composition and an average density not much less than that of Mars. Evidently lava once flowed here indicating that the. Other confirmed achondrite parent bodies include the Moon and Mars.
Achondrites fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If into two categories: Evolved and Primitive.
- Primitive achondrites are residues from partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting. They have achondritic textures but retain their broadly chondritic compositions. The primitive achondrites include AcapulcoitesPrimitive achondrite that belongs to a small group named after the Acapulco meteorite that was observed to fall in Mexico in 1976. Acapulcoites are made mostly of fine-grained olivine (Fo3-14), orthopyroxene(En86-97), Ca-rich pyroxene (En51Wo44), plagioclase (An12-31), Ni-Fe metal, and troilite. They are transitional between primordial chondritic matter and more differentiated, Brachinites, Lodranites, Ureilites, and Winonaites.
- Evolved achondrites experienced more extensive igneous processing than the primitive achondrites and are the results of melts. The evolved achondrites include Angrites, AubritesAubrites are named for the Aubres meteorite that fell in 1836 near Nyons, France. They are an evolved achondrite that is Ca-poor and composed mainly of enstatite (En100) and diopside (En50Wo50) with minor amounts of olivine (Fa0) and traces of plagioclase (An2-8). They contain large white crystals of enstatite as, the HED suite (Howardites, EucritesMost common type of achondrite meteorite and a member of the HED group. Eucrites are basalts composed primarily of pigeonite and anorthite (An60-98). Eucrites have been placed into three subgroups based on mineralogical and chemical differences. • Non-cumulate eucrites represent the upper crust that solidified on a magma ocean after and DiogenitesDiogenites belong to the evolved achondrite HED group that also includes howardites and eucrites. They are named after the Greek philosopher Diogenes of Apollonia, of the 5th century BCE, who was the first to suggest that meteorites come from outer space (a realization forgotten for over 2,000 years). They are), lunar meteoritesAchondrite meteorites from the surface of the Moon. Most were found in the hot deserts of northern Africa and Oman and others were found in the cold desert of Antarctica, although one, a 19-gram specimen, was recovered in 1990 from Calcalong Creek, Australia. These stones are of great importance because,, and martian meteorites.
Achondrite meteorites are often difficult to tell apart from terrestrial rocks because they formed on parent bodies that, like Earth, also experienced a wide range of igneous processes. Consequently, fusion crustMelted exterior of a meteorite that forms when it passes through Earth’s atmosphere. Friction with the air will raise a meteorite’s surface temperature upwards of 4800 K (8180 °F) and will melt (ablate) the surface minerals and flow backwards over the surface as shown in the Lafayette meteorite photograph below. is one of the best ways to identify achondrite meteorites in the field while hunting.
