HowarditeOne type of meteorite in the HED (Howardite, Eucrite, Diogenite) achondrite group. Howardites are named after the English chemist Edward Howard (1774-1816), one of the pioneers of meteoritics. Consisting mostly of eucritic and diogenitic clasts and fragments, howardites are polymict breccias. However, they can also contain dark clasts of carbonaceous Click on Term to Read More Fragmental brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material Click on Term to Read More Found June 1991 31° 10′ S., 127° 45′ E.
Two specimens with a total weight of 783 g were found on the Nullarbor Plain in Western Australia. The 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 Click on Term to Read More exhibits a typical howardite composition with a brecciated and compacted mixture of mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More and rock fragments, primarily diogeniteDiogenites 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 Click on Term to Read More and eucrite—with a cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. Click on Term to Read More/noncumulate eucriteMost 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 Click on Term to Read More ratio commensurate with that of known 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 Click on Term to Read More. A possible paired specimen, the ~60 g Mundrabilla 020, has recently been found. It has a pairing score of 76.7 based on the pairing criteria of petrography and geographic proximity.
Studies of orthopyroxeneOrthorhombic, low-Ca pyroxene common in chondrites. Its compositional range runs from all Mg-rich enstatite, MgSiO3 to Fe-rich ferrosilite, FeSiO3. These end-members form an almost complete solid solution where Mg2+ substitutes for Fe2+ up to about 90 mol. % and Ca substitutes no more than ~5 mol. % (higher Ca2+ contents occur Click on Term to Read More crystals from selected howardites indicate an iron and magnesium concentration in Old Homestead 001 that is consistent with that of the Mg-rich cumulate eucrites (Domeneghetti et al., 2000, 2004). Notably, certain howardites contain clasts of low-Mg, REE-, K-, and P-rich (KREEPy) lithologies representing compositionally evolved rocks (Barrat et al., 2012).
Late-stage cumulate eucriteMost 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 Click on Term to Read More material in certain howardites has been found to contain the first symplectic assemblages in such meteorites (Patzer and McSween, 2011). The absence of exsolved augiteHigh-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, that occurs in many igneous rocks, particularly those of basaltic composition. In order to be considered augite, the clinopyroxene must contain 20 to 45 mol % of calcium (Wo20 - 45). An important and unique Martian meteorite is NWA 8159, that has been classified as an augite basalt. Click on Term to Read More, the Ca-rich composition, and the relatively rapid cooling rate (1.4°C/day) all suggest an origin from a hot source rock that was excavated by impact. This scenario is supported by the high closure temperature of orthopyroxene of 619°C (Domeneghetti et al., 2000), consistent with quenching upon excavation of hot plutonicGeology: Igneous intrusive body that forms when magma is injected into host rocks and solidifies. Plutons occur in the crust of asteroids undergoing differentiation or planets. Named after Pluto, the Roman god of the underworld. Plutonic rocks are the rocks found within a pluton. Astronomy: Category of planet including all Click on Term to Read More rock. A more detailed scenario on the formation of the HED clan can be found on the Millbillillie page. The specimen pictured above is a 4.1 g partial slice.
