Achondrites

PART II

ACHONDRITES

Members of the achondrite classification can be divided into two distinct groups:

1. Primitive—igneous-textured meteorites that are nearly chondritic in bulk composition and retain geochemical and isotopic characteristics of their precursors. This includes the acapulcoite–lodraniteRare type of primitive achondrite named after the Lodran meteorite that fell in Pakistan in 1868. Initially, lodranites were grouped with the stony-iron meteorites because they contain silicates (olivine, orthopyroxene, and minor plagioclase) and Fe-Ni metal in nearly equal proportions. However, since discovery of the closely related acapulcoite group, lodranites clan, winonaites, and IAB silicates, and according to Fe/Mg–Fe/Mn plots it also includes the brachinites and ureilites (Goodrich and Delany, 2000). Brachinites and ureilites are generally residues from low degrees of 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 (< ~30%), and thus many are actually somewhat evolved meteorites.
2. Evolved—the primitive characteristics have become obliterated due to extensive igneous processing, i.e., melting and differentiationA process by which a generally homogeneous chondritic body containing mostly metal, silicates and sulfides will melt and form distinct (differentiated) layers of different densities.   When the melting process continues for a long enough period of time, the once chondritic body will re-partition into layers of different composition including (e.g., vestan [HEDs: 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 diogenites], martian [SNCs: shergottitesIgneous stony meteorite with a Martian origin consisting mainly of plagioclase (or a shocked glass of plagioclase composition) and pyroxene. They are the most abundant type of SNC meteorites and the type member is the Shergotty meteorite, which fell in India in 1865. Shergottites are igneous rocks of volcanic or, nakhlites, chassignitesThe group is part of the SNC martian trio and named after the meteorite seen to fall in Chassigny, France, in 1815. Its subsequent recovery led to it being one of the first meteorites to be recognized as a genuine rock from space. Chassigny resembles a terrestrial dunite - a, etc.], lunar).

The primitive achondrite group can be further divided into two subgroups:

1. Type A—members have some iron loss but preserve their main maficOne of the two broad categories of silicate minerals, the other being felsic, based on its magnesium (Mg) and/or iron (Fe) content. Mafic indicates silicate minerals that are predominantly comprised of Mg and/or Fe.The term is derived from those major constituents: Magnesium + Ferrum (Latin for iron) + ic (having chondritic silicateThe most abundant group of minerals in Earth's crust, the structure of silicates are dominated by the silica tetrahedron, SiO44-, with metal ions occurring between tetrahedra). The mesodesmic bonds of the silicon tetrahedron allow extensive polymerization and silicates are classified according to the amount of linking that occurs between the mineralogy (e.g. 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, lodranites, winonaites)
2. Type B—members have lost their primary REEOften abbreviated as “REE”, these 16 elements include (preceded by their atomic numbers): 21 scandium (Sc), 39 Yttrium (Y) and the 14 elements that comprise the lanthanides excluding 61 Promethium, an extremely rare and radioactive element. These elements show closely related geochemical behaviors associated with their filled 4f atomic orbital. and siderophile abundance ratios as well as some light silicate components, but preserve some primitive components such as Fe/Mg ratios, C- and O-isotopic ratios, and volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). abundances (e.g. brachinites, ureilites)

The Type A acapulcoite–lodranite clan has experienced a wide ranging thermal history. Of the most primitive acapulcoites, the least metamorphosed among them is GRA 98028, which exhibits relict 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, a very fine grain size, and contains no large FeS veins. The relict chondrules present in Monument Draw also reflect a relatively primitive nature. Further along the metamorphicRocks that have recrystallized in a solid state due to changes in temperature, pressure, and chemical environment. sequence is Dhofar 125, which exhibits early stages of melting and some loss of sulfides. Another typical acapulcoitePrimitive 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, Acapulco, has experienced a high degree of thermal metamorphism and has a highly recrystallized texture. Certain Antarctic meteorites exhibit loss of plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite and sulfide phases, and are transitional to lodranites. Finally, Lodran and the other lodranites have experienced the highest temperatures. They were crystallized from residual melt material depleted in the low-melting point components such as plagioclase, troiliteBrass colored non-magnetic mineral of iron sulfide, FeS, found in a variety of meteorites., and metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn. One further stage in the metamorphic continuum is represented by an Antarctic 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 that has trapped a partial melt component that was lost from the lodranite region. This meteorite has become an enriched acapulcoite.

Among the meteorite types that compose the evolved achondriteAn achondrite is a type of stony meteorite whose precursor was of chondritic origin and experienced metamorphic and igneous processes. They have a planetary or differentiated asteroidal origin where the chondritic parent body reached a sufficient size that through heating due to radioactive decay of 26Al (aluminum isotope) and gravitational group, the eucrites have been petrologically divided into a metamorphic sequence comprising seven types (after Takeda and Graham, 1991; Yamaguchi et al., 1996):

1. Type 1—most quickly cooled in the sequence; mesostasis-rich with glass phase and original chemistry preserved; exhibit pronounced Mg-Fe zoning in pyroxenes; represent the least altered basaltBasalt is the most common extrusive igneous rock on the terrestrial planets. For example, more than 90% of all volcanic rock on Earth is basalt. The term basalt is applied to most low viscosity dark silicate lavas, regardless of composition. Basalt is a mafic, extrusive and fine grained igneous rock studied; e.g., clasts in Y-75011, Y-75015, and Y-74450
2. Type 2—metastable Fe-rich pyroxenes are absent; mesostasisLast material to crystallize/solidify from a melt. Mesostasis can be found in both chondrules, in the matrix around chondrules, and in achondrites as interstitial fine-grained material such as plagioclase, and/or as glass between crystalline minerals.   glass is no longer clear; e.g., Pasamonte
3. Type 3—zoning from 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. to rim is less defined with an increase in Ca towards the rim; pyroxenes becoming cloudy; coarsening of pyroxenes resulting from 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. exsolutionSegregation, during cooling, of a homogeneous solid solution into two or more different solids. lamellae; e.g., clastA mineral or rock fragment embedded in another rock. in Y-790266
4. Type 4—only remnants of zoning still visible; cloudy pyroxenes present; mesostasis glass is recrystallized or absent; augite exsolution lamellae becoming resolvable in microprobe; e.g., Stannern, Nuevo Laredo
5. Type 5—homogenous host composition with readily resolvable exsolved pigeoniteLow-Ca clinopyroxene, (Ca,Mg,Fe)SiO3, found as a major mineral in eucrites and shergottites. In order to be considered pigeonite, the clinopyroxene must contain 5 to 20 mol % of calcium (Wo5 - 20). Chondrites of petrologic types 4 and below contain significant low-Ca clinopyroxene. During metamorphism to higher temperatures, all existing lamellae; pigeonites extensively clouded by reheating; mesostasis glass recrystallized or absent; e.g., Juvinas, Sioux Co., Lakangaon
6. Type 6—most slowly cooled eucrites in the sequence; the clinopyroxene pigeonite is partly inverted to 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 through slow cooling processes; pyroxenes contain Mg-rich cores and coarse augite exsolution lamellae; original mesostasis is absent; Ca is enriched in the rims; often have a brecciated texture; e.g., Millbillillie, Y-791186
7. Type 7—recognized as the most metamorphosed in the sequence (Yamaguchi et al., 1996); e.g., Palo Blanco Creek, Jonzac, Haraiya, A-87272

Achondrites represent about 8% of all meteorite falls. They originated on chondritic bodies that were subjected to some degree of igneous melting and recrystallization. Their parent bodies were large enough to melt and segregate the denser metals from the lighter silicates, generally forming a metallic core, a magnesium-rich 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 a calcium-rich 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. Of the various achondrites, three groups (howardites, eucrites, 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) are believed to have originated on the 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 and constitute the largest achondrite group. These represent the brecciated surface materials (howardites), the extrusiveRefers to igneous rocks erupted on a planetary body's surface./intrusive basalts (eucrites), the 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 cumulates (diogenites), and a deeper cumulateIgneous rock composed of crystals that have grown and accumulated (often by gravitational settling) in a cooling magma chamber. layer (olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion diogenites and dunites).

In addition, numerous meteorites comprising five main groups originated on Mars (shergottites, nakhlites, chassignites, etc.), and a growing number of meteorite finds are of lunar origin comprising lithologies and mixtures thereof from three distinct geochemical regions: the Feldspathic Highlands Terrane, the incompatible element-rich Procellarum KREEPLunar igneous rock rich in potassium (K), rare-earth elements (REE), phosphorus (P), thorium, and other incompatible elements. These elements are not incorporated into common rock-forming minerals during magma crystallization, and become enriched in the residual magma and the rocks that ultimately crystallize from it. Terrane, and the South Pole-Aitken Terrane (Jolliff et al., 2000).

The winonaites formed in the same region of the nebula, and possibly on the same parent body as silicate inclusions present in the IAB complex irons. There are still many theories proposed to explain the origins of the other achondrite groups, which include the second largest achondrite group, the ureilites, along with the less abundant acapulcoites, lodranites, brachinites, aubrites, and angrites.

© 1997–2019 by David Weir