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Achondrite
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Achondrite
— Type —
Cumberland Falls, achondrite (aubrite)
Compositional typeStony

An achondrite[1] is a stony meteorite that does not contain chondrules.[2][3] It consists of material similar to terrestrial basalts or plutonic rocks and has been differentiated and reprocessed to a lesser or greater degree due to melting and recrystallization on or within meteorite parent bodies.[4][5] As a result, achondrites have distinct textures and mineralogies indicative of igneous processes.[6]

Achondrites account for about 8% of meteorites overall, and the majority (about ) of them belong to the HED clan, possibly originating from the crust of asteroid Vesta. Other types include Martian, Lunar, and several types thought to originate from as-yet unidentified asteroids. These groups have been determined on the basis of e.g. the Fe/Mn chemical ratio and the 17O/18O oxygen isotope ratios, thought to be characteristic "fingerprints" for each parent body.[7]

Classification

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Achondrites are classified into the following groups:[8]

Primitive achondrites

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Primitive achondrites, also called PAC group, are so-called because their chemical composition is primitive in the sense that it is similar to the composition of chondrites, but their texture is igneous, indicative of melting processes. To this group belong:[8]

Asteroidal achondrites

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Asteroidal achondrites, also called evolved achondrites, are so-called because they have been differentiated on a parent body. This means that their mineralogical and chemical composition was changed by melting and crystallization processes. They are divided into several groups:[8]

Lunar meteorites

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Lunar meteorites are meteorites that originated from the Moon.

Martian meteorite

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Martian meteorites[10] are meteorites that originated from Mars. They are divided into three main groups, with two exceptions (see last two entries):

See also

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References

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Achondrite

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Achondrites are a class of stony meteorites defined by the absence of chondrules—small, spherical mineral grains characteristic of primitive chondritic meteorites—and their formation through igneous processes on differentiated parent bodies in the solar system. These meteorites originate from the crusts or mantles of asteroids, the Moon, Mars, and potentially other planetary objects that experienced partial or complete melting, leading to the segregation of metallic cores from silicate-rich layers. Composed primarily of silicate minerals such as pyroxene, olivine, and plagioclase, achondrites exhibit textures and compositions akin to terrestrial volcanic rocks, reflecting high-temperature crystallization from molten material. Unlike the more abundant chondrites, which preserve unaltered material from the solar nebula, achondrites represent secondary processing through heating and melting on larger parent bodies capable of sustaining internal heat from radioactive decay or impacts. Their identification on Earth is often difficult due to visual similarities with common igneous rocks, requiring evidence like a dark fusion crust from atmospheric entry, shock features, or geochemical analysis to confirm extraterrestrial origin. Achondrites constitute about 8% of all known meteorites, with many recovered as finds rather than observed falls, and they frequently appear as breccias—fragmented assemblages cemented together—indicating violent ejection from their parent bodies via collisions. Stony meteorites are broadly classified into chondrites and achondrites based on texture, , chemistry, and oxygen isotopes, with achondrites further subdivided into primitive and evolved types. Primitive achondrites, such as acapulcoites, lodranites, and ureilites, have undergone but retain bulk compositions close to chondrites, lacking distinct core-mantle separation. Evolved achondrites, in contrast, show significant and include major groups like the HED clan (howardites, eucrites, and diogenites), sourced from the asteroid ; the SNC group (shergottites, nakhlites, and chassignites) from Mars; lunar basalts and anorthosites from the ; and asteroidal types such as aubrites ( achondrites) and angrites. Notable examples include the Nakhla meteorite, a martian nakhlite that fell in in 1911, and the Allan Hills 84001, a martian orthopyroxenite famous for potential signs of ancient microbial life. Achondrites are invaluable in for revealing the mechanisms of differentiation in the early solar system, including how planetesimals accreted, melted, and evolved into layered bodies with metallic cores and mantles. They offer direct samples of igneous activity on airless worlds, informing models of volcanic processes on Mars and the , and linking meteorite compositions to asteroid spectra observed by telescopes. By studying achondrites, scientists reconstruct the thermal history and bombardment events that shaped terrestrial planets, providing constraints on the volatile budgets and geological diversity of the inner solar system.

Overview

Definition and Characteristics

Achondrites are a major class of stony meteorites defined by their lack of chondrules—small, spherical grains characteristic of chondrites—and their that closely resemble those of terrestrial basaltic or plutonic rocks. These meteorites originate from differentiated parent bodies where partial or complete allowed for the separation of molten material, leading to the formation of crystalline structures through cooling and recrystallization. Unlike primitive meteorites, achondrites exhibit no evidence of the primordial, unequilibrated components found in chondrites, instead showing signs of magmatic processing. The name "achondrite" originates from the Greek words a- (without) and chondros (grain or cartilage), highlighting the absence of chondrules, a term coined by German mineralogist Gustav Rose in 1864 when he distinguished these meteorites from chondritic ones during his cataloging of meteorite collections. This etymology underscores their structural distinction from the more granular chondrites. Achondrites also lack the abundant metal grains or nickel-iron alloys common in many chondrites, contributing to their overall appearance as coherent, rock-like fragments. Their typical bulk density ranges from 3.0 to 3.5 g/cm³, reflecting a composition dominated by silicate minerals with minimal porosity in fresh samples. In terms of abundance, achondrites account for approximately 8% of all observed meteorite falls, a proportion that highlights their relative rarity compared to chondrites but underscores their importance in understanding processes. This observed frequency in falls exceeds their representation in unpaired finds, as achondrites are less prone to fragmentation during than some fragile chondrite types.

Distinction from Chondrites

Chondrites represent the most primitive meteorites, characterized as undifferentiated materials that preserve the earliest compositions of the solar system, while achondrites are processed rocks derived from bodies that underwent melting and differentiation, lacking the defining features of their chondritic counterparts. Chondrites contain chondrules—small, spherical grains typically 0.1 to 1 mm in diameter, formed by the rapid cooling of molten droplets in the solar nebula—embedded within a fine-grained matrix, whereas achondrites entirely lack these chondrules, reflecting their igneous origins rather than nebular accretion. This absence of chondrules in achondrites distinguishes them as "non-chondritic" stony meteorites, comprising only a few percent of all falls compared to the abundant chondrites. Structurally, chondrites exhibit aggregational textures where chondrules are bound by a matrix, often interspersed with metal flecks and opaque nodules that indicate minimal alteration since formation. In contrast, achondrites display such as structures, with larger phenocrysts set in a finer groundmass, or equigranular crystals resulting from slow cooling of melts on their parent bodies. These textural differences highlight the chondrites' role as breccias of primordial components versus the achondrites' evidence of magmatic processing. From an evolutionary perspective, chondrites embody the initial accretion of solar system materials around 4.5 billion years ago, providing direct samples of the with little post-formation modification. Achondrites, however, signify subsequent thermal events that caused partial or complete melting on asteroidal parent bodies, leading to differentiation into crust, mantle, and core—processes that also produced iron meteorites as metallic remnants from the same bodies. This progression underscores achondrites' importance in tracing planetary beyond the primitive stage captured by chondrites. Representative examples illustrate these distinctions: ordinary chondrites, such as those in the H (high-iron), (low-iron), and (very low-iron) groups, feature prominent chondrules and metal grains, exemplifying undifferentiated accretion. Conversely, basaltic achondrites like eucrites showcase fine-grained, pyroxene-plagioclase assemblages with ophitic or subophitic textures, indicative of basaltic on a differentiated such as .

Physical and Chemical Properties

Texture and Mineralogy

Achondrites exhibit a range of primarily resulting from the cooling and of melts, distinguishing them from the chondrule-bearing structures of chondrites. These textures are typically crystalline, with holocrystalline varieties predominant in angrites, where fine- to medium-grained assemblages of and form without significant glassy components, indicative of rapid cooling on a parent body surface. Brecciated textures are common in polymict achondrites like howardites, composed of fragmented clasts cemented by impact-derived matrix, reflecting post- disruption events. Glassy phases are rare and generally limited to impact-melted varieties, where shock produces diaplectic glasses such as maskelynite from . The of achondrites is dominated by phases formed through magmatic processes, with pyroxenes as a primary constituent. Orthopyroxene, often with magnesium-rich compositions (En75), prevails in diogenites, forming coarse-grained cumulate textures up to several millimeters in size. , typically calcic varieties like (An80), is abundant in eucrites, intergrowing with pigeonite in basaltic textures. Forsteritic (Fo-rich, e.g., Fo73–90) appears in many types, such as angrites and diogenites, often as zoned crystals or cumulus grains, while minor oxides like occur as accessories in ultramafic varieties. Textural variations reflect differentiation histories, with coarse-grained cumulates in diogenites evidencing slow cooling in deeper crustal layers, contrasted by finer-grained volcanic textures in shergottites, where pigeonite and olivine form subhedral grains in a groundmass indicative of rapid eruption. Thin-section petrography remains the foundational analytical method for identifying these textures and minerals, involving polarized light microscopy to observe crystal habits, grain boundaries, and exsolution features, often supplemented by electron microprobe analysis for compositional mapping.

Composition and Isotopes

Achondrites exhibit silicate-rich bulk compositions, with SiO₂ contents typically ranging from 45 to 55 wt%, reflecting their derivation from differentiated parent bodies where and igneous processes concentrated silicates. The FeO/MgO ratios vary significantly among achondrite groups, indicating diverse oxidation states and magmatic histories; for instance, ureilites display notably low FeO/MgO ratios, often corresponding to with Fo values (molar Mg/(Mg+Fe)) exceeding 90 in many samples. Compared to chondrites, achondrites generally show depletions in volatile elements such as Na and , attributable to high-temperature processing that preferentially volatilized these moderately volatile components during parent body differentiation. Trace element abundances in achondrites further highlight their processed origins, with (REE) patterns often displaying that deviates from chondritic uniformity. In HED meteorites, for example, light REE enrichment relative to heavy REEs is common, signifying crustal-level magmatic differentiation processes such as fractional crystallization and of a mantle source. Isotopic compositions provide key evidence of achondrites' igneous heritage. Oxygen isotopes in achondrites plot along distinct mass-dependent fractionation lines, separate from those of chondrites; HED meteorites, in particular, align on a line parallel to but offset from the fractionation line by approximately 0.5‰ in Δ¹⁷O (HED Δ¹⁷O ≈ -0.24‰ vs. ≈ +0.26‰), consistent with equilibration on a common parent body under varying oxygen conditions. Cosmogenic nuclides, such as ²¹Ne and ³⁸Ar, accumulated during space exposure yield cosmic-ray exposure ages typically between 1 and 100 Ma, reflecting ejection events from parent bodies in the . Unlike primitive chondrites, achondrites lack presolar grains—nanoscale silicate and carbide particles predating the solar system—due to post-accretion heating and melting that destroyed these fragile relics through thermal metamorphism and igneous activity.

Origin and Formation

Planetary Differentiation

Planetary differentiation on achondrite parent bodies was primarily driven by radiogenic heating from the short-lived isotope ²⁶Al, which decayed rapidly in the early solar system approximately 4.5 billion years ago (Ga), providing sufficient energy to induce partial melting and subsequent separation of materials into metallic cores, silicate mantles, and basaltic crusts. This process began with the accretion of chondritic precursor materials, which were undifferentiated aggregates of dust and chondrules from the protoplanetary disk, followed by heating that elevated interior temperatures to 1000–1500°C, enabling widespread partial melting and the formation of magma oceans. As melting progressed, denser metallic components sank to form cores, while lighter silicates rose to create layered structures, with fractional crystallization in cooling magmas producing cumulate rocks at depth and extrusive lavas on the surface. The stages of differentiation unfolded rapidly after accretion: initial heating disrupted the chondritic structure without significant aqueous activity, leading to melting that homogenized and segregated components, unlike the water-mediated alteration seen in many chondrites. Evidence for these cooling sequences is preserved in zoned minerals, such as pigeonite pyroxenes in eucrites, where compositional gradients reflect sequential from evolving compositions during differentiation. The absence of hydrous minerals or alteration products in achondrites further supports a dry, high-temperature igneous environment, contrasting with the hydrated phyllosilicates and carbonates in aqueously altered chondrites. This entire differentiation process occurred on a compressed timescale of ~2–10 million years following calcium-aluminum-rich inclusion (CAI) formation at ~4.567 Ga, constrained by isotopic systems like ²⁶Al-²⁶Mg and ⁵³Mn-⁵³Cr in achondritic materials, which indicate core formation and magmatic activity shortly after CAIs. Such rapid evolution highlights the efficiency of ²⁶Al as a source in small planetesimals, allowing differentiation before significant collisional disruption.

Parent Bodies and Ejection Mechanisms

Achondrites originate from several differentiated parent bodies within the solar system, including , the , and Mars. The serves as the primary source for the howardite-eucrite-diogenite (HED) group of achondrites, a connection confirmed through spectroscopic and compositional matches established by NASA's Dawn mission during its 2011–2012 orbital observations. Other achondrite groups, such as ureilites, derive from a distinct parent body, inferred from inclusions and modeling to be a large (potentially Mercury- to Mars-sized) that underwent partial differentiation before catastrophic disruption. Lunar and martian achondrites, representing basaltic and ultramafic from these planetary bodies, provide direct samples of their crusts and mantles. Ejection of achondritic material occurs primarily through hypervelocity impacts that excavate and launch surface or near-surface rocks into space without complete melting. On Mars, oblique impacts generate , propelling fragments at velocities exceeding 5 km/s to escape the planet's , with most ejections dated to 0.7–20 million years ago based on cosmic ray exposure (CRE) analyses. Similar impact-driven mechanisms operate on the Moon, where cratering events loft material at speeds above 2.4 km/s, yielding CRE ages typically between 1 and 100 million years. For asteroidal sources like Vesta, family-forming collisions, such as the Rheasilvia basin impact, fragment the body and disperse material, with ejection velocities of several km/s enabling escape from the main . Following ejection, achondritic fragments undergo orbital evolution through gravitational interactions, including perturbations by and resonances such as the 3:1 or the ν6 secular resonance, which gradually transfer them from stable main-belt orbits to Earth-crossing paths over millions of years. Upon reaching Earth's vicinity, these meteoroids enter the atmosphere as bright fireballs, decelerating rapidly due to . Compact achondrites exhibit higher survival rates during entry compared to friable chondrites, as their coherent, resist fragmentation under peak dynamic pressures of 10–50 MPa. Key evidence linking achondrites to specific parent bodies and ejection events comes from pairing analyses, which group meteorites sharing similar CRE ages—indicating common launch times—and compatible orbital trajectories modeled via numerical simulations. For instance, HED meteorites cluster into several CRE age groups (e.g., 6–22 Ma), suggesting multiple impact ejections from Vesta, while martian meteorites form 5–8 launch pairs with CRE ages under 20 Ma, constraining source craters via backward integration of orbits. These methods, combined with isotopic and mineralogical signatures, affirm the differentiated origins of achondrites without relying on speculative transport scenarios.

Classification

Primitive Achondrites

Primitive achondrites represent a rare class of stony meteorites, comprising approximately 5% of all known achondrites, that experienced on their bodies but exhibit incomplete differentiation, thereby preserving some primitive, chondrite-like components such as bulk abundances and unequilibrated assemblages in certain cases. These meteorites bridge the gap between undifferentiated chondrites and fully differentiated achondrites, having undergone and limited melt extraction without complete separation of metallic, , and phases. The primary subgroups of primitive achondrites include acapulcoites-lodranites, winonaites, brachinites, and ureilites, each displaying distinct mineralogical and geochemical signatures indicative of varying degrees of . Acapulcoites and lodranites are closely related, metal-rich rocks derived from the same parent body, featuring protogranular to granular textures with significant Fe-Ni metal and , and evidence of 1-20% at temperatures around 950-1250°C. Winonaites exhibit links to EL chondrites through similar oxygen isotopic compositions and , including silicate-rich assemblages with accessory metals and sulfides. Brachinites are -dominated, ultramafic rocks with >10-30% , consisting primarily of forsteritic and clinopyroxene. Ureilites, the most abundant subgroup, are carbonaceous achondrites rich in , pyroxenes, and reduced carbon phases, often containing nanodiamonds formed by shock metamorphism during impact events on their parent body. Key characteristics of primitive achondrites include equilibrated, granoblastic textures resulting from high-temperature , with triple junctions at approximately 120° between mineral grains, and the presence of Fe-Ni metal and sulfide phases that vary in abundance depending on the —such as up to 7-8% Fe-Ni in some acapulcoites. Their oxygen isotopic compositions are generally close to those of chondrites, with Δ¹⁷O values ranging from -2.45‰ to +0.21‰ across groups, supporting genetic ties to primitive solar system materials; for instance, acapulcoites-lodranites cluster at Δ¹⁷O ≈ -1.07 ± 0.25‰, while brachinites are at -0.21 ± 0.23‰. These meteorites are thought to originate from multiple small asteroids, typically 25-65 km in , that were heated—likely by the decay of short-lived radionuclides like ²⁶Al—but did not achieve full , resulting in incomplete and retention of heterogeneous structures.

Asteroidal Achondrites

Asteroidal achondrites originate from the crusts and mantles of fully differentiated asteroids that underwent extensive and magmatic differentiation early in Solar System history. These meteorites exhibit from volcanic eruptions or plutonic intrusions, with mineral assemblages reflecting basaltic to ultramafic compositions. Unlike primitive achondrites, they represent products of complete , including core formation and crustal evolution. The primary groups—HED, aubrites, and angrites—account for the majority of known asteroidal achondrites and provide key insights into asteroidal and impact processing. The howardite-eucrite-diogenite (HED) clan forms the largest group of asteroidal achondrites, comprising the most abundant class in meteorite collections. Howardites are polymict breccias composed of clasts from eucritic and diogenitic lithologies, often incorporating material shocked and mixed by impacts. Eucrites include basaltic varieties, representing effusive lavas or shallow intrusions, and cumulate gabbros formed by crystal settling in deeper magmas; they consist mainly of pigeonite, , and minor silica phases. Diogenites are ultramafic rocks dominated by orthopyroxene (64–100 vol%), with some harzburgitic or dunitic subtypes containing up to 33 vol% , interpreted as lower crustal cumulates. These subgroups reflect a stratified parent body with a basaltic crust over an orthopyroxene-rich lower crust and mantle. The HED clan is linked to asteroid 4 Vesta as the parent body, evidenced by spectroscopic matches between Vesta's surface and HED mineralogies, as well as orbital data from NASA's Dawn mission confirming impact craters as sources of ejected material. Aubrites are highly reduced enstatite achondrites, characterized by nearly iron-free (Mg-rich orthopyroxene) as the dominant mineral (>75 vol%), alongside minor forsteritic , nickel-iron metal, , and unique phases like alabandite or daubreelite. Their light-colored, brecciated textures and low FeO/MgO ratios indicate formation under extremely reducing conditions, similar to enstatite chondrites but from a differentiated parent body that experienced and metal-silicate separation. Aubrites likely derive from a distinct enstatite chondrite-like , possibly within the Hungaria family, separate from the HED source; potential candidates include asteroid 3103 Egeria based on spectral analogies. Angrites represent a rare group of , calcium-rich basaltic achondrites, primarily composed of Al-Ti-diopside (fassaite), anorthitic , and calcic or kirschsteinite, with accessory and . Their or cumulate textures and depletion in moderately volatile elements (e.g., Na/Al ratios ~10 times lower than HED) point to origins as early crustal melts from a , volatile-poor parent body, crystallizing rapidly under oxidizing conditions relative to other achondrites. Angrites formed within ~2–6 million years of Solar System accretion, making them among the oldest known igneous meteorites; their parent body remains unidentified but is inferred to be a small, differentiated distinct from Vesta. A key distinguishing feature of asteroidal achondrites is their oxygen isotope systematics, which plot along group-specific mass lines on the δ¹⁷O vs. δ¹⁸O diagram: the eucrite fractionation line (EFL) for HED meteorites and the angrite fractionation line (AFL) for angrites, offset from the terrestrial fractionation line and those of lunar or martian meteorites, underscoring their asteroidal rather than planetary origins. Aubrites align closely with chondrites on the δ¹⁷O-δ¹⁸O plot, reflecting minimal oxygen exchange during differentiation. Notable anomalous asteroidal achondrites include Ibitira, classified as an unpaired eucrite due to its , lower Fe/Mn ratios in pyroxenes, and slightly offset oxygen isotopes from the main HED field, suggesting derivation from a Vesta-like but distinct parent body. HED meteorites dominate Antarctic collections, where they form a disproportionately high fraction (~5–10% of finds) compared to falls, owing to favorable preservation in ice; aubrites and angrites are far rarer, with only ~93 and ~35 known specimens, respectively, and no significant new asteroidal achondrite discoveries reported through 2025.

Lunar Meteorites

Lunar meteorites represent a subset of achondritic meteorites confirmed to originate from the , providing terrestrial samples of lunar materials independent of Apollo and Luna missions. As of November 2025, 273 lunar meteorites have been recognized, primarily recovered from hot and cold deserts such as and Northwest Africa. These achondrites are identified through a combination of petrographic, chemical, and isotopic analyses that match known lunar compositions, including oxygen ratios plotting near the Terrestrial Fractionation Line with Δ¹⁷O values typically around 0‰ but distinct from by 3–4 ppm, and noble gas signatures showing solar wind implantation evidenced by ⁴He/²⁰Ne ratios of 2.7 to 5.6 in regolith breccias, alongside cosmogenic nuclides like ¹⁰Be and ²⁶Al for exposure history. Lunar achondrites are broadly classified into three main types based on their and : feldspathic (anorthositic) breccias from the lunar highlands, basaltic meteorites from regions, and impact-melt breccias. Anorthositic examples, such as ALHA 81005, consist predominantly of plagioclase-rich rocks (>90% ) representing the ancient highland crust, with low iron and aluminum-rich compositions. basalts are divided into low-titanium (low-Ti, <6 wt% TiO₂, e.g., LAP 02205) and high-titanium (high-Ti, >8 wt% TiO₂) varieties, reflecting volcanic flows from lunar maria, while impact-melt breccias like NWA 773 feature fused clasts of basaltic and gabbroic material from shock events. Many samples exhibit KREEP-rich signatures, characterized by elevated concentrations of (K), rare earth elements (REE), and (P), such as up to 30 ppm in Sayh al Uhaymir 169, indicative of late-stage magmatic differentiation. These meteorites display crystallization ages predominantly between 3 and 4 billion years ago (Ga), aligning with the era of major lunar magmatic activity, and often feature brecciated textures from processes or impact gardening. As fragments ejected from the Moon's surface, they were launched by large basin-forming impacts, such as that forming the Imbrium basin, with data indicating transit times to of 1 to 10 million years. This ejection mechanism, combined with minimal alteration during space exposure, preserves pristine records of lunar geology.

Martian Meteorites

Martian meteorites, also known as SNC meteorites after the type specimens Shergotty, Nakhla, and Chassigny, represent a distinct group of achondrites originating from , providing the only direct samples of the planet's crust available on . These meteorites are primarily igneous rocks formed through volcanic processes, with crystallization ages spanning from the ancient Martian crust to relatively recent activity. As of November 2025, approximately 400 Martian meteorites have been officially classified, collectively weighing around 360 kg. The SNC meteorites are divided into several groups based on their , texture, and age. Shergottites, the most abundant group comprising approximately 80% of known specimens, are predominantly basaltic or lherzolitic rocks with young ages of less than 600 million years, reflecting recent Martian . Nakhlites are clinopyroxenites with ages around 1.3 billion years, while chassignites are dunites sharing a similar age of about 1.3 billion years. A notable outlier is ALH 84001, an orthopyroxenite with a age of approximately 4.1 billion years, representing one of the oldest known samples from Mars. Identification of these meteorites as Martian relies on unique geochemical signatures, including elevated δ¹³C values in carbonates and trapped atmospheric gases exhibiting a ¹⁴N/¹⁵N ratio consistent with Mars' current atmosphere, as confirmed by analyses of impact glass in specimens like EETA 79001. Additionally, their oxygen isotope compositions plot along the Martian line, distinct from terrestrial or lunar values. Physical characteristics include thin fusion crusts or rinds formed due to Mars' thin atmosphere, which results in minimal during compared to thicker atmospheres. Some, particularly nakhlites, show evidence of aqueous alteration, such as iddingsite veins formed by interaction with brines around 600-700 million years ago. These meteorites were ejected from Mars by recent impact events, with cosmic-ray exposure ages typically ranging from 11 to 20 million years, indicating launch from the surface within the last few tens of millions of years. Many are linked to impacts in the volcanic region, where young basaltic terrains align with the of shergottites and other groups.

History and Research

Early Discoveries and Classification

The first documented fall of an achondrite meteorite occurred on May 22, 1808, when the Stannern meteorite landed near Stonařov in what is now the , yielding approximately 52 kg of material later classified as a eucrite due to its basaltic composition and lack of chondrules. This event marked the initial recognition of achondritic material, though its significance was not fully appreciated until later petrographic studies in the . In 1864, German mineralogist Gustav Rose, director of the Mineralogical at the University of , formalized the distinction of achondrites in his seminal catalog "Beschreibung und Eintheilung der Meteoriten im mineralogischen Museum der Universität zu Berlin," coining the term "achondriten" (from Greek "a-" meaning without and "chondros" meaning grain) to describe stony meteorites lacking the rounded chondrules typical of chondrites. Rose's work built on earlier observations, such as those of the Stannern specimen, and introduced subgroups based on , including eucrites for plagioclase-pyroxene rocks and diogenites for orthopyroxene-rich varieties, exemplified by early finds like the 1866 material later associated with diogenitic textures. Rose's system laid the groundwork for separating achondrites from chondrites. By the late , catalogs like the 1897 edition by G. T. Prior at the explicitly distinguished achondrites by their igneous textures and absence of chondritic features. In the 20th century, detailed petrographic and chemical analyses refined Rose's categories into the modern Howardite-Eucrite-Diogenite (HED) clan by the mid-century. A major milestone came in the with U.S. Antarctic Search for Meteorites (ANSMET) expeditions, which dramatically increased achondrite recoveries—several dozen specimens by decade's end—enabling better statistical sampling and identification of rare types like lunar and martian achondrites. In the 1990s, spectroscopic studies solidified the link between HED achondrites and , based on matching basaltic compositions and spectral signatures observed from Earth-based telescopes.

Modern Studies and Techniques

Since the , advancements in in-situ analytical techniques have significantly enhanced the study of achondrites by enabling high-resolution characterization without extensive sample preparation. Secondary ion mass spectrometry (SIMS) has become a cornerstone for isotopic analysis, particularly for oxygen and other light elements, allowing researchers to map variations in achondritic materials that reveal differentiation processes on parent bodies. Similarly, computed (CT) scans have been employed to examine brecciated structures in achondrites, such as those in basaltic varieties, providing three-dimensional insights into internal textures and shock features that inform impact histories. Spacecraft missions have complemented laboratory efforts by supplying contextual data linking achondrites to their sources. The NASA Dawn mission, which orbited asteroid Vesta from 2011 to 2012, confirmed the connection between howardite-eucrite-diogenite (HED) achondrites and Vesta through spectroscopic matches in composition and mineralogy, including low volatile content and basaltic crust signatures. For Martian achondrites, the Perseverance rover's exploration of Jezero crater since 2021 has identified igneous rocks analogous to shergottites, offering surface context that highlights compositional differences from meteorite samples and underscores the need for returned samples to resolve discrepancies. As of 2025, Perseverance's sample caching continues to provide insights into Martian igneous processes relevant to SNC meteorites. Key methodological advances in the included the use of (Ni) and (Cr) stable isotopes for pairing meteorites, enabling precise grouping of fragments from the same fall based on mass-independent signatures that distinguish parent body origins. In the 2020s, micro-X-ray fluorescence (micro-XRF) spectrometry has emerged for non-destructive trace element mapping in achondrites, facilitating rapid field identification and detailed geochemical profiling of elements like rare earths in ungrouped samples. Recent classifications, such as ungrouped achondrites approved in 2024-2025, reflect ongoing refinement of existing groupings, with no major new types since the 2023 recognition of additional howardite variants. Major collections of achondrites have grown through systematic searches, with the Antarctic Search for Meteorites (ANSMET) program contributing significantly since the 1970s, recovering over 300 achondrites that represent approximately 1.5% of its total finds as of 2025, including rare Martian and lunar examples. Hot desert expeditions in and have yielded thousands of specimens, with alone documenting over 4,000 meteorites by 2024, many achondrites preserved by arid conditions despite surface . These collections are cataloged in the Meteoritical Bulletin Database, last updated in October 2025, which tracks over 3,600 new approvals from 2024 alone, standardizing nomenclature and data accessibility. Despite these progresses, challenges persist in achondrite , particularly from terrestrial exposure in observed falls, which can alter volatile contents and isotopic ratios, necessitating careful protocols. ambiguities also complicate inventories, as fragmented from shared events often exhibit subtle geochemical variations, requiring multi-isotope approaches to resolve without overcounting distinct meteorites.

Scientific Significance

Insights into Solar System Evolution

Achondrites offer critical evidence for the rapid differentiation of early solar system planetesimals, demonstrating widespread silicate melting and core-mantle separation within the first 10 million years after calcium-aluminum-rich inclusions (CAIs) formed. Chronological studies of basaltic achondrites reveal that crustal formation through occurred as early as approximately 4 million years after CAIs, driven primarily by the decay of short-lived radionuclides like aluminum-26. This timeline underscores a dynamic early solar system where heat from accretional impacts and triggered global melting on numerous protoplanetary bodies, leading to layered interiors akin to those of larger planets. The asteroid exemplifies a preserved , with howardite-eucrite-diogenite (HED) achondrites providing direct samples of its differentiated structure, including a metallic core, ultramafic mantle, and basaltic crust formed shortly after solar system inception. NASA's Dawn mission confirmed Vesta's link to HED meteorites through spectral and compositional matches, highlighting its survival amid the chaotic early dynamics. In the Grand Tack hypothesis, Jupiter's inward-then-outward migration scattered primitive material from the inner disk, implanting water-rich bodies outward while enabling dry, differentiated survivors like Vesta to accrete and evolve, as inferred from HED isotopic and signatures. Lunar and Martian achondrites further bridge gaps in in-situ sampling, with lunar meteorites revealing highland diversity beyond Apollo sites and shergottite-nakhlite-chassignite (SNC) meteorites exposing Martian mantle and crustal processes inaccessible to Viking landers. Comparative analyses of achondrites illuminate diversity, contrasting the volatile-depleted, anhydrous angrites—formed via rapid, dry melting on their parent body—with hydrated shergottites that bear evidence of in parental magmas, reflecting variable aqueous environments during differentiation. Nakhlites, in particular, preserve hydrous alteration products like clays and salts, indicating episodic water-rock interactions on early Mars that contributed to its geological evolution. These insights, supported by compositional data linking achondrite to planetary basalts, refine models of accretion and volatile delivery across the inner solar system. Sample returns from missions like (Ryugu) and (Bennu), though from primitive carbonaceous asteroids, contextualize achondrite parent body survival by elucidating the asteroid belt's heterogeneous origins and dynamical sculpting.

Notable Examples and Collections

One of the most iconic achondrites is Nakhla, a nakhlite that fell on June 28, 1911, near Nakhla, , making it the first meteorite recognized as Martian due to its chemical and isotopic similarities to Mars' atmosphere. This observed fall produced about 40 stones totaling around 10 kg, and its fresh fusion crust and augite-rich composition have made it a key specimen for studying Martian . Allan Hills 84001 (ALH 84001), discovered in on December 27, 1984, is another landmark Martian achondrite, an orthopyroxenite that sparked intense debate in 1996 over possible biogenic features like magnetite chains interpreted as fossilized , though later studies attributed them to inorganic processes. Weighing 1.93 kg, it provided early evidence of liquid water on Mars through its carbonate globules and has been extensively analyzed for its ~4.1 billion-year-old crystallization age. Sahara 99555, a pristine angrite found in May 1999 in the Desert, , represents one of the rarest achondrite types, with its 2.71 kg mass showcasing minimal and a quenched basaltic texture indicative of rapid cooling. Classified as an angrite, it offers insights into early differentiated planetesimals due to its high calcium and content, and its unbrecciated nature preserves volatile elements lost in other samples. Among asteroidal achondrites, Stannern stands out as an early eucrite fall on May 22, 1808, in Stonařov, Czech Republic, recovering 66 stones totaling 52 kg; its basaltic composition links it to asteroid 4 Vesta, and specimens are prominently displayed in museums for educational purposes. For lunar achondrites, Dhofar 081, found in January 2000 in Oman, is a feldspathic breccia weighing 174 g that samples the Moon's highlands, aiding studies of anorthositic crust formation through its plagioclase-rich clasts. Major institutional collections house significant achondrite holdings for research and preservation. The Smithsonian Institution's National Museum of Natural History in Washington, D.C., maintains over 55,000 meteorite specimens from more than 20,000 distinct falls and finds, including thousands of achondrites—particularly from Antarctic expeditions, which contribute over 23,000 samples and enable comparative studies of differentiation processes. The Natural History Museum in curates approximately 5,000 pieces from 2,000 meteorites, featuring historic achondrites like Stannern and supporting isotopic analyses of planetary origins. In , the Vernadsky Institute of Geochemistry and Analytical Chemistry holds a 250-year-old collection of over 1,000 meteorites, including rare achondrites like the Martian Zagami, vital for geochemical research on Soviet-era falls. The Northwest Africa (NWA) series, comprising hundreds of achondrites recovered from Moroccan and Algerian deserts since the 1990s, circulates extensively through private markets, with specimens like NWA 011 (an ungrouped basaltic achondrite) highlighting the role of commercial trade in distributing rare materials for global study. Recent additions include the 2023 approval of several achondrites in the Meteoritical Bulletin, expanding knowledge of diverse parent bodies.

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