Ordinary chondrite
Ordinary chondrites are the most common type of stony meteorites, comprising over 80% of all chondrite falls and representing primitive, undifferentiated remnants of the early solar system's building blocks. These meteorites are characterized by the presence of chondrules—millimeter-sized spherules of once-molten silicates—embedded in a fine-grained matrix, along with variable amounts of nickel-iron metal grains and troilite sulfide.[1][2] They originate primarily from S-type asteroids in the inner main asteroid belt and have undergone varying degrees of thermal metamorphism but little aqueous alteration.[3] Ordinary chondrites are classified into three chemical groups based on total iron content and oxidation state: H (high total iron, 25–31 wt%), L (low total iron, 20–25 wt%), and LL (low total iron and low metal, 19–22 wt%).[3] Each group is further subdivided by petrologic type (3 to 7), reflecting the extent of thermal metamorphism, with type 3 being the least altered (unequilibrated) and higher types showing increasing equilibration of minerals like olivine and pyroxene.[1] Mineralogically, they consist mainly of olivine ((Mg,Fe)2SiO4), low-calcium pyroxene (such as bronzite or hypersthene), plagioclase feldspar, and accessory metals and sulfides, with compositions closely approximating solar abundances for non-volatiles.[3] The H group is the most reduced, with abundant free metal, while the LL group formed under more oxidizing conditions, resulting in less metal and more oxidized silicates.[2] These meteorites provide critical insights into solar system formation, as their chondrules likely formed rapidly through heating events in the solar nebula around 4.56 billion years ago, and their unequilibrated subtypes preserve presolar grains and early isotopic signatures.[4] Although they contain trace organics and water (less than 0.1 wt%), ordinary chondrites are largely anhydrous compared to carbonaceous chondrites, highlighting diverse nebular conditions.[3] Notable examples include the Peekskill meteorite (H6), which fell in 1992 and was famously captured on video, underscoring the frequency of ordinary chondrite falls, and their study has linked specific groups to parent bodies such as asteroid 6 Hebe for H chondrites.[1]Definition and Classification
Overview and Characteristics
Ordinary chondrites represent the most abundant class of stony meteorites, accounting for approximately 87% of all chondrite finds documented in the Meteoritical Bulletin.[5] These primitive meteorites are aggregates of materials that accreted in the early solar system without significant melting or differentiation on their parent bodies, preserving a record of solar nebula conditions.[6] A defining feature of ordinary chondrites is the presence of chondrules, millimeter-sized spherical inclusions composed primarily of silicate minerals that formed as molten droplets and solidified through rapid cooling in the gas-rich environment of the protoplanetary disk.[7] These chondrules, often embedded in a finer-grained matrix, indicate high-temperature events followed by quick quenching, typically on timescales of hours to days, which halted further crystallization and preserved igneous textures.[8] Ordinary chondrites are divided into unequilibrated and equilibrated varieties based on their thermal history. Unequilibrated ordinary chondrites (petrologic types 3.0–3.9) retain heterogeneous mineral compositions reflective of minimal post-accretionary heating, directly sampling unaltered solar nebula material, whereas equilibrated ones (types 4–6) exhibit homogenized mineral chemistry due to metamorphic temperatures of 500–1000°C on their parent asteroids.[1] Their bulk elemental abundances closely approximate solar values for non-volatile elements, underscoring their role as benchmarks for solar system composition.[9] The recognition of ordinary chondrites as a distinct class emerged in the 19th century through analyses of observed falls, such as the LL5 chondrite from Krähenberg, Germany, in 1869, which helped establish their chemical and textural characteristics.[10] They are further subdivided into H (high iron), L (low iron), and LL (low iron, low metal) groups to reflect variations in total iron and metal content.[5]Chemical and Petrologic Groups
Ordinary chondrites are taxonomically subdivided into three chemical groups—H, L, and LL—primarily distinguished by their bulk iron contents and oxidation states, which reflect variations in the proportions of metallic iron-nickel and iron in silicates. The H group, representing approximately 39% of all ordinary chondrites as of 2025, is characterized by high total iron (typically 25–31 wt%) and high metallic iron content (15–19 wt%), with much of the iron present as kamacite and taenite.[11][12] In contrast, the L group (approximately 35%) features low total iron (20–25 wt%) and moderate metallic iron (4–10 wt%), while the LL group (approximately 11%) has the lowest total iron (19–22 wt%) and metallic iron (1–3 wt%), with a greater fraction of iron incorporated into oxidized forms within olivine and pyroxene.[11][12] These differences in iron partitioning are thought to arise from varying redox conditions during accretion onto distinct parent bodies. In addition to chemical grouping, ordinary chondrites are classified by petrologic type on a scale from 3 to 6, which indicates the degree of thermal metamorphism and equilibration experienced by the meteorite.[13] Type 3 chondrites are unequilibrated, preserving distinct chondrules and heterogeneous mineral compositions that reflect their primitive, minimally altered state.[13] As thermal processing increases, types 4 through 6 become progressively more equilibrated: type 4 shows initial recrystallization and homogenization of matrix minerals; type 5 exhibits well-defined chondrules with equilibrated olivines and pyroxenes; and type 6 represents full equilibration, with a recrystallized, granular texture and no discernible chondrules.[13] A rare type 7 designation applies to samples that have undergone extreme metamorphism beyond type 6, featuring partial melting and further grain growth, as documented in a few H and L chondrites.[14] The presence of chondrules in lower petrologic types serves as a key indicator of the material's primitive solar nebula heritage.[13] Further refinement of ordinary chondrite classification includes shock stages (S1–S6) and weathering grades (W0–W5), standardized by the Meteoritical Society to account for post-accretionary impacts and terrestrial alteration, respectively.[15] Shock stages range from S1 (unshocked, with sharp optical extinction in olivine) to S6 (extreme shock, involving melting and formation of high-pressure minerals like ringwoodite), based on petrographic observations of deformation features in silicates and plagioclase.[16] Weathering grades assess oxidation and hydration effects: W0 indicates no terrestrial alteration (fresh falls), while W5 denotes severe weathering with complete replacement of metal and troilite by iron oxides and hydroxides. For example, the Allegan meteorite, an H5 ordinary chondrite, exemplifies intermediate metamorphism with equilibrated textures and minimal weathering (W0) due to its observed fall in 1899.[17]Physical Properties
Texture and Microstructure
Ordinary chondrites display a distinctive brecciated texture at the macroscopic scale, featuring discrete chondrules set within a fine-grained matrix, with occasional irregular inclusions and metal-sulfide grains. This microstructure reflects accretion from diverse nebular components, where chondrules formed as molten droplets that cooled rapidly, preserving igneous textures indicative of high-temperature events in the early solar nebula. The overall fabric provides evidence for the dynamic conditions during parent body assembly, including mechanical mixing and minimal post-accretionary alteration in low petrologic types.[3] Chondrules dominate the texture, typically comprising 60-80 vol.% of the meteorite, and serve as primary indicators of nebular processing. The most prevalent type is porphyritic, accounting for 70-80% of chondrules, with phenocrysts of silicates embedded in a fine-grained or glassy groundmass that suggests incomplete melting followed by relatively slow cooling over hours to days. Less common varieties include barred olivine chondrules, marked by parallel olivine laths implying faster crystallization; radiating pyroxene types, exhibiting acicular pyroxene radiating from a central nucleus; and glassy chondrules, which retain largely uncrystallized melt compositions. These chondrules generally range in size from 0.1 to 2 mm in diameter, with variations reflecting local differences in heating intensity and duration during formation.[18][19][3] The matrix, constituting 20-40 vol.% of ordinary chondrites, fills spaces between chondrules and consists predominantly of fine-grained silicates with embedded presolar grains—nanometer- to micrometer-sized relics of stardust that survived nebular heating. This component's sub-micrometer grain size and heterogeneous distribution highlight its role as a low-temperature condensate or aggregate of subchondrule debris, capturing the pristine interstellar heritage while binding the coarser elements into a cohesive rock. In unequilibrated types, the matrix preserves nanoscale amorphous silicates and organics, underscoring its origin from cooler, outer disk regions.[1][3][20] Calcium-aluminum-rich inclusions (CAIs) occur rarely in ordinary chondrites relative to carbonaceous varieties, comprising less than 1 vol.% overall, but they are identifiable in Type 3 unequilibrated samples as small (typically <1 mm), irregularly shaped aggregates of refractory phases. These inclusions, often fragmented or rimmed by secondary minerals, indicate sporadic transport from hotter inner solar nebula zones where high-temperature condensation first occurred, providing chronological anchors for early solar system events. Their limited abundance in ordinary chondrites suggests selective sampling from a narrow nebular reservoir.[21][22] Textural evolution with petrologic type reveals the extent of parent body thermal metamorphism, transitioning from pristine to equilibrated states. In Type 3 ordinary chondrites, chondrules retain sharp boundaries and internal zoning, with the matrix appearing dark and fine-grained due to lack of significant heating. As petrologic type increases to 4-6, recrystallization blurs chondrule margins through grain growth and diffusion, homogenizing the microstructure and reducing textural contrast, which records progressive heating to 500-1000°C over extended timescales on the parent asteroid. This metamorphic overprint, without full melting, preserves the original nebular architecture while altering its details.[23][24][3]Density and Strength
Ordinary chondrites exhibit bulk densities ranging from 3.0 to 3.7 g/cm³, with variations primarily driven by differences in metal content across chemical groups. The H-group chondrites, which have the highest iron content, display the highest average bulk density of approximately 3.44 g/cm³, while the LL-group chondrites, with lower metal abundances, have the lowest average of about 3.29 g/cm³; L-group values fall in between at around 3.40 g/cm³.[25][26] Porosity significantly influences these density measurements, as void spaces reduce the overall bulk density. In freshly fallen ordinary chondrites, porosity typically ranges from 0 to 30%, with an average of about 7%, though unequilibrated ordinary chondrites (petrologic types 3.0–3.9) often exhibit higher values around 10% due to less compaction during parent body processes. Terrestrial weathering in found meteorites tends to decrease porosity, averaging less than 3%, as oxidation products fill pore spaces, whereas Antarctic finds may show elevated porosity from freeze-thaw cycles.[27][28] The mechanical strength of ordinary chondrites is characterized by compressive strengths of 105–203 MPa and tensile strengths of 18–31 MPa, reflecting their heterogeneous microstructure of silicates, metal, and sulfides. These properties vary with shock stage, as higher shock levels (e.g., S6) introduce fractures and melting that weaken the material, reducing compressive strength by up to an order of magnitude compared to lightly shocked (S1–S2) samples.[29][30] Compared to other meteorite classes, ordinary chondrites are denser than carbonaceous chondrites (typically 2.0–3.0 g/cm³) owing to their higher metal and lower volatile content, but less dense than iron meteorites (7.0–8.0 g/cm³), which consist primarily of metallic phases.[31][32][26]Mineralogy
Silicate Minerals
The silicate minerals in ordinary chondrites are primarily anhydrous phases that preserve records of early solar nebula processes and parent body metamorphism, dominating the non-opaque components with olivine and pyroxene comprising the bulk of the silicate fraction. These minerals occur predominantly within chondrules and matrix, reflecting heterogeneous nebular condensation and accretion before thermal equilibration on the parent body. Their compositions vary systematically across the H, L, and LL chemical groups, defined by increasing iron content, and show distinct heterogeneity in unequilibrated (type 3) samples compared to more processed (types 4–6) ones.[33] Olivine, the most abundant silicate mineral, constitutes 30–50 vol% of ordinary chondrites and serves as a key indicator of petrologic type and chemical group due to its sensitivity to metamorphic equilibration. In equilibrated ordinary chondrites (types 4–6), olivine compositions are homogeneous within each group: Fa16.5–20.8 (where Fa# = 100 × Mg/(Mg + Fe)) in H chondrites, Fa23–27 in L chondrites, and Fa27–33 in LL chondrites. These ranges reflect progressive iron enrichment from H to LL groups and are used for classification, with gaps separating the groups despite minor overlaps. In unequilibrated type 3 ordinary chondrites, olivine grains exhibit chemical zoning, with Mg-rich cores (Fa0–10) and Fe-enriched rims (up to Fa30), preserving nebular cooling histories and indicating incomplete homogenization during low-temperature metamorphism.[33][34][35] Low-Ca pyroxene, primarily orthoenstatite or clinoenstatite, is the second most abundant silicate, making up approximately 20–25 vol% and often intergrown with olivine in chondrules, where it contributes to the primitive, unequilibrated textures. Equilibrated samples show uniform compositions across groups: Fs14.5–19 (Fs# = 100 × Fe/(Mg + Fe)) and Wo0–5 in H chondrites, Fs19–23 and Wo0–5 in L chondrites, and Fs23–27 and Wo0–5 in LL chondrites, mirroring the iron trends seen in olivine. Type 4 chondrites display slightly lower Fs values due to partial equilibration, while type 3 samples reveal heterogeneous, zoned pyroxenes with variable Fe/Mg ratios that highlight nebular volatility gradients. These variations underscore the role of low-Ca pyroxene in tracing the degree of thermal processing on the parent body.[33][35] High-Ca pyroxene (diopside) and plagioclase are minor silicates, each comprising about 5–10 vol%, typically forming in chondrule mesostases or as interstitial phases during late-stage crystallization. High-Ca pyroxene has diopsidic compositions with Wo~35–45 and Fs5–15, showing little variation across groups but increasing abundance with petrologic type due to metamorphic recrystallization. Plagioclase, mostly as sodic feldspar (albite), ranges from An0–10 (An# = 100 × Ca/(Ca + Na + K)) in equilibrated chondrites, with grain sizes enlarging from <5 μm in type 3 to >50 μm in type 6, reflecting progressive annealing. In unequilibrated chondrites, plagioclase is finer-grained and more variable, often associated with glassy mesostases that link to chondrule formation processes.[35]Metallic and Sulfide Phases
Ordinary chondrites contain significant metallic phases primarily composed of iron-nickel alloys, which constitute a key opaque component and serve as indicators of the meteorites' thermal and redox history.[36] The dominant alloys are kamacite, a body-centered cubic phase with 5-10 wt% nickel, and taenite, a face-centered cubic phase with higher nickel content (typically 20-50 wt%).[36] These metals often form zoned structures, with kamacite rims surrounding taenite cores, reflecting cooling rates on the parent body.[37] The abundance of these metallic phases varies markedly among the chemical groups, highlighting differences in oxidation state. In H chondrites, metals comprise 15-20 vol%, dominated by kamacite with ~6.5 wt% Ni, indicating a relatively reduced environment.[36] L chondrites have intermediate abundances of ~8-10 vol%, while LL chondrites are more metal-poor at <5 vol%, with higher overall Ni contents (~30 wt% average) due to greater preservation of taenite.[36] This progressive decrease in metal content from H to LL reflects increasing oxidation during parent body metamorphism, where iron was incorporated into silicates and oxides.[38] Sulfide phases, chiefly troilite (FeS), are ubiquitous and typically make up 5-10 vol% of ordinary chondrites, often occurring as discrete grains or in polymineralic nodules intergrown with metals.[35] These nodules integrate into the matrix and chondrule textures, contributing substantially to the bulk iron budget alongside metallic phases.[39] Troilite grains in more oxidized LL chondrites frequently exhibit magnetite rims, a feature less common in reduced H chondrites, underscoring group-specific redox variations.[40] Trace opaque phases include phosphides and carbides such as schreibersite ((Fe,Ni)₃P) and cohenite (Fe₃C), present in minor amounts (<1 vol%) within metal grains or as isolated inclusions, formed under localized reducing conditions.[41] [42] Oxide minerals like chromite (FeCr₂O₄) and ilmenite (FeTiO₃) occur as small euhedral grains, with ilmenite showing increasing iron content from H to LL groups, further evidencing oxidation trends.[43] These phases collectively aid in classifying ordinary chondrite groups by their modal abundances and compositions, which correlate with parent body oxidation states.[38]Chemical Composition
Bulk Elemental Abundances
Ordinary chondrites exhibit bulk elemental abundances that reflect a relatively primitive composition, closely matching solar proportions for refractory lithophile elements while displaying systematic variations among the H, L, and LL groups due to differences in oxidation state and metal content. Major element oxides dominate the composition, with SiO₂ comprising approximately 40 wt%, MgO around 25 wt%, and total iron (as FeO and metallic Fe) between 19 and 31 wt%, varying by group: H (25–31 wt%), L (20–25 wt%), and LL (19–22 wt%), with the highest values in H-group chondrites owing to their greater proportion of metallic phases. These abundances are derived from analyses of multiple falls, ensuring representation of unweathered material. The H, L, and LL groups are primarily distinguished by total iron content and the partitioning between oxidized (FeO) and reduced (metallic Fe) forms, which decreases from H to LL. Refractory elements such as Al and Ca maintain near-solar levels across all groups, with Al₂O₃ at about 2.0-2.2 wt% and CaO at 1.8-2.0 wt%, indicating minimal fractionation during solar nebula condensation. In contrast, moderately volatile elements like Na and K are slightly depleted relative to solar values, typically by 10-20%, a feature common to non-CI chondrites. Siderophile trace elements show group-specific variations tied to metal abundance: Ni and Co are highest in H chondrites (Ni ≈ 1.6 wt%, Co ≈ 0.06 wt%), decreasing through L (Ni ≈ 1.4 wt%, Co ≈ 0.05 wt%) to LL (Ni ≈ 1.0 wt%, Co ≈ 0.04 wt%), while highly siderophile elements like Ir follow similar trends with enrichments in H by factors of up to 1.5 relative to LL. These patterns arise from nebular processes that segregated metal from silicates differently across groups. CI-chondrite normalized abundance plots for ordinary chondrites display flat patterns for refractory and moderately siderophile elements, underscoring their chondritic nature, but exhibit negative deviations for volatiles such as Na (≈0.85×CI) and K (≈0.80×CI), consistent with incomplete condensation in the inner solar nebula. The mineral contributions, particularly iron from metallic phases, influence these bulk patterns without altering the overall refractory-lithophile ratios. Average major element compositions (wt%) for the H, L, and LL groups, based on analyses of falls reported in the Meteoritical Bulletin Database, are summarized below:| Oxide | H-group | L-group | LL-group |
|---|---|---|---|
| SiO₂ | 40.0 | 40.5 | 41.0 |
| MgO | 25.0 | 24.5 | 24.0 |
| Al₂O₃ | 2.1 | 2.2 | 2.3 |
| CaO | 1.9 | 1.9 | 2.0 |
| FeO | 12.0 | 16.5 | 20.0 |
| Total Fe | 25.0 | 22.0 | 19.5 |