Ordinary Chondrite

Work in Progress

Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% of all falls (Meteoritical Society database 2018)1.

Meteorites & the Early Solar System: page 581 section 6.1
OC of type 5 or 6 with an apparent shock stage of S1, reveal evidence of annealing due to a previous intense shock. This observation demonstrates that ordinary chondrites experienced a more complex metamorphic history where shock heating also played a significant role.

Per the author’s analysis conducted on Dec 21, 2019 using the Meteoritical Society database, of 1193 total documented falls in the database, 374 were H chondrites, 95 were LL chondrites, and were 425 L chondrites. In total 74.9% of all falls are OC falls. If the transitional classes such as H/L (0.34%) and L/LL (0.92%) are counted, then the total percentage increases to 76.2%.

OC chondrite chondrules have densities between 3.15 and 3.26 g/cm3, as may be expected from their mineral and glass compositions2.

The properties of ordinary chondrites (OC) reflect both nebular and asteroidal processes.

Ordinary Chondrites consist primarily of olivine (40% to 50%), pyroxene (15% to 25%), Fe/Ni alloys of kamacite and taenite (2% to 25%), and other accessory minerals like feldspars, troilite, chromite, . Like all other chondritic meteorites (except CI chondrites), OCs contain chondrules that are distinct and sharply defined within the matrix for low petrologic types (type 3) or poorly defined for high petrologic types (type 6) that experienced high thermal metamorphism. Similarly, chondrules are progressively abraded and finally destroyed with increasing degrees of impact shock.

Reduced and oxidized iron is used diagnostically to determine the type of ordinary chondrite.

https://arxiv.org/ftp/arxiv/papers/1611/1611.08734.pdf

It is now understood that space weathering processes are responsible for the spectral mismatch between S-type bodies and OCs. Furthermore, both telescopic observations and the first asteroid sample return mission (Hayabusa) indicate that most S-type bodies have mineralogies similar to those of OCs. Importantly, the S-type/OC link has been delivering fundamental constraints on the formation and evolution of planetesimals over the recent years.

Ordinary chondrite meteorites (OCs) are silicate-rich meteorites composed mostly of olivine and low calcium pyroxene that are by far the most abundant meteorites (80% of all falls; Hutchison 2004). They have been subdivided into three groups (H, L and LL) based on variations in bulk composition, such as molecular ratios [FeO ⁄ (FeO+MgO)] in olivine and pyroxene (Mason 1963, Keil & Fredriksson 1964) and the ratio of metallic Fe to total Fe (Dodd et al. 1967) (Table 1).

H chondrites have the highest total abundance of Fe (and the highest Fe/Si atomic ratio) among OCs (H stands for « High » Fe). They are also the most reduced OCs, their iron being mostly in metallic form. When equilibrated, they exhibit the lowest FeO contents in their silicates.

L chondrites (Low in total iron) are intermediate between H and LLs in all these properties – somewhat closer to LLs in terms of total Fe.

LL chondrites contain the least total Fe, corresponding to the lowest Fe/Si atomic ratio, and they are the most oxidized OCs, with little metal and with the silicates that have the highest FeO contents when equilibrated. They are « Low » in total iron, « Low » in metal, hence the LL denomination.

H and LL chondrites also differ in terms of oxygen isotopes, with LL chondrites containing a very small but measurable depletion of 16O with respect to the other two isotopes compared to H chondrites.


From MetBull:

ordinary chondrite: A major class of chondrites, distinguished by sub-solar Mg/Si and refractory/Si ratios, oxygen isotope compositions that plot above the terrestrial fractionation line, and a large volume percentage of chondrules, with only 10-15 vol% fine-grained matrix.

H group: The high-iron (H) chemical group of ordinary chondrites, distinguished by their high siderophile element content, relatively small chondrules (~0.3 mm), and oxygen isotope compositions that are closer to the terrestrial fractionation line than those of other ordinary chondrites.

L group: The low-iron (L) chemical group of ordinary chondrites, distinguished by their relatively low siderophile element content, moderate sized chondrules (~0.7 mm), and oxygen isotope compositions that intermediate between H and LL group ordinary chondrites.

LL group: The low-iron, low metal (LL) chemical group of ordinary chondrites, distinguished by their low siderophile element content, fairly large chondrules (~0.9 mm), and oxygen isotope compositions that are further above the terrestrial fractionation line than those of other ordinary chondrites.

Subtypes of ordinary chondrites are determined using the petrographic criteria given in the table below.

CriterionPetrologic Type
3456
Olivine & Pyroxene Homogeneity> 5 % mean deviationhomogeneous
Feldspar
minor primary in 3; increase from <5 mm in 3 to >50 mm in 6

Chondrule & Matrix Glasspresent in decreasing abundance in 3; devitrified to absent in
4 to 6
Matrixfine grained, clastic, minor opaque in 3; recrystallized, coarsening in 4 to 6
Chondrule Definition & Abundancesharply defined in 3; become more diffuse and less abundant in 4 to 6

The division for subtypes 3.2 and below are made using the average concentration and standard deviation of Cr2O3 contents in olivine (Grossman & Brearley, 2005).