A very weathered mass of 4.5 kg was found by Harvey Nininger in Hamilton County, Kansas. Coolidge was originally classified as a metamorphosed member of the reduced subtype of the CV3 group due to certain similarities: 1) an abundance of FeNi-metal and troilite; 2) chondrules and refractory objects that are depleted in Na and K; and 3) a matrix composed of olivine (Noguchi, 1994). Nevertheless, significant differences exist between these chondrites; in contrast to the reduced CV3 chondrites, Coolidge matrix is depleted in Na, chondrule compositions are different, and thermal metamorphism was greater (temperatures to ~900°C for a short duration).
Thermoluminescence (TL) sensitivity analysis was applied to Coolidge, and it was resolved as a petrologic type 4. Kallemeyn and Rubin (1995) found features in the C4-ungrouped Loongana 001 indicative of a genetic relationship with Coolidge. For example, both meteorites have comparable low matrix abundances, comparable petrographic types, similar chondrule sizes, similar olivine Fa abundances, high refractory lithophile and refractory siderophile abundances, low abundances of all volatile element groups, and similarities in their opaque assemblages. After further in-depth analyses of Coolidge and Loongana 001 as well as Los Vientos 051 (C3.9), NWA 033 (C4), and NWA 13400 (C3.9), Metzler et al. (2021) assigned these five meteorites to a new CL group which derives its name from Loongana 001. Metzler et al. (2021) distinguished the CL group by certain petrologic, bulk composition, geochemical, and isotopic characteristics including the following:
FeNi-metal abundances are similar to CR chondrites
Chondrule size and abundance are similar to CV chondrites
CAI abundance ave. of ~1.4 vol% is between that of CV and CR chondrites
Higher volatile element depletions compared to other CC groups
Matrix abundance of 1721 vol% is lowest among CC groups
Matrix has lower Al2O3 and higher MgO and Cr2O3 compared to that in CV, CK, and CR chondrites
Metamorphic equilibration of olivine with ave. Fa12.514.7 mol% (Loongana and NWA 13400, respectively)
ε54Cr values similar to CV, CK, and CO chondrites; ε50Ti values similar to CR chondrites
Previous petrographic studies of Coolidge, Loongana 001, and HaH 073 (C4-ung) defined a petrologic type range of 3.84, implying that these meteorites are not simply metamorphosed samples of CV3 material. The percentage mean deviation (PMD) of olivine fayalite values was used by Metzler et al. (2021) as an indicator of the degree of equilibration of the five CL meteorites. Coolidge, Loongana, and NWA 033 have PMD values below 5% and are designated petrologic type 4, while those for LoV 051 and NWA 13400 are between 5% and 10% and are designated petrologic subtype 3.9. In addition, they determined that all five CL meteorites have features consistent with shock stage S2, and all have experienced moderate (W1/2 for LoV 051) to heavy (W3/4 for NWA 033) terrestrial weathering.
An O-isotopic plot of HaH 073 lies on the CCAM line close to Coolidge, and it plots close to the CL chondrite NWA 033 in the diagram below (note misspelling of HaH 073 as HaH 173). HaH 073 has many of the same features as Coolidge and Loongana 001, but it has some significant differences as well; e.g., more ferroan olivine, slightly higher matrix abundance, slightly smaller chondrule size, not all volatiles have low abundances, and slightly different O-isotopic values. Based on these differences and other abundance ratios, Huber et al. (2006) were unable to establish that HaH 073 was part of the CoolidgeLoongana grouplet. In-depth studies of HaH 073, Coolidge, Loongana 001, and Sah 00182 (C3-ung) conducted by Choe et al. (2010) led them to conclude that, given heterogeneous volatile element abundances, these meteorites are likely related. In addition, NWA 779 (C3-ung) was considered by Weckwerth et al. (2001) to be a potential member of the CoolidgeLoongana grouplet. All of these meteorites were subsequently investigated by Metzler et al. (2021) and extensively compared to the newly established parameters which define the CL group. They concluded that differences are apparent in these characteristics for each of the meteorites, some of which are very significant differences, and they can all be excluded as members of the CL group; however, HaH 073 could be distantly related in a way that is yet to be determined.
Diagram credit: Greenwood et al., GCA, vol. 277, p. 381 (2020, open access link)
'Linking asteroids and meteorites to the primordial planetesimal population'
Only the karoondaites (CK) have petrologic type 4 members in common with some of the CL meteorites, but current oxidation state and bulk composition studies solidly place these latter meteorites in their own distinct carbonaceous chondrite group. For example, the matrix of CK4 chondrites consists of both olivine and sodic plagioclase, while no plagioclase is present in the Coolidge matrix. CK chondrites are not depleted in Na or K. In addition, Coolidge has a finer-grained matrix and exhibits compositional differences in spinel minerals. In contrast to Coolidge, CK chondrites are more highly oxidized as attested by the presence of magnetite with high-Ni sulfides instead of FeNi-metal and troilite. The CL group is also separated by its highly fractionated moderately volatile elements, higher than all other meteorite groups. The finding of Al-rich inclusions unique from other carbonaceous chondrites also supports this classification.
Torrano et al. (2021) presented coupled diagrams to demonstrate that Coolidge represents a distinct meteorite among the known carbonaceous chondrite groups. On a coupled δ18O vs. δ17O diagram, they showed that Coolidge overlaps the CO, CV, and CK fields (see top diagram below), while on a coupled ε54Cr vs. ε50Ti diagram, Coolidge plots close to the limits of the CO and CK chondrite fields (see middle diagram below). The coupled ε54Cr vs. ε50Ti diagram is important in its ability to resolve the CL group from the other CC groups as also demonstrated well by this diagram from Metzler et al. (2021). With respect to Δ17O vs. ε50Ti and ε54Cr, Torrano et al. (2021) determined that Coolidge plots close to the CO, CV, and CK chondrite fields (see bottom diagram below).
δ18O vs. δ17O Isotope Plot for Coolidge
ε54Cr vs. ε50Ti Isotope Plot for Coolidge
Δ17O vs. ε50Ti and ε54Cr Isotope Plot for Coolidge
Diagrams credit: Torrano et al., GCA, vol. 301 (2021, open accesslink)
'The relationship between CM and CO chondrites: Insights from combined analyses of titanium, chromium, and oxygen isotopes in CM, CO, and ungrouped chondrites'
Comparative analyses of the known asteroid types and a suite of ungrouped and rare meteorites in multiple forms (bulk, powder, polished section), including the CL chondrites Coolidge and LoV 051, were conducted by Krämer Ruggiu et al. (2021) utilizing petrographic, spectroscopic, and albedo data. They concluded that the best matches to these two meteorites are the S and Sr-type asteroids (see diagram below).
Comparison of Meteorite and Asteroid Spectra
Diagram credit: Krämer Ruggiu et al.Icarus, vol. 362, art. 114393 (2021, open accesslink)
'Visible-infrared spectroscopy of ungrouped and rare meteorites brings further constraints on meteorite-asteroid connections'
Currently all known CL group meteorites are petrologic type 3.9 to 4, but lower subtypes could be identified in the future. Coolidge has a CRE age of 37.7 m.y. The specimen of Coolidge shown above is a 1.1 g cut fragment, while the image below is an excellent petrographic thin section micrograph of Coolidge, shown courtesy of Peter Marmet.