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 which suggested to them that a genetic relationship exists (i.e., same parent body) 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 (C3.9), 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.
Furthermore, Metzler et al. (2021) have 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)
Olivine is nearly equilibrated, with mean fayalite (Fa) values between 12.5 mol% (Loongana 001) and 14.7 mol% (NWA 13400) as a metamorphic effect
ε54Cr values similar to CV, CK, and CO chondrites; ε50Ti values similar to CR chondrites
Petrographic studies of Coolidge, Loongana 001, and HaH 073 (W3, S1) have defined a petrologic type range of 3.84, implying that these meteorites are not simply metamorphosed samples of CV3 material. Studies of HaH 073 show an O-isotopic plot on the CCAM line close to Coolidge, and it has many of the same features as Coolidge and Loongana 001. However, in comparison to Coolidge and Loongana 001, HaH 073 has some significant differences; 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 as it was known at the time. However, in-depth studies of HaH 073, Coolidge, Loongana 001, and Sah 00182 by Choe et al. (2010) led them to conclude that, given heterogeneous volatile element abundances, these meteorites are likely related. Two other meteorites, NWA 779 and Sah 00177 (photo courtesy of L. Labenne), exhibit low-fayalite olivine compositions and refractory element abundances characteristic of the CL group, and thus one or both could conceivably be recognized in the future as additional members.
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). With respect to Δ17O vs. ε50Ti and ε54Cr, they 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'
Currently all known CL group meteorites are petrologic type 3.9 to 4, but lower subtypes may 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.