K-chondrite grouplet, Type 3.6
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Fell June 4, 1890
12° 23' N., 78° 31' E.

At 8:00 A.M., sonic booms were heard and two stones were recovered in Tamil Nadu, Salem district, at Kangankarai (Madras Railway), India (C. A. Silberrad, Indian Civil Service, 1932). One stone was broken into pieces while the other, weighing 347 g, was preserved. Most of this meteorite is accounted for in only four institutions, with the above specimen being obtained through the Natural History Museum in London by an American meteorite dealer.

Kakangari (British Museum spelling) was initially termed an anomalous chondrite. It is a petrologic type 3.6 (3.5–3.8 in Berlin, 2009) chondrite that belongs to none of the known chondrite groups, but has some characteristics in common with both ordinary and carbonaceous chondrite groups. Along with the similar petrologic type 3 LEW 87232 (23 g) and the most recently classified type 4 NWA 10085 (52.2 g; Utas et al., 2017, #2906), these three meteorites form a unique grouplet. A fourth meteorite, Lea County 002, was previously considered to share a genetic relationship with the other three K chondrites. However, this has been called into question by some investigators in light of several significant differences between Lea County 002 and the others: (1) matrix abundance of 33 vol% vs. 60–77 vol%, respectively; (2) chondrule size of 1.0–1.25 mm (not including one unusually large 5.3-mm-diameter chondrule) vs. 0.25–0.5 mm, respectively; and (3) CAIs in Lea County 002 differ in mineralogy from those in Kakangari and LEW 87232 (Weisberg et al., 1996; Prinz et al., 1991); CAI data are not yet reported for NWA 10085. Although Lea County 002 is a very small meteorite that may not be representative of its parent lithology, and while it is extensively weathered making characterization difficult, it was considered to be mineralogically and isotopically more similar to the CR chondrite group (Krot et al., 2005 and references therein). It should be noted that NWA 10085 most closely resembles Lea County 002, and future recoveries of new K chondrites should help resolve the full range of data that defines this grouplet.

The Kakangari grouplet has unique petrologic and O-isotopic characteristics that distinguish it from other chondrite groups, including the following:

Chondrules in Kakangari constitute ~25 vol% and consist mostly of chondrule fragments, with only ~56% of the chondrules having remained intact. In a study employing four Kakangari thin sections, Barosch et al. (2019, 2020) measured only the complete chondrules and calculated an average diameter of ~0.7 mm. The chondrules and chondrule fragments are mostly type-I FeO-poor chondrules similar to those in EH3 chondrites, and they are also FeNi–FeS-rich. In addition, AOAs and refractory material in the form of irregularly shaped CAIs are present. The relative proportion of chondrule textural-types, predominantly porphyritic (86%) with minor abundances of both radial pyroxene (9%) and agglomeratic chondrules (5%), is unlike that in other chondrite groups; however, the abundance of POP chondrules (43%) is most similar to that in ordinary chondrites. Some porphyritic chondrules contain large metal–sulfide intergrowths having ragged outlines, while others contain small metal–sulfide beads within olivines and groundmass. Most chondrules and fragments also have metal–sulfide rims, but some have fine-grained, igneous-textured silicate rims. Barosch et al. (2019, 2020) analyzed four Kakangari sections and determined that the abundance of zoned silicate rims (~7%) is much lower than in ordinary chondrites (~40%) and carbonaceous chondrites (~80%). Layered rims are found on some chondrules, implying that they have experienced at least three melting events with two accretionary episodes. Spongy troilite usually forms the final layer on chondrules and chondrule fragments.

The matrix mineralogy preserves the thermal history of Kakangari, revealing peak metamorphic temperatures of <500°C, with an estimated cooling rate of at least 10 K/m.y. (Nagashima et al., 2015 and references therein). Kakangari matrix is composed of both relatively large clastic grains (enstatite, olivine, and clinopyroxene) and aggregates of ultrafine-grained, magnesium-rich, crystalline enstatite and olivine, along with lesser amounts of albite, anorthite, Cr-spinel, troilite, and FeNi-metal (Floss and Stadermann, 2012; Nagashima et al., 2015). The precursor material was likely a mixture of nebular and presolar amorphous or partially crystalline dust. Following thermal metamorphism, which probably caused the destruction of primary presolar grains, a period of rapid cooling ensued resulting in the production of orthoenstatite and clinoenstatite in the matrix. The presence of solar-wind-implanted He and Ne suggest a residence within a regolith setting, increasing the probability that the meteorite is a breccia (Srinivasan and Anders, 1977).

Chondrules and matrix have similar compositions and both experienced reduction processes, primarily in the nebula, with the chondrules being less reduced than matrix silicates (Berlin, 2009). Following thermal metamorphism on the parent body, a sulfidization event occurred affecting mostly Ni-poor metal. This was succeeded by an increase in oxidizing conditions under which ferrihydrite and chlorapatite were produced as replacements for kamacite in the presence of water (see illustration in Berlin, 2009, p. 166). Barosch et al. (2019) determined that Kakangari chondrule and matrix components exhibit complementarity with respect to Al/Mg, Al/Ca, and possibly Fe/Mg ratios, but atypically this is not reflected in the Mg/Si ratios wherein both components are nearly the same (see example complementarity diagrams below). To explain the similar Mg/Si ratios, it was suggested by Barosch et al. (2020) that Kakangari chondrule and matrix components were equilibrated during parent body metamorphism. On the other hand, recent studies have addressed the chondrule–matrix complementarity issue and have concluded that advocacy for such a genetic relationship is unnecessary to explain the elemental ratios observed in these two components (e.g., Alexander, 2019; Patzer et al., 2020; Patzer et al., 2021).

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Diagram credit: Barosch et al., 82nd MetSoc, #6305 (2019)

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Diagram credit: Barosch et al., EPSL, vol. 542, Article 116286 (2020)
'Formation of chondrules and matrix in Kakangari chondrites'

Mineralogical analysis of Kakangari by Nagashima et al. (2015) revealed a majority of FeO-poor type-I chondrules, with only rare FeO-enriched chondrules (not typical type-II). Although silicates in Kakangari show evidence for reduction processes, silicates in E chondrites were reduced to a greater degree than those in Kakangari as demonstrated by the more Mg-rich chondrules and the higher content of Cr and Ti in troilite of E chondrites (Berlin et al., 2007). Weisberg et al. (1996) determined that the olivine and pyroxene in Kakangari chondrules have a compositional range of Fa0.4-8.0, Fs0.4-10.0, with an average of Fa3.9, Fs5.8. They also found that the composition of LEW 87232 chondrules is Fa1.2, Fs3.3, while data of Zolensky et al. (1989) show the composition of Lea County 002 chondrules to be Fa2.0, Fs4.0. Kakangari and LEW 87232 matrix values for olivine and pyroxene are even more magnesian than those for the chondrules. Values for the K chondrite grouplet as a whole were calculated by Weisberg et al. (1996) using the averages of Kakangari chondrules and matrix in a weighted ratio of 23:77 corresponding to that observed in Kakangari; the resulting values are Fa2.2, Fs4.4.

It was suggested that the FeO-rich silicates observed in both the E and K chondrite groups had a common precursor, despite their differences in degree of reduction. In a broader sense, the K chondrites have a combination of properties that do not fit the existing systematics of either the E, O, R, or C chondrites for which isotopic, chemical, and petrologic characteristics vary smoothly relative to their heliocentric distance of formation. For example, the chondrule/matrix ratio is negatively correlated to the oxidation state through the series E>H>L>LL>C3>CM2>CI1, but the K chondrites do not fit into this sequence. Redox analysis suggests that Kakangari contains a unique suite of chondrules (Berlin et al., 2007). It is notable that the models employed by Neumann et al. (2018) indicate a small grain size (0.2 µm) for the precursor material of the acapulcoite–lodranite (AL) parent body, consistent with a large matrix component. Considering the high matrix volume present in K chondrites, along with the mineralogical and geochemical data for this rare group, they speculated that the K chondrites might represent the thin chondritic outer layer of the AL parent body. In support of this hypothesis is a study in which Layak and Rai (2021) compared a sampling of acapulcoite–lodranite clan meteorites to samples of H, R, K, CB, and CR chondrites with respect to mineralogy, density, modeled bulk composition, trace element concentrations, modal mineral abundances, and oxygen isotopes. Only the K chondrites were found to be consistent with all of these parameters and can be considered a potential precursor material of the acapulcoite–lodranite parent body (see diagram below).

O-isotopes for ACA-LOD Clan and H and K Chondrites
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Diagram credit: Layak and Rai, 52nd LPSC, #1983 (2021)

Generally, the O-isotope composition of the bulk chondrules in Kakangari are 16O-poor and plot along the terrestrial fractionation line on an oxygen three-isotope diagram (ave. olivine: Δ17O +0.0 [±0.8] ‰; ave. low-Ca pyroxene: +0.2 [±0.9] ‰), while that of the matrix olivine and enstatite grains are slightly 16O-enriched by 2.6‰ (Nagashima et al., 2011, 2015). Analyses show that the matrix grains actually exhibit a bimodal distribution with a majority that are 16O-rich (Δ17O ~ –23.5 [±2.9] ‰) and others that are 16O-poor (Δ17O –0.1 [±1.7] ‰). In accordance with these values, the O-isotope composition of olivines composing a coarse-grained igneous rim on a porphyritic chondrule is 16O-rich (Δ17O ~ –24‰). As indicated by the O-isotopic data, Nagashima et al. (2015) deduced that Kakangari chondrule and matrix components likely formed from the same nebular reservoir, and that the observed 16O-enrichment in the bulk matrix compared to the 16O-poor bulk chondrules is due to the presence of 16O-rich grains in the matrix.

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Diagram credit: Weisberg et al., GCA, vol. 60, p. 9 (1996)
'The K (Kakangari) chondrite grouplet'

A NanoSIMS study of Kakangari conducted by Floss and Stadermann (2009, 2012) revealed a single C-anomalous grain (13C-rich), probably SiC from an AGB star, which corresponds to an overall abundance of ~4 ppm in the meteorite. Kakangari also contains relatively low abundances of presolar silicate/oxide grains (< ~5 ppm). Furthermore, there is a complete lack of both O-anomalous presolar grains and amorphous silicates in the matrix. These findings attest to the fact that Kakangari experienced secondary processing, probably on the parent body, and thus cannot be considered a primitive meteorite. It was posited by Barosch et al. (2020) that Kakangari chondrules experienced open system gas–melt exchange, fragmentation, and remelting prior to accretion of the parent body.

The fusion crust of Kakangari is described as dull and dark brown, exhibiting a close textured, locally ribbed and netted texture (S. Ghosh and A. Dube, 1999). The photos above show a 1.15 g surface fragment of Kakangari displaying both the fusion-crusted side and the matrix-rich interior.

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Photo courtesy of Geological Survey of India, Catalogue Series No. 3, S. Ghosh and A. Dube (© 1999)

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Specimen acquired December 9, 1997