Five stones with black, bubbly crust were found by a member of the Geological Survey of Egypt in a farm field on the Nile River near Luxor. Today only a single 23 kg specimen remains preserved.
Recent studies have determined that systematic changes occur in amoeboid olivine aggregates (AOAs) with increasing subtype, which is directly linked to increasing aqueous and thermal metamorphism (Chizmadia et al., 2002; Grossman and Rubin, 2006). For example, textures and morphologies of AOAs show changes, olivine in AOIs becomes progressively FeO-rich, troilite becomes more prevalent, and trace elements become more equilibrated. Because of their smaller grain size, olivines in AOAs are better indicators of alteration processes (such as the substitution of Fe for Mg) than are the chondrules, which were previously utilized to determine subtype. As a result of this study, a refinement in the subtypes of the CO3 chondrites was proposed; the CO carbonaceous chondrite group would span a petrographic sequence from type 3.0, represented by Colony, to type 3.8, represented by Isna.
Chizmadia and Bravo-Ruiz (2013) employed a similar method to that of Grossman and Rubin (2006) to classify CO3 chondrites by the degree of aqueous alteration, in which they utilized the Fe-Mg composition and distribution in olivines in AOAs. However, Chizmadia and Bravo-Ruiz extended their study to comprise the entire metamorphic range. Based on their study, they proposed that Isna should be assigned to petrologic type 3.75. They also better resolved Colony as type 3.05, and assigned the MET 00694 pairing group to the highest CO3 petrologic type of 3.8.
In a different analysis of CO3 petrologic types conducted by Bonal et al. (2005), they found that an accurate comparison could be made between the metamorphic grades of the CO and the ordinary chondrites using Raman spectrometry combined with petrographic analysis. Their method is based on the structural order of the chondritic organic matter, which was initially accreted in the same proportions in both CO and ordinary chondrites. From their data, the CO group currently spans a petrographic sequence from 3.03 as represented by ALHA77307, to 3.7 as represented by both Warrenton and Isna.
Isna represents the most highly metamorphosed CO3 both thermally and aqueously. The peak metamorphic temperature of Isna was ~530°C, reflected by its higher Ni and Co content, lower Cr in kamacite concentration, coarser texture, decreased abundance of presolar diamonds, and increased TL sensitivity due to feldspar crystallization from chondrule glass. Compared to other CO chondrites, Isna has an exceptionally low cosmic ray exposure age of ~0.15 m.y. A high solar noble gas content is evidence for a regolith origin.
A high hydrothermal alteration is reflected in the increased size and abundance of rimmed AOAs, an apparant increase in chondrule size, and melilite-rich refractory inclusions altered to fine-grained, spinelpyroxene inclusions. In addition, 16O compositions are lowest in the highest petrographic subtypes, reflecting an increased oxygen exchange in an 16O-poor water reservoir during hydrothermal metamorphism.
Some theories have linked this group to the K-type asteroids of the Eos family, a group that possesses S-type spectra in visible wavelengths and C-type spectra in the near-infrared. Based on reflectance spectra and albedo, the asteroids 221 Eos (104 km diameter) and 653 Berenike (39 km diameter) were found to be good analogs for the olivine-rich CO3 meteorites. These asteroids are located near 3 AU at the 9:4 resonance, where meteorites are expected to become Earth-crossers on timescales greater than tens of millions of years. However, utilizing more advanced spectrographic techniques comparing a broader spectrum, it was determined that a better fit to the Eos family was the ungrouped achondrite Colony page. The above specimen is a 4.5 g partial slice.