Qidong

L/LL5

Fell July 2, 1982
32° 05′ N., 121° 30′ E. A single stone of 1,275 g fell at 5:45 P.M. in Qidong County, Jiangsu province, China. After detonations were heard, the meteorite was found lying in a field. Qidong has a recrystallized texture but chondrules are still obvious, consistent with a petrologic grade of 5. Shock features include extensive fracturing and undulose extinction in silicates, with mosaisicm recognized in some olivines. Fine-grained troilite/metal assemblages are abundant throughout. The vast majority of metal grains are taenite.

Qidong follows metal–silicate trends that are different from those of the recognized ordinary chondrite groups. Evidence for a new intermediate L/LL-chondrite group is becoming well documented. The average olivine fayalite content of 25.7 mole% places Qidong at the extreme higher end of the L group (22.7–25.6 mole%), biased towards the LL group. Still, the Fa in one olivine studied was 28.4 mole%, within the range of the LL group. The average ferrosilite content of 21.5 mole% is at the extreme higher end of the L group (18.7–21.8 mole%), also biased towards the LL group. Qidong has an FeNi-metal abundance of 4.7 wt% placing it at the extreme lower end of the L group (4.4–11.7 wt%) and in the middle of the LL group (3.0–6.0 wt%). Another classification parameter useful for distinguishing between the L and LL groups is the Co abundance (mg/g) in matrix kamacite. An average of 15 mg/g was measured for Qidong, placing it at the extreme lower end of the LL range (15–110) and well outside that of the L range (6.7–8.2).

It was demonstrated by Szurgot (2016) that the mean atomic weight (Amean) of meteorites can be used to resolve the OC groups, including the intermediate groups L/LL and H/L. Amean values can also be predicted through various equations based on other parameters such as atomic Fe/Si ratio, grain density, and magnetic susceptibility, and these Amean values all consistently resolve these groups into the ordered sequence LL < L/LL < L < H/L < H. Furthermore, it was demonstrated that Amean values are lower for unequilibrated type 3 samples than for equilibrated samples within each OC group due to the presence of water; Amean values for petrologic types 4–6 are indistinguishable within each group.
Diagram credit: M. Szurgot, 47th LPSC, #2180 (2016)
Amean based on chemical composition (Eq. 1), Fe/Si atomic ratio (Eq. 2), and grain density (Eq. 3) The discovery of several anomalous olivine and pyroxene grains having abnormal Mg# suggests that Qidong is a fragmental breccia. The various components of the breccia must have been mixed by a late impact event after the main period of metamorphism had ended. Final lithification was accomplished through subsequent smaller impact shock events.

Although Qidong is published as a classification of L5, a growing body of evidence is leading to the conclusion that it is another member of an intermediate chondrite group. This group includes L/LL3 Esperance, L/LL4 Seemore Downs, Bjurböle, and Cynthiana, L/LL5/6 Sahara 97021, L/LL6 Acfer 041, and several others. A few meteorites are only partially resolved into the L/LL group, including L/LL3 Inman, L/LL5 Knyahinya, and L/LL6 Holbrook and Sultanpur. The main mass of Qidong is held at the Purple Mountain Observatory, Academia Sinica, in Nanjing, China. The Qidong specimen shown above weighs 2.16 g, and it was cut from a 35 g fragment purchased at the 2001 Macovich Meteorite Auction in Tucson.