At 12:30 P.M. in Estonian SSR, sonic booms were heard and stones fell at Aukoma, Kurla, Wahhe, and Sawiauk. The weight of these stones was ~14 kg, 7.5 kg, 1.5 kg, and 0.25 kg. These falls are also known by the names of Pilistvere and Pillistvere.
The Van SchmusWood (1967) scheme for petrographic type has been modified for enstatite chondrites, establishing both a textural type (37), reflecting peak metamorphic temperature, and a mineralogical type (αδ), pertaining to the cooling history (Zhang and Sears, 1996; Quirico et al., 2011). Under this classification scheme, Pillistfer has thermometers that give it a classification of EL6β.
Enstatite chondrites were formed in a highly reducing environment. Therefore, they contain virtually no metal in the oxide formmuch less by comparison to other chondrites and to the terrestrial planets. Iron in EL6 chondrites is depleted and isotopically fractionated compared to less metamorphosed EL3 and EH chondrites (Wang et al., 2013). A trace element analysis utilizing nonmagnetic micron-scale grains from Pillistfer was conducted by Lavrentjeva and Lyul (2017). They found depletions in siderophile elements and enrichments in lithophile elements, which indicates that nebular metalsilicate fractionation of precursor material occurred, as well as redistribution during parent body metamorphism. The mineral sinoite (silicon oxynitride) has been found to occur in Pillistfer and many other EL chondrites that have a high bulk N content. Sinoite is associated with crystallization from an impact melt, or alternatively, with metamorphic processes. This suggests that Pillistfer experienced a period of high, possibly melt-forming temperatures. A rapid cooling phase was initiated consistent with 0.8°C/day (Kissin, 1989). This was followed by a period of annealing, and then a final shock to stage S2.
An isochron age for Pillistfer representing the KAr system closure was calculated by Bogard et al. (2010) to be 4,541 (±7) m.y. ago., a similar age to that of several equilibrated E chondrites. A comparison of the younger ArAr ages measured for ordinary chondrites suggests that E chondrites cooled more quickly, possibly reflecting a smaller parent body size, a lower initial heating level, a shallower burial, and/or a collisional disruption prior to KAr closure. More recently, employing a broader range of EL chondrite petrologic types (i.e., formation temperatures), Hopp et al. (2013, 2014) determined a lower corrected age range for metamorphic cooling of EL5 and EL6 meteorites of 4.484.51 b.y. In a similar manner, the ArAr isochron age for an EL3 chondrite reflected a younger age, possibly representing a late-stage impact ~4.434.47 b.y. ago. This better constrained age range would allow for a more extended period of time for parent body cooling and a relaxation of the constraints on the parent body size. However, since the KAr closure for the EL parent body occurred 30 m.y. earlier than that of the H-chondrite parent body, the size of the EL parent body was most likely significantly smaller than the H parent body.
Oxygen isotopic studies place the formation of enstatite chondrites on the terrestrial fractionation line, which is taken by some to mean that they formed within the inner Solar System. Based on MnCr isotope systematics and its correlation with heliocentric distance, Shukolyukov and Lugmair (2004) concluded that E chondrites originated ~1.01.4 AU from the Sun before being perturbed into their present locations in the asteroid belt. Similarly, Nakashima et al. (2006) calculated a heliocentric distance of >1.1 and 1.3 AU for two EL3 chondrites (ALH 85119 and MAC 88136, respectively) on the basis of their implanted solar noble gas concentrations.
In contrast, the identification of the E-asteroid group, including Hungaria at 1.94 AU, Nysa at 2.42 AU, and Angelina at 2.68 AU, suggests that the actual solar region of formation could lie at a greater heliocentric distance. Cosmochemists are presently trying to construct a suitable theory involving oxygen depletion in this E-asteroid region of the Solar System to explain the conflicting theories. The specimen pictured above is a 5.7 g partial slice showing the abundant free metal that characterizes this group.