Benešov (a)        49°46’ N, 14°38’ E

Stredocesky, Czech Republic

Found: 9 Apr 2011; possibly fell 7 May 1991

Classification: Ordinary chondrite (LL3.5)

History: A -19.5 absolute magnitude fireball was recorded by 3 all-sky and 2 spectral cameras at 3 Czech stations of the European Fireball Network on May 7, 1991, at 23:03:48 UT. Data on atmospheric trajectory, heliocentric orbit, fragmentation history, composition and possible impact location were subsequently determined (Spurny, 1994; Borovicka and Spurny, 1996; Borovicka et al. 1998a; Borovicka et al. 1998b). Despite great efforts no meteorite was found in the weeks and years after the fall. The fireball observations were re-analyzed by P. Spurny in 2011 and a revised impact location was determined. The new impact area was probed using metal detectors and three small meteorites were discovered - two named as Benešov (a) and one named as Benešov (b). The two Benešov (a) fragments were 250 m apart. Benešov (b) was found between them.

Physical characteristics: Weathered fragments lacking fusion crust. The meteorites resemble the terrestrial stones and slag found in the field.

Petrography: (Jakub Haloda, PCU) The meteorite is a polymict breccia containing two lithologies with different texture, chemical, and mineralogical compositions. The largest portion is an LL3.5 chondrite. An achondritic clast was also found within the thick section. This clast (4.8 × 2.6 mm) is cemented to the LL3.5 chondrite lithology by an irregular vein of impact melt. Olivine, pyroxene and plagioclase in the LL3.5 lithology show weak shock features corresponding to shock stage S3. The achondritic clast shows features corresponding to stage S4 (olivine mosaicism, plagioclase is partly isotropic).

Geochemistry: LL3.5: The chondrules and matrix are unequilibrated. Chondrules (0.2-1.9 mm) consist mostly of olivine (Fa_6-32), low-Ca pyroxene (Fs_1-37Wo_0.7-3) and alkali-bearing glass exhibiting a wide range of chemical compositions. The chondrule olivine contains on average 0.08 wt% Cr₂O₃. Type I chondrules contain 0.17 to 1.1 wt% CaO and Type II 0.04 to 1.1 wt%. Fine-grained matrix contains olivine (Fa_21-40), low-Ca pyroxene (Fs_1-32 Wo_0.7-3.5), predominantely weathered alkalic glass, Fe-Ni phases and troilite. The achondritic lithology is composed predominantly of olivine (Fo_90-69) and low-Ca pyroxene (En_79-65Wo_1.2-4.8). Anorthitic plagioclase (An_86-82Ab_18-14) and high-Ca pyroxene (En_56-48Wo_43-32) are also present.

Classification: Polymict breccia, LL3.5 with achondritic clast.

Specimens: 7.72 g (LL3.5 with achondritic clast), 1.99 g (LL3.5), both at CzAS