Meteoritical Bulletin: Entry for Silistra

Writeup from MB 100:

[NOTE: This provisional writeup was superseded 15 Sept 2017]

Silistra 44°7’N, 27°16’E

Silistra, Bulgaria

Fell: 19 July 1917

Classification: Ungrouped achondrite

History: A bolide was observed in the sky over Elena, Razgrad, and Silistra, Bulgaria, and Tulcea, Romania (fall direction SW - NE) at around 7 am on July 19, 1917. Several fragments were recovered but a single one reached the Mineralogy Museum of the University of Sofia (Dimov, 1975). Coordinates given are for Silistra town.

Petrography: No crystals are observed on the SEM section. CT-scan and µXRF indicate very minor amount of dense crystals ~100 µm size.

Geochemistry: Oxygen isotopic composition lies on the HED fractionation line: (J. Gattacceca, C. Sonzogni, CEREGE).

Classification: The material is composed almost entirely of vesicular glass, possibly related to eucrites.

Specimens: The main mass remaining after analysis is 0.125 g and resides at USof (curator E. Neykova). One polished micro-section and 10 mg are at CEREGE..

Silistra 44°7’N, 27°16’E

Silistra, Bulgaria

Fell: 19 July 1917

Classification: Ungrouped achondrite

Physical characteristics: A single 0.15 g fragment (1.1 × 0.4 cm) was preserved; one side shows a smooth rounded fusion crust, the other a cut or broken inner surface. Porosity is estimated to be 86%. The material is composed of vesicular glass and looks like a dark pumice. µCT-scan indicates homogeneous distribution of bubbles with moderate elongation. Bubbles diameter ranges from 10 μm to 3 mm, with median diameter of 70 μm. Based on fusion crust shape the full initial object can be estimated to be ~2 cc, 0.8 g object with a shape similar to core tektite.

Petrography: No crystals are observed on the SEM section. CT-scan and µXRF indicate very minor amount of dense crystals ~100 µm size identified as chromite and ilmenite based on Cr and Ti spots on the µXRF map.

Geochemistry: Oxygen isotopic composition was measured in CEREGE on two 1.5 mg powdered samples and lies on the HED fractionation line: δ¹⁷O = 2.64 and 2.50 ‰, δ¹⁸O = 5.55 and 5.30 ‰, and Δ¹⁷O = -0.27 and -0.27‰ per mil (J. Gattacceca, C. Sonzogni, CEREGE). HED falls were measured during the same session and fit with reference values. EMPA analysis (mean of 14 points) of the glass yield composition similar to cumulate eucrite: Fe/Mn= 29, Fe/Mg= 0.8 (atomic ratio); SiO₂ 48.6, FeO 15.2, Al₂O₃ 14.2,CaO 9.9, MgO 9.2, MnO 0.5, Cr₂O₃ 0.4, NaO 0.3, TiO₂ 0.1 (wt.%). K, Ni, S, Cl were undetectable. Average total at 98.2% suggest minor volatiles and Fe³⁺. Rare gas analyses (B. Marty, CRPG) indicate strong atmospheric contamination but significant content of cosmogenic ²¹Ne, pointing toward an apparent residence time in space of 0.2 Ma. Cosmogenic ³He is also present. Rare gas data indicate strong heating during atmospheric entry, but probably below the fusion temperature.

Classification: The material is composed almost entirely of vesicular glass, possibly related to eucrites based on oxygen isotopes and composition.

Specimens: The main mass remaining after analysis is 0.125 g and resides at USof (curator E. Neykova). One polished micro-section and 10 mg are at CEREGE.