Shidian meteorite, a new fall analog of near-Earth asteroid (101955) Bennu

Shidian is a recent meteorite which fell in Yunnan province, China, on November 27, 2017, and has been classified as a CM2 chondrite. Petrography, mineralogy, oxygen and chromium isotopic composition, reflectance spectrum, and density studies of Shidian are reported in this study. Clasts with different aqueous alteration degree, two type 1 clasts with nontypical CM petrography, and one metamorphic clast are observed in Shidian. Mineralogically, Shidian main body consists of phyllosilicates (similar to 70 vol%), forsterite (similar to 13 vol%), fayalitic olivine, carbonates, sulfide, high-Ca pyroxene, magnetite framboids, and Fe-Ni metal. The average electron microprobe analysis (EMPA) analytical totals of phyllosilicates are 84.07 +/- 1.75 wt%, with average FeO/SiO2 of tochilinite-cronstedtite intergrowths (TCIs) in different clasts ranging from 1.18 to 3.29. The bulk geochemical composition is characterized by flat rare earth element pattern, and by depletion of highly volatile elements. The whole rock oxygen isotopic composition is -0.51 +/- 0.73 parts per thousand, 5.44 +/- 1.01 parts per thousand, and -3.38 +/- 0.20 parts per thousand for delta O-17, delta O-18, and Delta O-17, respectively, with bulk chromium isotopic composition as epsilon Cr-54 = 1.00 +/- 0.11. The grain density, bulk density, and porosity are 2.758 +/- 0.008 g cm(-3), 2.500 +/- 0.004 g cm(-3), and 9.37 +/- 0.59%, respectively. The reflectance spectrum shows "blue" (negative) continuum slope across the visible and near-infrared range, with characteristic absorption features (such as 0.765, 0.923, and 1.160 mu m for phyllosilicates). These characteristics indicate that Shidian is an unheated, brecciated CM chondrite and may be an analog of asteroid Bennu.