FIRST FIND OF WADSLEYITE AND RINGWOODITE IN THE ALFIANELLO L6 ORDINARY CHONDRITE

Ordinary chondrites (OCs) are the most abundant (>80%) group of meteorites recovered on Earth and also among the most primitive materials in the solar system. They are composed of an assemblage of chondrules (60-80 vol.%) and fine grained matrix. Subjected to collisional processes on small bodies, OCs have suffered variable degrees of shock metamorphism. Olivine, pyroxene and plagioclase might have been transformed into high pressure polymorphs, such as ringwoodite (spinel structure), majorite (hollandite structure) and jadeite, respectively, or contain other evidence of shock metamorphism, e.g., planar deformation features, shock darkening, amorphization, etc. The Alfianello (Brescia, Italy) meteorite is an L6 ordinary chondrite fallen on February 16, 1883. Petrological investigation performed by indicated hat it mostly consists of olivine and low Ca pyroxene, while plagioclase, merrillite and apatite occur in minor amounts. Other accessory phases include chromite and ilmenite, nickel-iron alloys and sulphides. Plagioclase is commonly transformed into maskelynite and rarely occurs as birefringent crystals. According to the shock classification proposed by [2], the deformation features observed in the investigated sample from the Alfianello meteorite indicate high shock, equivalent to shock degree S5. Experimental: The sample studied here is a petrographic thin section of a fragment 7 x 10 mm in size displaying no fusion crust from the Alfianello chondrite, kindly provided by the Museo di Storia Naturale of the University of Florence. The investigated sample has been examined with a variety of analytical techniques: optical microscopy, Field Emission Scanning Electron Microscopy (FE-SEM), Electron MicroProbe Analysis (EMPA) and Raman spectroscopy, with the aim of further characterising shock effects and confirm the classification of this meteorites. Moreover, a portion of an olivine grain displaying a fine microstructure was extracted by FIB obtaining a lamella thin ~100 nm in order to perform further investigation by TEM