Unusual forms of magnetite in the Orgueil carbonaceous chondrite

Abstract— The Orgueil CI carbonaceous chondrite contains magnetite (Fe₃O₄) that displays a rich and seemingly unique variety of morphologies. While many have already been described, recent images provide far greater definition and, thus, provide more unambiguous identification of trapezohedral crystals and possibly also trisoctahedral forms. Although it is an extremely abundant terrestrial mineral and commonly occurs in well-developed crystals, neither of these forms have been recognized previously from magnetite. The barrel-shaped arrays of magnetite discs (“plaquettes”) are also unique. It appears as if the circumstances for the formation of these unusual crystals were somehow exceptional. Their morphologies and association with sulfates and carbonates suggest an aqueous origin.