On the occurrences and formation mechanisms of cliftonites: The case of Campo del Cielo iron meteorite

The origin and shock history of IAB meteorites are still a matter of debate. From the literature, the study of carbon phases can provide further information on the thermal conditions to which the meteorites have been exposed. Within this framework, the occurrences of cliftonites -classically defined as graphite aggregates- in a sample of the Campo del Cielo iron meteorite, were explored through optical and electron microscopy, and Raman spectroscopy. The assessment of carbon organization within the cliftonites, allowed us to propose a partial melting process of the kamacitic matrix followed by its resolidification and the optimization of the shape orientation relationships by the host crystal. This scenario combined with the cliftonites characterization at the micro and nanoscale led us to suggest a growth mechanism for the cliftonites in our Campo del Cielo sample, while the results from previous studies could account for cliftonites formation under diverse temperature -pressure-composition conditions as it is expected among different specimens and within them.