Data Repository 1: Analytical methods 1.1. Sample preparation Chromitite samples were collected from Tehuitzingo serpentinites. Thin and thick polished sections were prepared at the Serveis de lámina prima of the University of Barcelona. The thin sections were polished using diamond abrasive paste of 1 µm particle size. Due to the fact that most of the identified diamonds are bigger than 1 µm, we can exclude the possibility of them being the result of contamination by polishing material. A second set of duplicate thin sections from samples containing diamond was prepared using Al 2 O 3 as a carbon-free polishing medium to exclude contamination. An ultrasonic bath was used to remove any remaining polishing material from the sample. 1.2. Field emission scanning electron microscopy (FE-SEM) The samples were examined under a Field Emission Scanning Electron
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