> Other, smaller chunks in the Bennu sample have light-coloured reflective coatings, making them seem brighter than the darker pebbles. But, under a scanning electron microscope, they were found to be covered in a highly brittle layer that broke easily, revealing a dark interior. Chemical analysis showed that this light-coloured surface skin contained magnesium, sodium and phosphate. This combination is rarely if ever seen in meteorites, Lauretta said. “It’s a head-scratcher right now.”

Can anyone expound on which part of this is rare or what would be more expected?

I'm guessing that if most meteorite samples are collected after impact on earth such a brittle crust would be destroyed before observation.