It's rare to find volatile compounds like phosphate and sodium on the surface of a meteorite because they leach out and vaporize. It would make sense on a comet

It's rare to find volatile compounds like phosphate and sodium on the surface of a meteorite because they leach out and vaporize. It would make sense on a comet which form in the outer zone of our protoplanetary disk and shed their surface continuously due to all the water, but not a magnesium-rich asteroid that would have formed closer to the center of the solar system where the sun would have boiled volatile compounds off quickly, without a way for replacement compounds to migrate to the surface.