Metallic iron asteroids of spectral type "M" are considered to be the "parent body" of iron-nickel meteorites. Examples of type M asteroids are 16 Psyche or 22 Kalliope.

Since iron meteorites contain a large amount of platin group elements, iron asteroids could turn out to be "flying orebodies" in the forseeable future.

But as it turns out, many M type asteroids have very low densities: 16 Psyche has 1.8 g/ccm (which is very low compared to about 7-8 g/ccm for iron meteorites), with a small diameter it has still 3.3 g/ccm. Kalliope has a density of about 2.4 g/ccm, still relatively low compared to iron meteorites.

There are two possible solutions to this "problem" (maybe you can think of another one?):

1. High "macroporosity" - assuming the asteroids contain a lot of empty space, up to 75% (!), depending on the density. Problem: If these (large and massive) asteroids are rubble piles, you would expect a lognormal size distribution of their components, the "gaps" between the larger "boulders" could be very easily filled by smaller components. Even if you had no smaller components, with the densest "sphere packing" you get 26% free space at maximum. We know that the the porosity must be a large scale one because all the iron meteorites we find here on earth are "dense" and not porous.

2. The M type asteroids aren't "metallic iron asteroids", but metal-silicate ones with densities around 4 g/ccm. In that case, you the "macroporosity" can be lower, around 30%. This is still large, considering that...
1. all metal-silicate meteorites here on earth (e.g. pallasites) are dense, so we are speaking of large scale "open space" between the components of the asteroid, which brings back the "sphere packing problem" explained above, and...
2. these asteroids are huge, compared to others. even if they were "rubble piles", you would expect them to collapse under their own gravity (at least for the smaller components)

This second solution seems more plausible, but raises another question: where, then, are the "parent bodies" of the metallic iron meteorites?

An interesting question.

I don't have an answer, but some suggestions on how to go about finding an answer (some, or all, of this work may have already been done):

- how long is it since metallic iron meteorites were part of larger bodies? I imagine cosmic ray tracks, by depth, would give some indications.

- what orbital families do M asteroids and metallic iron meteorites seems to be associated with? IIRC, studies of these can sometimes yield likely ages of parent body breakup, as well as the largest member.

The "cosmic ages" of iron meteorites are (on average) larger than for all other meteorite classes. They go up to 500 million years, where your garden variety ordinary chondrite spends up to 10, 20, 30 million years in space.

AFAIK, there are no associations of M asteroids with their possible "mantle parts" in asteroid families. 44 Nysa was once considered to be the core of its family, but as it turned out, "its family" is a family for itself, and Nysa doesn't really belong to them.