Mesosiderites are stony-iron meteorites made from roughly equal parts iron-nickel metal and silicate minerals such as pyroxene and plagioclase. Our piece of OSM 019 is around 40:60 metal to silicate, and is the first stony-iron meteorite we've ever classified!

Both the silicate mineral and the metal portion of this meteorite is interesting to us. The silicate minerals here have the same basic composition as those in the HED meteorites, but are ever so slightly different. The metal portion can be broken down into iron and iron-nickel metal, and we also found areas of an iron-nickel phosphide ([Fe,Ni]₃ P) called schreibersite that appears in between individual iron metal crystals.

Backscattered electron (BSE) image of OSM 019. In this greyscale: darkest grey = plagioclase feldspar; dark grey = pyroxen; light grey = iron-nickel metal; lightest grey = schreibersite.

Mineral map of OSM 019. In this colour scheme: green = iron metal; yellow = iron-nickel metal; pink = schreibersite; dark blue = pyroxene; red = plagioclase feldspar; orange = troilite; turquoise = apatite; white = silica.

We mentioned earlier that the silicate minerals in mesosiderites like this one are largely the same as those we find in HED meteorites, but not quite. Researchers have also found that mesosiderites and HED meteorites have the same oxygen isotope compositions which means that they formed in the same part of the Solar System. Naturally, this raises the question that are mesosiderites (or at least the silicate minerals) and HEDs are from the same parent body?

This question has been rattling around the planetary science community for years, but a recent study by S.I. Lelarge and colleagues suggests that those little differences in silicate mineral composition reflect two different parent bodies - one for HEDs and one for mesosiderites - from a similar patch of space. Basaltic magmatism is very common throughout the Solar System, so it's easy to see how two different asteroids could produce similar rocks and when we double checked our data of OSM 019 we did indeed find that the pyroxenes had a different iron-manganese ratio to HED meteorites. To summarise, our classification data supports the idea that mesosiderites have their own parent body and didn't share it with the HEDs!